New upstream version 1.62.1+dfsg1

[rustc.git] / compiler / rustc_lint / src / types.rs

use crate::{LateContext, LateLintPass, LintContext};
use rustc_ast as ast;
use rustc_attr as attr;
use rustc_data_structures::fx::FxHashSet;
use rustc_errors::Applicability;
use rustc_hir as hir;
use rustc_hir::def_id::DefId;
use rustc_hir::{is_range_literal, Expr, ExprKind, Node};
use rustc_middle::ty::layout::{IntegerExt, LayoutOf, SizeSkeleton};
use rustc_middle::ty::subst::SubstsRef;
use rustc_middle::ty::{self, AdtKind, DefIdTree, Ty, TyCtxt, TypeFoldable};
use rustc_span::source_map;
use rustc_span::symbol::sym;
use rustc_span::{Span, Symbol, DUMMY_SP};
use rustc_target::abi::{Abi, WrappingRange};
use rustc_target::abi::{Integer, TagEncoding, Variants};
use rustc_target::spec::abi::Abi as SpecAbi;

use std::cmp;
use std::iter;
use std::ops::ControlFlow;
use tracing::debug;

declare_lint! {
    /// The `unused_comparisons` lint detects comparisons made useless by
    /// limits of the types involved.
    ///
    /// ### Example
    ///
    /// ```rust
    /// fn foo(x: u8) {
    ///     x >= 0;
    /// }
    /// ```
    ///
    /// {{produces}}
    ///
    /// ### Explanation
    ///
    /// A useless comparison may indicate a mistake, and should be fixed or
    /// removed.
    UNUSED_COMPARISONS,
    Warn,
    "comparisons made useless by limits of the types involved"
}

declare_lint! {
    /// The `overflowing_literals` lint detects literal out of range for its
    /// type.
    ///
    /// ### Example
    ///
    /// ```rust,compile_fail
    /// let x: u8 = 1000;
    /// ```
    ///
    /// {{produces}}
    ///
    /// ### Explanation
    ///
    /// It is usually a mistake to use a literal that overflows the type where
    /// it is used. Either use a literal that is within range, or change the
    /// type to be within the range of the literal.
    OVERFLOWING_LITERALS,
    Deny,
    "literal out of range for its type"
}

declare_lint! {
    /// The `variant_size_differences` lint detects enums with widely varying
    /// variant sizes.
    ///
    /// ### Example
    ///
    /// ```rust,compile_fail
    /// #![deny(variant_size_differences)]
    /// enum En {
    ///     V0(u8),
    ///     VBig([u8; 1024]),
    /// }
    /// ```
    ///
    /// {{produces}}
    ///
    /// ### Explanation
    ///
    /// It can be a mistake to add a variant to an enum that is much larger
    /// than the other variants, bloating the overall size required for all
    /// variants. This can impact performance and memory usage. This is
    /// triggered if one variant is more than 3 times larger than the
    /// second-largest variant.
    ///
    /// Consider placing the large variant's contents on the heap (for example
    /// via [`Box`]) to keep the overall size of the enum itself down.
    ///
    /// This lint is "allow" by default because it can be noisy, and may not be
    /// an actual problem. Decisions about this should be guided with
    /// profiling and benchmarking.
    ///
    /// [`Box`]: https://doc.rust-lang.org/std/boxed/index.html
    VARIANT_SIZE_DIFFERENCES,
    Allow,
    "detects enums with widely varying variant sizes"
}

#[derive(Copy, Clone)]
pub struct TypeLimits {
    /// Id of the last visited negated expression
    negated_expr_id: Option<hir::HirId>,
}

impl_lint_pass!(TypeLimits => [UNUSED_COMPARISONS, OVERFLOWING_LITERALS]);

impl TypeLimits {
    pub fn new() -> TypeLimits {
        TypeLimits { negated_expr_id: None }
    }
}

/// Attempts to special-case the overflowing literal lint when it occurs as a range endpoint.
/// Returns `true` iff the lint was overridden.
fn lint_overflowing_range_endpoint<'tcx>(
    cx: &LateContext<'tcx>,
    lit: &hir::Lit,
    lit_val: u128,
    max: u128,
    expr: &'tcx hir::Expr<'tcx>,
    parent_expr: &'tcx hir::Expr<'tcx>,
    ty: &str,
) -> bool {
    // We only want to handle exclusive (`..`) ranges,
    // which are represented as `ExprKind::Struct`.
    let mut overwritten = false;
    if let ExprKind::Struct(_, eps, _) = &parent_expr.kind {
        if eps.len() != 2 {
            return false;
        }
        // We can suggest using an inclusive range
        // (`..=`) instead only if it is the `end` that is
        // overflowing and only by 1.
        if eps[1].expr.hir_id == expr.hir_id && lit_val - 1 == max {
            cx.struct_span_lint(OVERFLOWING_LITERALS, parent_expr.span, |lint| {
                let mut err = lint.build(&format!("range endpoint is out of range for `{}`", ty));
                if let Ok(start) = cx.sess().source_map().span_to_snippet(eps[0].span) {
                    use ast::{LitIntType, LitKind};
                    // We need to preserve the literal's suffix,
                    // as it may determine typing information.
                    let suffix = match lit.node {
                        LitKind::Int(_, LitIntType::Signed(s)) => s.name_str(),
                        LitKind::Int(_, LitIntType::Unsigned(s)) => s.name_str(),
                        LitKind::Int(_, LitIntType::Unsuffixed) => "",
                        _ => bug!(),
                    };
                    let suggestion = format!("{}..={}{}", start, lit_val - 1, suffix);
                    err.span_suggestion(
                        parent_expr.span,
                        "use an inclusive range instead",
                        suggestion,
                        Applicability::MachineApplicable,
                    );
                    err.emit();
                    overwritten = true;
                }
            });
        }
    }
    overwritten
}

// For `isize` & `usize`, be conservative with the warnings, so that the
// warnings are consistent between 32- and 64-bit platforms.
fn int_ty_range(int_ty: ty::IntTy) -> (i128, i128) {
    match int_ty {
        ty::IntTy::Isize => (i64::MIN.into(), i64::MAX.into()),
        ty::IntTy::I8 => (i8::MIN.into(), i8::MAX.into()),
        ty::IntTy::I16 => (i16::MIN.into(), i16::MAX.into()),
        ty::IntTy::I32 => (i32::MIN.into(), i32::MAX.into()),
        ty::IntTy::I64 => (i64::MIN.into(), i64::MAX.into()),
        ty::IntTy::I128 => (i128::MIN, i128::MAX),
    }
}

fn uint_ty_range(uint_ty: ty::UintTy) -> (u128, u128) {
    let max = match uint_ty {
        ty::UintTy::Usize => u64::MAX.into(),
        ty::UintTy::U8 => u8::MAX.into(),
        ty::UintTy::U16 => u16::MAX.into(),
        ty::UintTy::U32 => u32::MAX.into(),
        ty::UintTy::U64 => u64::MAX.into(),
        ty::UintTy::U128 => u128::MAX,
    };
    (0, max)
}

fn get_bin_hex_repr(cx: &LateContext<'_>, lit: &hir::Lit) -> Option<String> {
    let src = cx.sess().source_map().span_to_snippet(lit.span).ok()?;
    let firstch = src.chars().next()?;

    if firstch == '0' {
        match src.chars().nth(1) {
            Some('x' | 'b') => return Some(src),
            _ => return None,
        }
    }

    None
}

fn report_bin_hex_error(
    cx: &LateContext<'_>,
    expr: &hir::Expr<'_>,
    ty: attr::IntType,
    repr_str: String,
    val: u128,
    negative: bool,
) {
    let size = Integer::from_attr(&cx.tcx, ty).size();
    cx.struct_span_lint(OVERFLOWING_LITERALS, expr.span, |lint| {
        let (t, actually) = match ty {
            attr::IntType::SignedInt(t) => {
                let actually = if negative {
                    -(size.sign_extend(val) as i128)
                } else {
                    size.sign_extend(val) as i128
                };
                (t.name_str(), actually.to_string())
            }
            attr::IntType::UnsignedInt(t) => {
                let actually = size.truncate(val);
                (t.name_str(), actually.to_string())
            }
        };
        let mut err = lint.build(&format!("literal out of range for `{}`", t));
        if negative {
            // If the value is negative,
            // emits a note about the value itself, apart from the literal.
            err.note(&format!(
                "the literal `{}` (decimal `{}`) does not fit into \
                 the type `{}`",
                repr_str, val, t
            ));
            err.note(&format!("and the value `-{}` will become `{}{}`", repr_str, actually, t));
        } else {
            err.note(&format!(
                "the literal `{}` (decimal `{}`) does not fit into \
                 the type `{}` and will become `{}{}`",
                repr_str, val, t, actually, t
            ));
        }
        if let Some(sugg_ty) =
            get_type_suggestion(cx.typeck_results().node_type(expr.hir_id), val, negative)
        {
            if let Some(pos) = repr_str.chars().position(|c| c == 'i' || c == 'u') {
                let (sans_suffix, _) = repr_str.split_at(pos);
                err.span_suggestion(
                    expr.span,
                    &format!("consider using the type `{}` instead", sugg_ty),
                    format!("{}{}", sans_suffix, sugg_ty),
                    Applicability::MachineApplicable,
                );
            } else {
                err.help(&format!("consider using the type `{}` instead", sugg_ty));
            }
        }
        err.emit();
    });
}

// This function finds the next fitting type and generates a suggestion string.
// It searches for fitting types in the following way (`X < Y`):
//  - `iX`: if literal fits in `uX` => `uX`, else => `iY`
//  - `-iX` => `iY`
//  - `uX` => `uY`
//
// No suggestion for: `isize`, `usize`.
fn get_type_suggestion(t: Ty<'_>, val: u128, negative: bool) -> Option<&'static str> {
    use ty::IntTy::*;
    use ty::UintTy::*;
    macro_rules! find_fit {
        ($ty:expr, $val:expr, $negative:expr,
         $($type:ident => [$($utypes:expr),*] => [$($itypes:expr),*]),+) => {
            {
                let _neg = if negative { 1 } else { 0 };
                match $ty {
                    $($type => {
                        $(if !negative && val <= uint_ty_range($utypes).1 {
                            return Some($utypes.name_str())
                        })*
                        $(if val <= int_ty_range($itypes).1 as u128 + _neg {
                            return Some($itypes.name_str())
                        })*
                        None
                    },)+
                    _ => None
                }
            }
        }
    }
    match t.kind() {
        ty::Int(i) => find_fit!(i, val, negative,
                      I8 => [U8] => [I16, I32, I64, I128],
                      I16 => [U16] => [I32, I64, I128],
                      I32 => [U32] => [I64, I128],
                      I64 => [U64] => [I128],
                      I128 => [U128] => []),
        ty::Uint(u) => find_fit!(u, val, negative,
                      U8 => [U8, U16, U32, U64, U128] => [],
                      U16 => [U16, U32, U64, U128] => [],
                      U32 => [U32, U64, U128] => [],
                      U64 => [U64, U128] => [],
                      U128 => [U128] => []),
        _ => None,
    }
}

fn lint_int_literal<'tcx>(
    cx: &LateContext<'tcx>,
    type_limits: &TypeLimits,
    e: &'tcx hir::Expr<'tcx>,
    lit: &hir::Lit,
    t: ty::IntTy,
    v: u128,
) {
    let int_type = t.normalize(cx.sess().target.pointer_width);
    let (min, max) = int_ty_range(int_type);
    let max = max as u128;
    let negative = type_limits.negated_expr_id == Some(e.hir_id);

    // Detect literal value out of range [min, max] inclusive
    // avoiding use of -min to prevent overflow/panic
    if (negative && v > max + 1) || (!negative && v > max) {
        if let Some(repr_str) = get_bin_hex_repr(cx, lit) {
            report_bin_hex_error(
                cx,
                e,
                attr::IntType::SignedInt(ty::ast_int_ty(t)),
                repr_str,
                v,
                negative,
            );
            return;
        }

        let par_id = cx.tcx.hir().get_parent_node(e.hir_id);
        if let Node::Expr(par_e) = cx.tcx.hir().get(par_id) {
            if let hir::ExprKind::Struct(..) = par_e.kind {
                if is_range_literal(par_e)
                    && lint_overflowing_range_endpoint(cx, lit, v, max, e, par_e, t.name_str())
                {
                    // The overflowing literal lint was overridden.
                    return;
                }
            }
        }

        cx.struct_span_lint(OVERFLOWING_LITERALS, e.span, |lint| {
            let mut err = lint.build(&format!("literal out of range for `{}`", t.name_str()));
            err.note(&format!(
                "the literal `{}` does not fit into the type `{}` whose range is `{}..={}`",
                cx.sess()
                    .source_map()
                    .span_to_snippet(lit.span)
                    .expect("must get snippet from literal"),
                t.name_str(),
                min,
                max,
            ));
            if let Some(sugg_ty) =
                get_type_suggestion(cx.typeck_results().node_type(e.hir_id), v, negative)
            {
                err.help(&format!("consider using the type `{}` instead", sugg_ty));
            }
            err.emit();
        });
    }
}

fn lint_uint_literal<'tcx>(
    cx: &LateContext<'tcx>,
    e: &'tcx hir::Expr<'tcx>,
    lit: &hir::Lit,
    t: ty::UintTy,
) {
    let uint_type = t.normalize(cx.sess().target.pointer_width);
    let (min, max) = uint_ty_range(uint_type);
    let lit_val: u128 = match lit.node {
        // _v is u8, within range by definition
        ast::LitKind::Byte(_v) => return,
        ast::LitKind::Int(v, _) => v,
        _ => bug!(),
    };
    if lit_val < min || lit_val > max {
        let parent_id = cx.tcx.hir().get_parent_node(e.hir_id);
        if let Node::Expr(par_e) = cx.tcx.hir().get(parent_id) {
            match par_e.kind {
                hir::ExprKind::Cast(..) => {
                    if let ty::Char = cx.typeck_results().expr_ty(par_e).kind() {
                        cx.struct_span_lint(OVERFLOWING_LITERALS, par_e.span, |lint| {
                            lint.build("only `u8` can be cast into `char`")
                                .span_suggestion(
                                    par_e.span,
                                    "use a `char` literal instead",
                                    format!("'\\u{{{:X}}}'", lit_val),
                                    Applicability::MachineApplicable,
                                )
                                .emit();
                        });
                        return;
                    }
                }
                hir::ExprKind::Struct(..) if is_range_literal(par_e) => {
                    let t = t.name_str();
                    if lint_overflowing_range_endpoint(cx, lit, lit_val, max, e, par_e, t) {
                        // The overflowing literal lint was overridden.
                        return;
                    }
                }
                _ => {}
            }
        }
        if let Some(repr_str) = get_bin_hex_repr(cx, lit) {
            report_bin_hex_error(
                cx,
                e,
                attr::IntType::UnsignedInt(ty::ast_uint_ty(t)),
                repr_str,
                lit_val,
                false,
            );
            return;
        }
        cx.struct_span_lint(OVERFLOWING_LITERALS, e.span, |lint| {
            lint.build(&format!("literal out of range for `{}`", t.name_str()))
                .note(&format!(
                    "the literal `{}` does not fit into the type `{}` whose range is `{}..={}`",
                    cx.sess()
                        .source_map()
                        .span_to_snippet(lit.span)
                        .expect("must get snippet from literal"),
                    t.name_str(),
                    min,
                    max,
                ))
                .emit();
        });
    }
}

fn lint_literal<'tcx>(
    cx: &LateContext<'tcx>,
    type_limits: &TypeLimits,
    e: &'tcx hir::Expr<'tcx>,
    lit: &hir::Lit,
) {
    match *cx.typeck_results().node_type(e.hir_id).kind() {
        ty::Int(t) => {
            match lit.node {
                ast::LitKind::Int(v, ast::LitIntType::Signed(_) | ast::LitIntType::Unsuffixed) => {
                    lint_int_literal(cx, type_limits, e, lit, t, v)
                }
                _ => bug!(),
            };
        }
        ty::Uint(t) => lint_uint_literal(cx, e, lit, t),
        ty::Float(t) => {
            let is_infinite = match lit.node {
                ast::LitKind::Float(v, _) => match t {
                    ty::FloatTy::F32 => v.as_str().parse().map(f32::is_infinite),
                    ty::FloatTy::F64 => v.as_str().parse().map(f64::is_infinite),
                },
                _ => bug!(),
            };
            if is_infinite == Ok(true) {
                cx.struct_span_lint(OVERFLOWING_LITERALS, e.span, |lint| {
                    lint.build(&format!("literal out of range for `{}`", t.name_str()))
                        .note(&format!(
                            "the literal `{}` does not fit into the type `{}` and will be converted to `{}::INFINITY`",
                            cx.sess()
                                .source_map()
                                .span_to_snippet(lit.span)
                                .expect("must get snippet from literal"),
                            t.name_str(),
                            t.name_str(),
                        ))
                        .emit();
                });
            }
        }
        _ => {}
    }
}

impl<'tcx> LateLintPass<'tcx> for TypeLimits {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx hir::Expr<'tcx>) {
        match e.kind {
            hir::ExprKind::Unary(hir::UnOp::Neg, ref expr) => {
                // propagate negation, if the negation itself isn't negated
                if self.negated_expr_id != Some(e.hir_id) {
                    self.negated_expr_id = Some(expr.hir_id);
                }
            }
            hir::ExprKind::Binary(binop, ref l, ref r) => {
                if is_comparison(binop) && !check_limits(cx, binop, &l, &r) {
                    cx.struct_span_lint(UNUSED_COMPARISONS, e.span, |lint| {
                        lint.build("comparison is useless due to type limits").emit();
                    });
                }
            }
            hir::ExprKind::Lit(ref lit) => lint_literal(cx, self, e, lit),
            _ => {}
        };

        fn is_valid<T: cmp::PartialOrd>(binop: hir::BinOp, v: T, min: T, max: T) -> bool {
            match binop.node {
                hir::BinOpKind::Lt => v > min && v <= max,
                hir::BinOpKind::Le => v >= min && v < max,
                hir::BinOpKind::Gt => v >= min && v < max,
                hir::BinOpKind::Ge => v > min && v <= max,
                hir::BinOpKind::Eq | hir::BinOpKind::Ne => v >= min && v <= max,
                _ => bug!(),
            }
        }

        fn rev_binop(binop: hir::BinOp) -> hir::BinOp {
            source_map::respan(
                binop.span,
                match binop.node {
                    hir::BinOpKind::Lt => hir::BinOpKind::Gt,
                    hir::BinOpKind::Le => hir::BinOpKind::Ge,
                    hir::BinOpKind::Gt => hir::BinOpKind::Lt,
                    hir::BinOpKind::Ge => hir::BinOpKind::Le,
                    _ => return binop,
                },
            )
        }

        fn check_limits(
            cx: &LateContext<'_>,
            binop: hir::BinOp,
            l: &hir::Expr<'_>,
            r: &hir::Expr<'_>,
        ) -> bool {
            let (lit, expr, swap) = match (&l.kind, &r.kind) {
                (&hir::ExprKind::Lit(_), _) => (l, r, true),
                (_, &hir::ExprKind::Lit(_)) => (r, l, false),
                _ => return true,
            };
            // Normalize the binop so that the literal is always on the RHS in
            // the comparison
            let norm_binop = if swap { rev_binop(binop) } else { binop };
            match *cx.typeck_results().node_type(expr.hir_id).kind() {
                ty::Int(int_ty) => {
                    let (min, max) = int_ty_range(int_ty);
                    let lit_val: i128 = match lit.kind {
                        hir::ExprKind::Lit(ref li) => match li.node {
                            ast::LitKind::Int(
                                v,
                                ast::LitIntType::Signed(_) | ast::LitIntType::Unsuffixed,
                            ) => v as i128,
                            _ => return true,
                        },
                        _ => bug!(),
                    };
                    is_valid(norm_binop, lit_val, min, max)
                }
                ty::Uint(uint_ty) => {
                    let (min, max): (u128, u128) = uint_ty_range(uint_ty);
                    let lit_val: u128 = match lit.kind {
                        hir::ExprKind::Lit(ref li) => match li.node {
                            ast::LitKind::Int(v, _) => v,
                            _ => return true,
                        },
                        _ => bug!(),
                    };
                    is_valid(norm_binop, lit_val, min, max)
                }
                _ => true,
            }
        }

        fn is_comparison(binop: hir::BinOp) -> bool {
            matches!(
                binop.node,
                hir::BinOpKind::Eq
                    | hir::BinOpKind::Lt
                    | hir::BinOpKind::Le
                    | hir::BinOpKind::Ne
                    | hir::BinOpKind::Ge
                    | hir::BinOpKind::Gt
            )
        }
    }
}

declare_lint! {
    /// The `improper_ctypes` lint detects incorrect use of types in foreign
    /// modules.
    ///
    /// ### Example
    ///
    /// ```rust
    /// extern "C" {
    ///     static STATIC: String;
    /// }
    /// ```
    ///
    /// {{produces}}
    ///
    /// ### Explanation
    ///
    /// The compiler has several checks to verify that types used in `extern`
    /// blocks are safe and follow certain rules to ensure proper
    /// compatibility with the foreign interfaces. This lint is issued when it
    /// detects a probable mistake in a definition. The lint usually should
    /// provide a description of the issue, along with possibly a hint on how
    /// to resolve it.
    IMPROPER_CTYPES,
    Warn,
    "proper use of libc types in foreign modules"
}

declare_lint_pass!(ImproperCTypesDeclarations => [IMPROPER_CTYPES]);

declare_lint! {
    /// The `improper_ctypes_definitions` lint detects incorrect use of
    /// [`extern` function] definitions.
    ///
    /// [`extern` function]: https://doc.rust-lang.org/reference/items/functions.html#extern-function-qualifier
    ///
    /// ### Example
    ///
    /// ```rust
    /// # #![allow(unused)]
    /// pub extern "C" fn str_type(p: &str) { }
    /// ```
    ///
    /// {{produces}}
    ///
    /// ### Explanation
    ///
    /// There are many parameter and return types that may be specified in an
    /// `extern` function that are not compatible with the given ABI. This
    /// lint is an alert that these types should not be used. The lint usually
    /// should provide a description of the issue, along with possibly a hint
    /// on how to resolve it.
    IMPROPER_CTYPES_DEFINITIONS,
    Warn,
    "proper use of libc types in foreign item definitions"
}

declare_lint_pass!(ImproperCTypesDefinitions => [IMPROPER_CTYPES_DEFINITIONS]);

#[derive(Clone, Copy)]
crate enum CItemKind {
    Declaration,
    Definition,
}

struct ImproperCTypesVisitor<'a, 'tcx> {
    cx: &'a LateContext<'tcx>,
    mode: CItemKind,
}

enum FfiResult<'tcx> {
    FfiSafe,
    FfiPhantom(Ty<'tcx>),
    FfiUnsafe { ty: Ty<'tcx>, reason: String, help: Option<String> },
}

crate fn nonnull_optimization_guaranteed<'tcx>(tcx: TyCtxt<'tcx>, def: ty::AdtDef<'tcx>) -> bool {
    tcx.has_attr(def.did(), sym::rustc_nonnull_optimization_guaranteed)
}

/// `repr(transparent)` structs can have a single non-ZST field, this function returns that
/// field.
pub fn transparent_newtype_field<'a, 'tcx>(
    tcx: TyCtxt<'tcx>,
    variant: &'a ty::VariantDef,
) -> Option<&'a ty::FieldDef> {
    let param_env = tcx.param_env(variant.def_id);
    variant.fields.iter().find(|field| {
        let field_ty = tcx.type_of(field.did);
        let is_zst = tcx.layout_of(param_env.and(field_ty)).map_or(false, |layout| layout.is_zst());
        !is_zst
    })
}

/// Is type known to be non-null?
fn ty_is_known_nonnull<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>, mode: CItemKind) -> bool {
    let tcx = cx.tcx;
    match ty.kind() {
        ty::FnPtr(_) => true,
        ty::Ref(..) => true,
        ty::Adt(def, _) if def.is_box() && matches!(mode, CItemKind::Definition) => true,
        ty::Adt(def, substs) if def.repr().transparent() && !def.is_union() => {
            let marked_non_null = nonnull_optimization_guaranteed(tcx, *def);

            if marked_non_null {
                return true;
            }

            // Types with a `#[repr(no_niche)]` attribute have their niche hidden.
            // The attribute is used by the UnsafeCell for example (the only use so far).
            if def.repr().hide_niche() {
                return false;
            }

            def.variants()
                .iter()
                .filter_map(|variant| transparent_newtype_field(cx.tcx, variant))
                .any(|field| ty_is_known_nonnull(cx, field.ty(tcx, substs), mode))
        }
        _ => false,
    }
}

/// Given a non-null scalar (or transparent) type `ty`, return the nullable version of that type.
/// If the type passed in was not scalar, returns None.
fn get_nullable_type<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
    let tcx = cx.tcx;
    Some(match *ty.kind() {
        ty::Adt(field_def, field_substs) => {
            let inner_field_ty = {
                let first_non_zst_ty = field_def
                    .variants()
                    .iter()
                    .filter_map(|v| transparent_newtype_field(cx.tcx, v));
                debug_assert_eq!(
                    first_non_zst_ty.clone().count(),
                    1,
                    "Wrong number of fields for transparent type"
                );
                first_non_zst_ty
                    .last()
                    .expect("No non-zst fields in transparent type.")
                    .ty(tcx, field_substs)
            };
            return get_nullable_type(cx, inner_field_ty);
        }
        ty::Int(ty) => tcx.mk_mach_int(ty),
        ty::Uint(ty) => tcx.mk_mach_uint(ty),
        ty::RawPtr(ty_mut) => tcx.mk_ptr(ty_mut),
        // As these types are always non-null, the nullable equivalent of
        // Option<T> of these types are their raw pointer counterparts.
        ty::Ref(_region, ty, mutbl) => tcx.mk_ptr(ty::TypeAndMut { ty, mutbl }),
        ty::FnPtr(..) => {
            // There is no nullable equivalent for Rust's function pointers -- you
            // must use an Option<fn(..) -> _> to represent it.
            ty
        }

        // We should only ever reach this case if ty_is_known_nonnull is extended
        // to other types.
        ref unhandled => {
            debug!(
                "get_nullable_type: Unhandled scalar kind: {:?} while checking {:?}",
                unhandled, ty
            );
            return None;
        }
    })
}

/// Check if this enum can be safely exported based on the "nullable pointer optimization". If it
/// can, return the type that `ty` can be safely converted to, otherwise return `None`.
/// Currently restricted to function pointers, boxes, references, `core::num::NonZero*`,
/// `core::ptr::NonNull`, and `#[repr(transparent)]` newtypes.
/// FIXME: This duplicates code in codegen.
crate fn repr_nullable_ptr<'tcx>(
    cx: &LateContext<'tcx>,
    ty: Ty<'tcx>,
    ckind: CItemKind,
) -> Option<Ty<'tcx>> {
    debug!("is_repr_nullable_ptr(cx, ty = {:?})", ty);
    if let ty::Adt(ty_def, substs) = ty.kind() {
        let field_ty = match &ty_def.variants().raw[..] {
            [var_one, var_two] => match (&var_one.fields[..], &var_two.fields[..]) {
                ([], [field]) | ([field], []) => field.ty(cx.tcx, substs),
                _ => return None,
            },
            _ => return None,
        };

        if !ty_is_known_nonnull(cx, field_ty, ckind) {
            return None;
        }

        // At this point, the field's type is known to be nonnull and the parent enum is Option-like.
        // If the computed size for the field and the enum are different, the nonnull optimization isn't
        // being applied (and we've got a problem somewhere).
        let compute_size_skeleton = |t| SizeSkeleton::compute(t, cx.tcx, cx.param_env).unwrap();
        if !compute_size_skeleton(ty).same_size(compute_size_skeleton(field_ty)) {
            bug!("improper_ctypes: Option nonnull optimization not applied?");
        }

        // Return the nullable type this Option-like enum can be safely represented with.
        let field_ty_abi = &cx.layout_of(field_ty).unwrap().abi;
        if let Abi::Scalar(field_ty_scalar) = field_ty_abi {
            match field_ty_scalar.valid_range(cx) {
                WrappingRange { start: 0, end }
                    if end == field_ty_scalar.size(&cx.tcx).unsigned_int_max() - 1 =>
                {
                    return Some(get_nullable_type(cx, field_ty).unwrap());
                }
                WrappingRange { start: 1, .. } => {
                    return Some(get_nullable_type(cx, field_ty).unwrap());
                }
                WrappingRange { start, end } => {
                    unreachable!("Unhandled start and end range: ({}, {})", start, end)
                }
            };
        }
    }
    None
}

impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
    /// Check if the type is array and emit an unsafe type lint.
    fn check_for_array_ty(&mut self, sp: Span, ty: Ty<'tcx>) -> bool {
        if let ty::Array(..) = ty.kind() {
            self.emit_ffi_unsafe_type_lint(
                ty,
                sp,
                "passing raw arrays by value is not FFI-safe",
                Some("consider passing a pointer to the array"),
            );
            true
        } else {
            false
        }
    }

    /// Checks if the given field's type is "ffi-safe".
    fn check_field_type_for_ffi(
        &self,
        cache: &mut FxHashSet<Ty<'tcx>>,
        field: &ty::FieldDef,
        substs: SubstsRef<'tcx>,
    ) -> FfiResult<'tcx> {
        let field_ty = field.ty(self.cx.tcx, substs);
        if field_ty.has_opaque_types() {
            self.check_type_for_ffi(cache, field_ty)
        } else {
            let field_ty = self.cx.tcx.normalize_erasing_regions(self.cx.param_env, field_ty);
            self.check_type_for_ffi(cache, field_ty)
        }
    }

    /// Checks if the given `VariantDef`'s field types are "ffi-safe".
    fn check_variant_for_ffi(
        &self,
        cache: &mut FxHashSet<Ty<'tcx>>,
        ty: Ty<'tcx>,
        def: ty::AdtDef<'tcx>,
        variant: &ty::VariantDef,
        substs: SubstsRef<'tcx>,
    ) -> FfiResult<'tcx> {
        use FfiResult::*;

        if def.repr().transparent() {
            // Can assume that at most one field is not a ZST, so only check
            // that field's type for FFI-safety.
            if let Some(field) = transparent_newtype_field(self.cx.tcx, variant) {
                self.check_field_type_for_ffi(cache, field, substs)
            } else {
                // All fields are ZSTs; this means that the type should behave
                // like (), which is FFI-unsafe
                FfiUnsafe {
                    ty,
                    reason: "this struct contains only zero-sized fields".into(),
                    help: None,
                }
            }
        } else {
            // We can't completely trust repr(C) markings; make sure the fields are
            // actually safe.
            let mut all_phantom = !variant.fields.is_empty();
            for field in &variant.fields {
                match self.check_field_type_for_ffi(cache, &field, substs) {
                    FfiSafe => {
                        all_phantom = false;
                    }
                    FfiPhantom(..) if def.is_enum() => {
                        return FfiUnsafe {
                            ty,
                            reason: "this enum contains a PhantomData field".into(),
                            help: None,
                        };
                    }
                    FfiPhantom(..) => {}
                    r => return r,
                }
            }

            if all_phantom { FfiPhantom(ty) } else { FfiSafe }
        }
    }

    /// Checks if the given type is "ffi-safe" (has a stable, well-defined
    /// representation which can be exported to C code).
    fn check_type_for_ffi(&self, cache: &mut FxHashSet<Ty<'tcx>>, ty: Ty<'tcx>) -> FfiResult<'tcx> {
        use FfiResult::*;

        let tcx = self.cx.tcx;

        // Protect against infinite recursion, for example
        // `struct S(*mut S);`.
        // FIXME: A recursion limit is necessary as well, for irregular
        // recursive types.
        if !cache.insert(ty) {
            return FfiSafe;
        }

        match *ty.kind() {
            ty::Adt(def, substs) => {
                if def.is_box() && matches!(self.mode, CItemKind::Definition) {
                    if ty.boxed_ty().is_sized(tcx.at(DUMMY_SP), self.cx.param_env) {
                        return FfiSafe;
                    } else {
                        return FfiUnsafe {
                            ty,
                            reason: "box cannot be represented as a single pointer".to_string(),
                            help: None,
                        };
                    }
                }
                if def.is_phantom_data() {
                    return FfiPhantom(ty);
                }
                match def.adt_kind() {
                    AdtKind::Struct | AdtKind::Union => {
                        let kind = if def.is_struct() { "struct" } else { "union" };

                        if !def.repr().c() && !def.repr().transparent() {
                            return FfiUnsafe {
                                ty,
                                reason: format!("this {} has unspecified layout", kind),
                                help: Some(format!(
                                    "consider adding a `#[repr(C)]` or \
                                             `#[repr(transparent)]` attribute to this {}",
                                    kind
                                )),
                            };
                        }

                        let is_non_exhaustive =
                            def.non_enum_variant().is_field_list_non_exhaustive();
                        if is_non_exhaustive && !def.did().is_local() {
                            return FfiUnsafe {
                                ty,
                                reason: format!("this {} is non-exhaustive", kind),
                                help: None,
                            };
                        }

                        if def.non_enum_variant().fields.is_empty() {
                            return FfiUnsafe {
                                ty,
                                reason: format!("this {} has no fields", kind),
                                help: Some(format!("consider adding a member to this {}", kind)),
                            };
                        }

                        self.check_variant_for_ffi(cache, ty, def, def.non_enum_variant(), substs)
                    }
                    AdtKind::Enum => {
                        if def.variants().is_empty() {
                            // Empty enums are okay... although sort of useless.
                            return FfiSafe;
                        }

                        // Check for a repr() attribute to specify the size of the
                        // discriminant.
                        if !def.repr().c() && !def.repr().transparent() && def.repr().int.is_none()
                        {
                            // Special-case types like `Option<extern fn()>`.
                            if repr_nullable_ptr(self.cx, ty, self.mode).is_none() {
                                return FfiUnsafe {
                                    ty,
                                    reason: "enum has no representation hint".into(),
                                    help: Some(
                                        "consider adding a `#[repr(C)]`, \
                                                `#[repr(transparent)]`, or integer `#[repr(...)]` \
                                                attribute to this enum"
                                            .into(),
                                    ),
                                };
                            }
                        }

                        if def.is_variant_list_non_exhaustive() && !def.did().is_local() {
                            return FfiUnsafe {
                                ty,
                                reason: "this enum is non-exhaustive".into(),
                                help: None,
                            };
                        }

                        // Check the contained variants.
                        for variant in def.variants() {
                            let is_non_exhaustive = variant.is_field_list_non_exhaustive();
                            if is_non_exhaustive && !variant.def_id.is_local() {
                                return FfiUnsafe {
                                    ty,
                                    reason: "this enum has non-exhaustive variants".into(),
                                    help: None,
                                };
                            }

                            match self.check_variant_for_ffi(cache, ty, def, variant, substs) {
                                FfiSafe => (),
                                r => return r,
                            }
                        }

                        FfiSafe
                    }
                }
            }

            ty::Char => FfiUnsafe {
                ty,
                reason: "the `char` type has no C equivalent".into(),
                help: Some("consider using `u32` or `libc::wchar_t` instead".into()),
            },

            ty::Int(ty::IntTy::I128) | ty::Uint(ty::UintTy::U128) => FfiUnsafe {
                ty,
                reason: "128-bit integers don't currently have a known stable ABI".into(),
                help: None,
            },

            // Primitive types with a stable representation.
            ty::Bool | ty::Int(..) | ty::Uint(..) | ty::Float(..) | ty::Never => FfiSafe,

            ty::Slice(_) => FfiUnsafe {
                ty,
                reason: "slices have no C equivalent".into(),
                help: Some("consider using a raw pointer instead".into()),
            },

            ty::Dynamic(..) => {
                FfiUnsafe { ty, reason: "trait objects have no C equivalent".into(), help: None }
            }

            ty::Str => FfiUnsafe {
                ty,
                reason: "string slices have no C equivalent".into(),
                help: Some("consider using `*const u8` and a length instead".into()),
            },

            ty::Tuple(..) => FfiUnsafe {
                ty,
                reason: "tuples have unspecified layout".into(),
                help: Some("consider using a struct instead".into()),
            },

            ty::RawPtr(ty::TypeAndMut { ty, .. }) | ty::Ref(_, ty, _)
                if {
                    matches!(self.mode, CItemKind::Definition)
                        && ty.is_sized(self.cx.tcx.at(DUMMY_SP), self.cx.param_env)
                } =>
            {
                FfiSafe
            }

            ty::RawPtr(ty::TypeAndMut { ty, .. })
                if match ty.kind() {
                    ty::Tuple(tuple) => tuple.is_empty(),
                    _ => false,
                } =>
            {
                FfiSafe
            }

            ty::RawPtr(ty::TypeAndMut { ty, .. }) | ty::Ref(_, ty, _) => {
                self.check_type_for_ffi(cache, ty)
            }

            ty::Array(inner_ty, _) => self.check_type_for_ffi(cache, inner_ty),

            ty::FnPtr(sig) => {
                if self.is_internal_abi(sig.abi()) {
                    return FfiUnsafe {
                        ty,
                        reason: "this function pointer has Rust-specific calling convention".into(),
                        help: Some(
                            "consider using an `extern fn(...) -> ...` \
                                    function pointer instead"
                                .into(),
                        ),
                    };
                }

                let sig = tcx.erase_late_bound_regions(sig);
                if !sig.output().is_unit() {
                    let r = self.check_type_for_ffi(cache, sig.output());
                    match r {
                        FfiSafe => {}
                        _ => {
                            return r;
                        }
                    }
                }
                for arg in sig.inputs() {
                    let r = self.check_type_for_ffi(cache, *arg);
                    match r {
                        FfiSafe => {}
                        _ => {
                            return r;
                        }
                    }
                }
                FfiSafe
            }

            ty::Foreign(..) => FfiSafe,

            // While opaque types are checked for earlier, if a projection in a struct field
            // normalizes to an opaque type, then it will reach this branch.
            ty::Opaque(..) => {
                FfiUnsafe { ty, reason: "opaque types have no C equivalent".into(), help: None }
            }

            // `extern "C" fn` functions can have type parameters, which may or may not be FFI-safe,
            //  so they are currently ignored for the purposes of this lint.
            ty::Param(..) | ty::Projection(..) if matches!(self.mode, CItemKind::Definition) => {
                FfiSafe
            }

            ty::Param(..)
            | ty::Projection(..)
            | ty::Infer(..)
            | ty::Bound(..)
            | ty::Error(_)
            | ty::Closure(..)
            | ty::Generator(..)
            | ty::GeneratorWitness(..)
            | ty::Placeholder(..)
            | ty::FnDef(..) => bug!("unexpected type in foreign function: {:?}", ty),
        }
    }

    fn emit_ffi_unsafe_type_lint(
        &mut self,
        ty: Ty<'tcx>,
        sp: Span,
        note: &str,
        help: Option<&str>,
    ) {
        let lint = match self.mode {
            CItemKind::Declaration => IMPROPER_CTYPES,
            CItemKind::Definition => IMPROPER_CTYPES_DEFINITIONS,
        };

        self.cx.struct_span_lint(lint, sp, |lint| {
            let item_description = match self.mode {
                CItemKind::Declaration => "block",
                CItemKind::Definition => "fn",
            };
            let mut diag = lint.build(&format!(
                "`extern` {} uses type `{}`, which is not FFI-safe",
                item_description, ty
            ));
            diag.span_label(sp, "not FFI-safe");
            if let Some(help) = help {
                diag.help(help);
            }
            diag.note(note);
            if let ty::Adt(def, _) = ty.kind() {
                if let Some(sp) = self.cx.tcx.hir().span_if_local(def.did()) {
                    diag.span_note(sp, "the type is defined here");
                }
            }
            diag.emit();
        });
    }

    fn check_for_opaque_ty(&mut self, sp: Span, ty: Ty<'tcx>) -> bool {
        struct ProhibitOpaqueTypes<'a, 'tcx> {
            cx: &'a LateContext<'tcx>,
        }

        impl<'a, 'tcx> ty::fold::TypeVisitor<'tcx> for ProhibitOpaqueTypes<'a, 'tcx> {
            type BreakTy = Ty<'tcx>;

            fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
                match ty.kind() {
                    ty::Opaque(..) => ControlFlow::Break(ty),
                    // Consider opaque types within projections FFI-safe if they do not normalize
                    // to more opaque types.
                    ty::Projection(..) => {
                        let ty = self.cx.tcx.normalize_erasing_regions(self.cx.param_env, ty);

                        // If `ty` is an opaque type directly then `super_visit_with` won't invoke
                        // this function again.
                        if ty.has_opaque_types() {
                            self.visit_ty(ty)
                        } else {
                            ControlFlow::CONTINUE
                        }
                    }
                    _ => ty.super_visit_with(self),
                }
            }
        }

        if let Some(ty) = ty.visit_with(&mut ProhibitOpaqueTypes { cx: self.cx }).break_value() {
            self.emit_ffi_unsafe_type_lint(ty, sp, "opaque types have no C equivalent", None);
            true
        } else {
            false
        }
    }

    fn check_type_for_ffi_and_report_errors(
        &mut self,
        sp: Span,
        ty: Ty<'tcx>,
        is_static: bool,
        is_return_type: bool,
    ) {
        // We have to check for opaque types before `normalize_erasing_regions`,
        // which will replace opaque types with their underlying concrete type.
        if self.check_for_opaque_ty(sp, ty) {
            // We've already emitted an error due to an opaque type.
            return;
        }

        // it is only OK to use this function because extern fns cannot have
        // any generic types right now:
        let ty = self.cx.tcx.normalize_erasing_regions(self.cx.param_env, ty);

        // C doesn't really support passing arrays by value - the only way to pass an array by value
        // is through a struct. So, first test that the top level isn't an array, and then
        // recursively check the types inside.
        if !is_static && self.check_for_array_ty(sp, ty) {
            return;
        }

        // Don't report FFI errors for unit return types. This check exists here, and not in
        // `check_foreign_fn` (where it would make more sense) so that normalization has definitely
        // happened.
        if is_return_type && ty.is_unit() {
            return;
        }

        match self.check_type_for_ffi(&mut FxHashSet::default(), ty) {
            FfiResult::FfiSafe => {}
            FfiResult::FfiPhantom(ty) => {
                self.emit_ffi_unsafe_type_lint(ty, sp, "composed only of `PhantomData`", None);
            }
            // If `ty` is a `repr(transparent)` newtype, and the non-zero-sized type is a generic
            // argument, which after substitution, is `()`, then this branch can be hit.
            FfiResult::FfiUnsafe { ty, .. } if is_return_type && ty.is_unit() => {}
            FfiResult::FfiUnsafe { ty, reason, help } => {
                self.emit_ffi_unsafe_type_lint(ty, sp, &reason, help.as_deref());
            }
        }
    }

    fn check_foreign_fn(&mut self, id: hir::HirId, decl: &hir::FnDecl<'_>) {
        let def_id = self.cx.tcx.hir().local_def_id(id);
        let sig = self.cx.tcx.fn_sig(def_id);
        let sig = self.cx.tcx.erase_late_bound_regions(sig);

        for (input_ty, input_hir) in iter::zip(sig.inputs(), decl.inputs) {
            self.check_type_for_ffi_and_report_errors(input_hir.span, *input_ty, false, false);
        }

        if let hir::FnRetTy::Return(ref ret_hir) = decl.output {
            let ret_ty = sig.output();
            self.check_type_for_ffi_and_report_errors(ret_hir.span, ret_ty, false, true);
        }
    }

    fn check_foreign_static(&mut self, id: hir::HirId, span: Span) {
        let def_id = self.cx.tcx.hir().local_def_id(id);
        let ty = self.cx.tcx.type_of(def_id);
        self.check_type_for_ffi_and_report_errors(span, ty, true, false);
    }

    fn is_internal_abi(&self, abi: SpecAbi) -> bool {
        matches!(
            abi,
            SpecAbi::Rust | SpecAbi::RustCall | SpecAbi::RustIntrinsic | SpecAbi::PlatformIntrinsic
        )
    }
}

impl<'tcx> LateLintPass<'tcx> for ImproperCTypesDeclarations {
    fn check_foreign_item(&mut self, cx: &LateContext<'_>, it: &hir::ForeignItem<'_>) {
        let mut vis = ImproperCTypesVisitor { cx, mode: CItemKind::Declaration };
        let abi = cx.tcx.hir().get_foreign_abi(it.hir_id());

        if !vis.is_internal_abi(abi) {
            match it.kind {
                hir::ForeignItemKind::Fn(ref decl, _, _) => {
                    vis.check_foreign_fn(it.hir_id(), decl);
                }
                hir::ForeignItemKind::Static(ref ty, _) => {
                    vis.check_foreign_static(it.hir_id(), ty.span);
                }
                hir::ForeignItemKind::Type => (),
            }
        }
    }
}

impl<'tcx> LateLintPass<'tcx> for ImproperCTypesDefinitions {
    fn check_fn(
        &mut self,
        cx: &LateContext<'tcx>,
        kind: hir::intravisit::FnKind<'tcx>,
        decl: &'tcx hir::FnDecl<'_>,
        _: &'tcx hir::Body<'_>,
        _: Span,
        hir_id: hir::HirId,
    ) {
        use hir::intravisit::FnKind;

        let abi = match kind {
            FnKind::ItemFn(_, _, header, ..) => header.abi,
            FnKind::Method(_, sig, ..) => sig.header.abi,
            _ => return,
        };

        let mut vis = ImproperCTypesVisitor { cx, mode: CItemKind::Definition };
        if !vis.is_internal_abi(abi) {
            vis.check_foreign_fn(hir_id, decl);
        }
    }
}

declare_lint_pass!(VariantSizeDifferences => [VARIANT_SIZE_DIFFERENCES]);

impl<'tcx> LateLintPass<'tcx> for VariantSizeDifferences {
    fn check_item(&mut self, cx: &LateContext<'_>, it: &hir::Item<'_>) {
        if let hir::ItemKind::Enum(ref enum_definition, _) = it.kind {
            let t = cx.tcx.type_of(it.def_id);
            let ty = cx.tcx.erase_regions(t);
            let Ok(layout) = cx.layout_of(ty) else { return };
            let Variants::Multiple {
                    tag_encoding: TagEncoding::Direct, tag, ref variants, ..
                } = &layout.variants else {
                return
            };

            let tag_size = tag.size(&cx.tcx).bytes();

            debug!(
                "enum `{}` is {} bytes large with layout:\n{:#?}",
                t,
                layout.size.bytes(),
                layout
            );

            let (largest, slargest, largest_index) = iter::zip(enum_definition.variants, variants)
                .map(|(variant, variant_layout)| {
                    // Subtract the size of the enum tag.
                    let bytes = variant_layout.size().bytes().saturating_sub(tag_size);

                    debug!("- variant `{}` is {} bytes large", variant.ident, bytes);
                    bytes
                })
                .enumerate()
                .fold((0, 0, 0), |(l, s, li), (idx, size)| {
                    if size > l {
                        (size, l, idx)
                    } else if size > s {
                        (l, size, li)
                    } else {
                        (l, s, li)
                    }
                });

            // We only warn if the largest variant is at least thrice as large as
            // the second-largest.
            if largest > slargest * 3 && slargest > 0 {
                cx.struct_span_lint(
                    VARIANT_SIZE_DIFFERENCES,
                    enum_definition.variants[largest_index].span,
                    |lint| {
                        lint.build(&format!(
                            "enum variant is more than three times \
                                          larger ({} bytes) than the next largest",
                            largest
                        ))
                        .emit();
                    },
                );
            }
        }
    }
}

declare_lint! {
    /// The `invalid_atomic_ordering` lint detects passing an `Ordering`
    /// to an atomic operation that does not support that ordering.
    ///
    /// ### Example
    ///
    /// ```rust,compile_fail
    /// # use core::sync::atomic::{AtomicU8, Ordering};
    /// let atom = AtomicU8::new(0);
    /// let value = atom.load(Ordering::Release);
    /// # let _ = value;
    /// ```
    ///
    /// {{produces}}
    ///
    /// ### Explanation
    ///
    /// Some atomic operations are only supported for a subset of the
    /// `atomic::Ordering` variants. Passing an unsupported variant will cause
    /// an unconditional panic at runtime, which is detected by this lint.
    ///
    /// This lint will trigger in the following cases: (where `AtomicType` is an
    /// atomic type from `core::sync::atomic`, such as `AtomicBool`,
    /// `AtomicPtr`, `AtomicUsize`, or any of the other integer atomics).
    ///
    /// - Passing `Ordering::Acquire` or `Ordering::AcqRel` to
    ///   `AtomicType::store`.
    ///
    /// - Passing `Ordering::Release` or `Ordering::AcqRel` to
    ///   `AtomicType::load`.
    ///
    /// - Passing `Ordering::Relaxed` to `core::sync::atomic::fence` or
    ///   `core::sync::atomic::compiler_fence`.
    ///
    /// - Passing `Ordering::Release` or `Ordering::AcqRel` as the failure
    ///   ordering for any of `AtomicType::compare_exchange`,
    ///   `AtomicType::compare_exchange_weak`, or `AtomicType::fetch_update`.
    ///
    /// - Passing in a pair of orderings to `AtomicType::compare_exchange`,
    ///   `AtomicType::compare_exchange_weak`, or `AtomicType::fetch_update`
    ///   where the failure ordering is stronger than the success ordering.
    INVALID_ATOMIC_ORDERING,
    Deny,
    "usage of invalid atomic ordering in atomic operations and memory fences"
}

declare_lint_pass!(InvalidAtomicOrdering => [INVALID_ATOMIC_ORDERING]);

impl InvalidAtomicOrdering {
    fn inherent_atomic_method_call<'hir>(
        cx: &LateContext<'_>,
        expr: &Expr<'hir>,
        recognized_names: &[Symbol], // used for fast path calculation
    ) -> Option<(Symbol, &'hir [Expr<'hir>])> {
        const ATOMIC_TYPES: &[Symbol] = &[
            sym::AtomicBool,
            sym::AtomicPtr,
            sym::AtomicUsize,
            sym::AtomicU8,
            sym::AtomicU16,
            sym::AtomicU32,
            sym::AtomicU64,
            sym::AtomicU128,
            sym::AtomicIsize,
            sym::AtomicI8,
            sym::AtomicI16,
            sym::AtomicI32,
            sym::AtomicI64,
            sym::AtomicI128,
        ];
        if let ExprKind::MethodCall(ref method_path, args, _) = &expr.kind
            && recognized_names.contains(&method_path.ident.name)
            && let Some(m_def_id) = cx.typeck_results().type_dependent_def_id(expr.hir_id)
            && let Some(impl_did) = cx.tcx.impl_of_method(m_def_id)
            && let Some(adt) = cx.tcx.type_of(impl_did).ty_adt_def()
            // skip extension traits, only lint functions from the standard library
            && cx.tcx.trait_id_of_impl(impl_did).is_none()
            && let parent = cx.tcx.parent(adt.did())
            && cx.tcx.is_diagnostic_item(sym::atomic_mod, parent)
            && ATOMIC_TYPES.contains(&cx.tcx.item_name(adt.did()))
        {
            return Some((method_path.ident.name, args));
        }
        None
    }

    fn matches_ordering(cx: &LateContext<'_>, did: DefId, orderings: &[Symbol]) -> bool {
        let tcx = cx.tcx;
        let atomic_ordering = tcx.get_diagnostic_item(sym::Ordering);
        orderings.iter().any(|ordering| {
            tcx.item_name(did) == *ordering && {
                let parent = tcx.parent(did);
                Some(parent) == atomic_ordering
                    // needed in case this is a ctor, not a variant
                    || tcx.opt_parent(parent) == atomic_ordering
            }
        })
    }

    fn opt_ordering_defid(cx: &LateContext<'_>, ord_arg: &Expr<'_>) -> Option<DefId> {
        if let ExprKind::Path(ref ord_qpath) = ord_arg.kind {
            cx.qpath_res(ord_qpath, ord_arg.hir_id).opt_def_id()
        } else {
            None
        }
    }

    fn check_atomic_load_store(cx: &LateContext<'_>, expr: &Expr<'_>) {
        use rustc_hir::def::{DefKind, Res};
        use rustc_hir::QPath;
        if let Some((method, args)) = Self::inherent_atomic_method_call(cx, expr, &[sym::load, sym::store])
            && let Some((ordering_arg, invalid_ordering)) = match method {
                sym::load => Some((&args[1], sym::Release)),
                sym::store => Some((&args[2], sym::Acquire)),
                _ => None,
            }
            && let ExprKind::Path(QPath::Resolved(_, path)) = ordering_arg.kind
            && let Res::Def(DefKind::Ctor(..), ctor_id) = path.res
            && Self::matches_ordering(cx, ctor_id, &[invalid_ordering, sym::AcqRel])
        {
            cx.struct_span_lint(INVALID_ATOMIC_ORDERING, ordering_arg.span, |diag| {
                if method == sym::load {
                    diag.build("atomic loads cannot have `Release` or `AcqRel` ordering")
                        .help("consider using ordering modes `Acquire`, `SeqCst` or `Relaxed`")
                        .emit()
                } else {
                    debug_assert_eq!(method, sym::store);
                    diag.build("atomic stores cannot have `Acquire` or `AcqRel` ordering")
                        .help("consider using ordering modes `Release`, `SeqCst` or `Relaxed`")
                        .emit();
                }
            });
        }
    }

    fn check_memory_fence(cx: &LateContext<'_>, expr: &Expr<'_>) {
        if let ExprKind::Call(ref func, ref args) = expr.kind
            && let ExprKind::Path(ref func_qpath) = func.kind
            && let Some(def_id) = cx.qpath_res(func_qpath, func.hir_id).opt_def_id()
            && matches!(cx.tcx.get_diagnostic_name(def_id), Some(sym::fence | sym::compiler_fence))
            && let ExprKind::Path(ref ordering_qpath) = &args[0].kind
            && let Some(ordering_def_id) = cx.qpath_res(ordering_qpath, args[0].hir_id).opt_def_id()
            && Self::matches_ordering(cx, ordering_def_id, &[sym::Relaxed])
        {
            cx.struct_span_lint(INVALID_ATOMIC_ORDERING, args[0].span, |diag| {
                diag.build("memory fences cannot have `Relaxed` ordering")
                    .help("consider using ordering modes `Acquire`, `Release`, `AcqRel` or `SeqCst`")
                    .emit();
            });
        }
    }

    fn check_atomic_compare_exchange(cx: &LateContext<'_>, expr: &Expr<'_>) {
        if let Some((method, args)) = Self::inherent_atomic_method_call(cx, expr, &[sym::fetch_update, sym::compare_exchange, sym::compare_exchange_weak])
            && let Some((success_order_arg, failure_order_arg)) = match method {
                sym::fetch_update => Some((&args[1], &args[2])),
                sym::compare_exchange | sym::compare_exchange_weak => Some((&args[3], &args[4])),
                _ => None,
            }
            && let Some(fail_ordering_def_id) = Self::opt_ordering_defid(cx, failure_order_arg)
        {
            // Helper type holding on to some checking and error reporting data. Has
            // - (success ordering,
            // - list of failure orderings forbidden by the success order,
            // - suggestion message)
            type OrdLintInfo = (Symbol, &'static [Symbol], &'static str);
            const RELAXED: OrdLintInfo = (sym::Relaxed, &[sym::SeqCst, sym::Acquire], "ordering mode `Relaxed`");
            const ACQUIRE: OrdLintInfo = (sym::Acquire, &[sym::SeqCst], "ordering modes `Acquire` or `Relaxed`");
            const SEQ_CST: OrdLintInfo = (sym::SeqCst, &[], "ordering modes `Acquire`, `SeqCst` or `Relaxed`");
            const RELEASE: OrdLintInfo = (sym::Release, RELAXED.1, RELAXED.2);
            const ACQREL: OrdLintInfo = (sym::AcqRel, ACQUIRE.1, ACQUIRE.2);
            const SEARCH: [OrdLintInfo; 5] = [RELAXED, ACQUIRE, SEQ_CST, RELEASE, ACQREL];

            let success_lint_info = Self::opt_ordering_defid(cx, success_order_arg)
                .and_then(|success_ord_def_id| -> Option<OrdLintInfo> {
                    SEARCH
                        .iter()
                        .copied()
                        .find(|(ordering, ..)| {
                            Self::matches_ordering(cx, success_ord_def_id, &[*ordering])
                        })
                });
            if Self::matches_ordering(cx, fail_ordering_def_id, &[sym::Release, sym::AcqRel]) {
                // If we don't know the success order is, use what we'd suggest
                // if it were maximally permissive.
                let suggested = success_lint_info.unwrap_or(SEQ_CST).2;
                cx.struct_span_lint(INVALID_ATOMIC_ORDERING, failure_order_arg.span, |diag| {
                    let msg = format!(
                        "{}'s failure ordering may not be `Release` or `AcqRel`",
                        method,
                    );
                    diag.build(&msg)
                        .help(&format!("consider using {} instead", suggested))
                        .emit();
                });
            } else if let Some((success_ord, bad_ords_given_success, suggested)) = success_lint_info {
                if Self::matches_ordering(cx, fail_ordering_def_id, bad_ords_given_success) {
                    cx.struct_span_lint(INVALID_ATOMIC_ORDERING, failure_order_arg.span, |diag| {
                        let msg = format!(
                            "{}'s failure ordering may not be stronger than the success ordering of `{}`",
                            method,
                            success_ord,
                        );
                        diag.build(&msg)
                            .help(&format!("consider using {} instead", suggested))
                            .emit();
                    });
                }
            }
        }
    }
}

impl<'tcx> LateLintPass<'tcx> for InvalidAtomicOrdering {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
        Self::check_atomic_load_store(cx, expr);
        Self::check_memory_fence(cx, expr);
        Self::check_atomic_compare_exchange(cx, expr);
    }
}