src/librustc_lint/types.rs

   1 #![allow(non_snake_case)]
   2
   3 use crate::{LateContext, LateLintPass, LintContext};
   4 use rustc_ast as ast;
   5 use rustc_attr as attr;
   6 use rustc_data_structures::fx::FxHashSet;
   7 use rustc_errors::Applicability;
   8 use rustc_hir as hir;
   9 use rustc_hir::{is_range_literal, ExprKind, Node};
  10 use rustc_index::vec::Idx;
  11 use rustc_middle::mir::interpret::{sign_extend, truncate};
  12 use rustc_middle::ty::layout::{IntegerExt, SizeSkeleton};
  13 use rustc_middle::ty::subst::SubstsRef;
  14 use rustc_middle::ty::{self, AdtKind, Ty, TypeFoldable};
  15 use rustc_span::source_map;
  16 use rustc_span::symbol::sym;
  17 use rustc_span::{Span, DUMMY_SP};
  18 use rustc_target::abi::Abi;
  19 use rustc_target::abi::{Integer, LayoutOf, TagEncoding, VariantIdx, Variants};
  20 use rustc_target::spec::abi::Abi as SpecAbi;
  21
  22 use std::cmp;
  23 use tracing::debug;
  24
  25 declare_lint! {
  26     UNUSED_COMPARISONS,
  27     Warn,
  28     "comparisons made useless by limits of the types involved"
  29 }
  30
  31 declare_lint! {
  32     OVERFLOWING_LITERALS,
  33     Deny,
  34     "literal out of range for its type"
  35 }
  36
  37 declare_lint! {
  38     VARIANT_SIZE_DIFFERENCES,
  39     Allow,
  40     "detects enums with widely varying variant sizes"
  41 }
  42
  43 #[derive(Copy, Clone)]
  44 pub struct TypeLimits {
  45     /// Id of the last visited negated expression
  46     negated_expr_id: Option<hir::HirId>,
  47 }
  48
  49 impl_lint_pass!(TypeLimits => [UNUSED_COMPARISONS, OVERFLOWING_LITERALS]);
  50
  51 impl TypeLimits {
  52     pub fn new() -> TypeLimits {
  53         TypeLimits { negated_expr_id: None }
  54     }
  55 }
  56
  57 /// Attempts to special-case the overflowing literal lint when it occurs as a range endpoint.
  58 /// Returns `true` iff the lint was overridden.
  59 fn lint_overflowing_range_endpoint<'tcx>(
  60     cx: &LateContext<'tcx>,
  61     lit: &hir::Lit,
  62     lit_val: u128,
  63     max: u128,
  64     expr: &'tcx hir::Expr<'tcx>,
  65     parent_expr: &'tcx hir::Expr<'tcx>,
  66     ty: &str,
  67 ) -> bool {
  68     // We only want to handle exclusive (`..`) ranges,
  69     // which are represented as `ExprKind::Struct`.
  70     let mut overwritten = false;
  71     if let ExprKind::Struct(_, eps, _) = &parent_expr.kind {
  72         if eps.len() != 2 {
  73             return false;
  74         }
  75         // We can suggest using an inclusive range
  76         // (`..=`) instead only if it is the `end` that is
  77         // overflowing and only by 1.
  78         if eps[1].expr.hir_id == expr.hir_id && lit_val - 1 == max {
  79             cx.struct_span_lint(OVERFLOWING_LITERALS, parent_expr.span, |lint| {
  80                 let mut err = lint.build(&format!("range endpoint is out of range for `{}`", ty));
  81                 if let Ok(start) = cx.sess().source_map().span_to_snippet(eps[0].span) {
  82                     use ast::{LitIntType, LitKind};
  83                     // We need to preserve the literal's suffix,
  84                     // as it may determine typing information.
  85                     let suffix = match lit.node {
  86                         LitKind::Int(_, LitIntType::Signed(s)) => s.name_str().to_string(),
  87                         LitKind::Int(_, LitIntType::Unsigned(s)) => s.name_str().to_string(),
  88                         LitKind::Int(_, LitIntType::Unsuffixed) => "".to_string(),
  89                         _ => bug!(),
  90                     };
  91                     let suggestion = format!("{}..={}{}", start, lit_val - 1, suffix);
  92                     err.span_suggestion(
  93                         parent_expr.span,
  94                         &"use an inclusive range instead",
  95                         suggestion,
  96                         Applicability::MachineApplicable,
  97                     );
  98                     err.emit();
  99                     overwritten = true;
 100                 }
 101             });
 102         }
 103     }
 104     overwritten
 105 }
 106
 107 // For `isize` & `usize`, be conservative with the warnings, so that the
 108 // warnings are consistent between 32- and 64-bit platforms.
 109 fn int_ty_range(int_ty: ast::IntTy) -> (i128, i128) {
 110     match int_ty {
 111         ast::IntTy::Isize => (i64::MIN as i128, i64::MAX as i128),
 112         ast::IntTy::I8 => (i8::MIN as i64 as i128, i8::MAX as i128),
 113         ast::IntTy::I16 => (i16::MIN as i64 as i128, i16::MAX as i128),
 114         ast::IntTy::I32 => (i32::MIN as i64 as i128, i32::MAX as i128),
 115         ast::IntTy::I64 => (i64::MIN as i128, i64::MAX as i128),
 116         ast::IntTy::I128 => (i128::MIN as i128, i128::MAX),
 117     }
 118 }
 119
 120 fn uint_ty_range(uint_ty: ast::UintTy) -> (u128, u128) {
 121     match uint_ty {
 122         ast::UintTy::Usize => (u64::MIN as u128, u64::MAX as u128),
 123         ast::UintTy::U8 => (u8::MIN as u128, u8::MAX as u128),
 124         ast::UintTy::U16 => (u16::MIN as u128, u16::MAX as u128),
 125         ast::UintTy::U32 => (u32::MIN as u128, u32::MAX as u128),
 126         ast::UintTy::U64 => (u64::MIN as u128, u64::MAX as u128),
 127         ast::UintTy::U128 => (u128::MIN, u128::MAX),
 128     }
 129 }
 130
 131 fn get_bin_hex_repr(cx: &LateContext<'_>, lit: &hir::Lit) -> Option<String> {
 132     let src = cx.sess().source_map().span_to_snippet(lit.span).ok()?;
 133     let firstch = src.chars().next()?;
 134
 135     if firstch == '0' {
 136         match src.chars().nth(1) {
 137             Some('x' | 'b') => return Some(src),
 138             _ => return None,
 139         }
 140     }
 141
 142     None
 143 }
 144
 145 fn report_bin_hex_error(
 146     cx: &LateContext<'_>,
 147     expr: &hir::Expr<'_>,
 148     ty: attr::IntType,
 149     repr_str: String,
 150     val: u128,
 151     negative: bool,
 152 ) {
 153     let size = Integer::from_attr(&cx.tcx, ty).size();
 154     cx.struct_span_lint(OVERFLOWING_LITERALS, expr.span, |lint| {
 155         let (t, actually) = match ty {
 156             attr::IntType::SignedInt(t) => {
 157                 let actually = sign_extend(val, size) as i128;
 158                 (t.name_str(), actually.to_string())
 159             }
 160             attr::IntType::UnsignedInt(t) => {
 161                 let actually = truncate(val, size);
 162                 (t.name_str(), actually.to_string())
 163             }
 164         };
 165         let mut err = lint.build(&format!("literal out of range for {}", t));
 166         err.note(&format!(
 167             "the literal `{}` (decimal `{}`) does not fit into \
 168              the type `{}` and will become `{}{}`",
 169             repr_str, val, t, actually, t
 170         ));
 171         if let Some(sugg_ty) =
 172             get_type_suggestion(&cx.typeck_results().node_type(expr.hir_id), val, negative)
 173         {
 174             if let Some(pos) = repr_str.chars().position(|c| c == 'i' || c == 'u') {
 175                 let (sans_suffix, _) = repr_str.split_at(pos);
 176                 err.span_suggestion(
 177                     expr.span,
 178                     &format!("consider using `{}` instead", sugg_ty),
 179                     format!("{}{}", sans_suffix, sugg_ty),
 180                     Applicability::MachineApplicable,
 181                 );
 182             } else {
 183                 err.help(&format!("consider using `{}` instead", sugg_ty));
 184             }
 185         }
 186         err.emit();
 187     });
 188 }
 189
 190 // This function finds the next fitting type and generates a suggestion string.
 191 // It searches for fitting types in the following way (`X < Y`):
 192 //  - `iX`: if literal fits in `uX` => `uX`, else => `iY`
 193 //  - `-iX` => `iY`
 194 //  - `uX` => `uY`
 195 //
 196 // No suggestion for: `isize`, `usize`.
 197 fn get_type_suggestion(t: Ty<'_>, val: u128, negative: bool) -> Option<&'static str> {
 198     use rustc_ast::IntTy::*;
 199     use rustc_ast::UintTy::*;
 200     macro_rules! find_fit {
 201         ($ty:expr, $val:expr, $negative:expr,
 202          $($type:ident => [$($utypes:expr),*] => [$($itypes:expr),*]),+) => {
 203             {
 204                 let _neg = if negative { 1 } else { 0 };
 205                 match $ty {
 206                     $($type => {
 207                         $(if !negative && val <= uint_ty_range($utypes).1 {
 208                             return Some($utypes.name_str())
 209                         })*
 210                         $(if val <= int_ty_range($itypes).1 as u128 + _neg {
 211                             return Some($itypes.name_str())
 212                         })*
 213                         None
 214                     },)+
 215                     _ => None
 216                 }
 217             }
 218         }
 219     }
 220     match t.kind {
 221         ty::Int(i) => find_fit!(i, val, negative,
 222                       I8 => [U8] => [I16, I32, I64, I128],
 223                       I16 => [U16] => [I32, I64, I128],
 224                       I32 => [U32] => [I64, I128],
 225                       I64 => [U64] => [I128],
 226                       I128 => [U128] => []),
 227         ty::Uint(u) => find_fit!(u, val, negative,
 228                       U8 => [U8, U16, U32, U64, U128] => [],
 229                       U16 => [U16, U32, U64, U128] => [],
 230                       U32 => [U32, U64, U128] => [],
 231                       U64 => [U64, U128] => [],
 232                       U128 => [U128] => []),
 233         _ => None,
 234     }
 235 }
 236
 237 fn lint_int_literal<'tcx>(
 238     cx: &LateContext<'tcx>,
 239     type_limits: &TypeLimits,
 240     e: &'tcx hir::Expr<'tcx>,
 241     lit: &hir::Lit,
 242     t: ast::IntTy,
 243     v: u128,
 244 ) {
 245     let int_type = t.normalize(cx.sess().target.ptr_width);
 246     let (min, max) = int_ty_range(int_type);
 247     let max = max as u128;
 248     let negative = type_limits.negated_expr_id == Some(e.hir_id);
 249
 250     // Detect literal value out of range [min, max] inclusive
 251     // avoiding use of -min to prevent overflow/panic
 252     if (negative && v > max + 1) || (!negative && v > max) {
 253         if let Some(repr_str) = get_bin_hex_repr(cx, lit) {
 254             report_bin_hex_error(cx, e, attr::IntType::SignedInt(t), repr_str, v, negative);
 255             return;
 256         }
 257
 258         let par_id = cx.tcx.hir().get_parent_node(e.hir_id);
 259         if let Node::Expr(par_e) = cx.tcx.hir().get(par_id) {
 260             if let hir::ExprKind::Struct(..) = par_e.kind {
 261                 if is_range_literal(par_e)
 262                     && lint_overflowing_range_endpoint(cx, lit, v, max, e, par_e, t.name_str())
 263                 {
 264                     // The overflowing literal lint was overridden.
 265                     return;
 266                 }
 267             }
 268         }
 269
 270         cx.struct_span_lint(OVERFLOWING_LITERALS, e.span, |lint| {
 271             lint.build(&format!("literal out of range for `{}`", t.name_str()))
 272                 .note(&format!(
 273                     "the literal `{}` does not fit into the type `{}` whose range is `{}..={}`",
 274                     cx.sess()
 275                         .source_map()
 276                         .span_to_snippet(lit.span)
 277                         .expect("must get snippet from literal"),
 278                     t.name_str(),
 279                     min,
 280                     max,
 281                 ))
 282                 .emit();
 283         });
 284     }
 285 }
 286
 287 fn lint_uint_literal<'tcx>(
 288     cx: &LateContext<'tcx>,
 289     e: &'tcx hir::Expr<'tcx>,
 290     lit: &hir::Lit,
 291     t: ast::UintTy,
 292 ) {
 293     let uint_type = t.normalize(cx.sess().target.ptr_width);
 294     let (min, max) = uint_ty_range(uint_type);
 295     let lit_val: u128 = match lit.node {
 296         // _v is u8, within range by definition
 297         ast::LitKind::Byte(_v) => return,
 298         ast::LitKind::Int(v, _) => v,
 299         _ => bug!(),
 300     };
 301     if lit_val < min || lit_val > max {
 302         let parent_id = cx.tcx.hir().get_parent_node(e.hir_id);
 303         if let Node::Expr(par_e) = cx.tcx.hir().get(parent_id) {
 304             match par_e.kind {
 305                 hir::ExprKind::Cast(..) => {
 306                     if let ty::Char = cx.typeck_results().expr_ty(par_e).kind {
 307                         cx.struct_span_lint(OVERFLOWING_LITERALS, par_e.span, |lint| {
 308                             lint.build("only `u8` can be cast into `char`")
 309                                 .span_suggestion(
 310                                     par_e.span,
 311                                     &"use a `char` literal instead",
 312                                     format!("'\\u{{{:X}}}'", lit_val),
 313                                     Applicability::MachineApplicable,
 314                                 )
 315                                 .emit();
 316                         });
 317                         return;
 318                     }
 319                 }
 320                 hir::ExprKind::Struct(..) if is_range_literal(par_e) => {
 321                     let t = t.name_str();
 322                     if lint_overflowing_range_endpoint(cx, lit, lit_val, max, e, par_e, t) {
 323                         // The overflowing literal lint was overridden.
 324                         return;
 325                     }
 326                 }
 327                 _ => {}
 328             }
 329         }
 330         if let Some(repr_str) = get_bin_hex_repr(cx, lit) {
 331             report_bin_hex_error(cx, e, attr::IntType::UnsignedInt(t), repr_str, lit_val, false);
 332             return;
 333         }
 334         cx.struct_span_lint(OVERFLOWING_LITERALS, e.span, |lint| {
 335             lint.build(&format!("literal out of range for `{}`", t.name_str()))
 336                 .note(&format!(
 337                     "the literal `{}` does not fit into the type `{}` whose range is `{}..={}`",
 338                     cx.sess()
 339                         .source_map()
 340                         .span_to_snippet(lit.span)
 341                         .expect("must get snippet from literal"),
 342                     t.name_str(),
 343                     min,
 344                     max,
 345                 ))
 346                 .emit()
 347         });
 348     }
 349 }
 350
 351 fn lint_literal<'tcx>(
 352     cx: &LateContext<'tcx>,
 353     type_limits: &TypeLimits,
 354     e: &'tcx hir::Expr<'tcx>,
 355     lit: &hir::Lit,
 356 ) {
 357     match cx.typeck_results().node_type(e.hir_id).kind {
 358         ty::Int(t) => {
 359             match lit.node {
 360                 ast::LitKind::Int(v, ast::LitIntType::Signed(_) | ast::LitIntType::Unsuffixed) => {
 361                     lint_int_literal(cx, type_limits, e, lit, t, v)
 362                 }
 363                 _ => bug!(),
 364             };
 365         }
 366         ty::Uint(t) => lint_uint_literal(cx, e, lit, t),
 367         ty::Float(t) => {
 368             let is_infinite = match lit.node {
 369                 ast::LitKind::Float(v, _) => match t {
 370                     ast::FloatTy::F32 => v.as_str().parse().map(f32::is_infinite),
 371                     ast::FloatTy::F64 => v.as_str().parse().map(f64::is_infinite),
 372                 },
 373                 _ => bug!(),
 374             };
 375             if is_infinite == Ok(true) {
 376                 cx.struct_span_lint(OVERFLOWING_LITERALS, e.span, |lint| {
 377                     lint.build(&format!("literal out of range for `{}`", t.name_str()))
 378                         .note(&format!(
 379                             "the literal `{}` does not fit into the type `{}` and will be converted to `std::{}::INFINITY`",
 380                             cx.sess()
 381                                 .source_map()
 382                                 .span_to_snippet(lit.span)
 383                                 .expect("must get snippet from literal"),
 384                             t.name_str(),
 385                             t.name_str(),
 386                         ))
 387                         .emit();
 388                 });
 389             }
 390         }
 391         _ => {}
 392     }
 393 }
 394
 395 impl<'tcx> LateLintPass<'tcx> for TypeLimits {
 396     fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx hir::Expr<'tcx>) {
 397         match e.kind {
 398             hir::ExprKind::Unary(hir::UnOp::UnNeg, ref expr) => {
 399                 // propagate negation, if the negation itself isn't negated
 400                 if self.negated_expr_id != Some(e.hir_id) {
 401                     self.negated_expr_id = Some(expr.hir_id);
 402                 }
 403             }
 404             hir::ExprKind::Binary(binop, ref l, ref r) => {
 405                 if is_comparison(binop) && !check_limits(cx, binop, &l, &r) {
 406                     cx.struct_span_lint(UNUSED_COMPARISONS, e.span, |lint| {
 407                         lint.build("comparison is useless due to type limits").emit()
 408                     });
 409                 }
 410             }
 411             hir::ExprKind::Lit(ref lit) => lint_literal(cx, self, e, lit),
 412             _ => {}
 413         };
 414
 415         fn is_valid<T: cmp::PartialOrd>(binop: hir::BinOp, v: T, min: T, max: T) -> bool {
 416             match binop.node {
 417                 hir::BinOpKind::Lt => v > min && v <= max,
 418                 hir::BinOpKind::Le => v >= min && v < max,
 419                 hir::BinOpKind::Gt => v >= min && v < max,
 420                 hir::BinOpKind::Ge => v > min && v <= max,
 421                 hir::BinOpKind::Eq | hir::BinOpKind::Ne => v >= min && v <= max,
 422                 _ => bug!(),
 423             }
 424         }
 425
 426         fn rev_binop(binop: hir::BinOp) -> hir::BinOp {
 427             source_map::respan(
 428                 binop.span,
 429                 match binop.node {
 430                     hir::BinOpKind::Lt => hir::BinOpKind::Gt,
 431                     hir::BinOpKind::Le => hir::BinOpKind::Ge,
 432                     hir::BinOpKind::Gt => hir::BinOpKind::Lt,
 433                     hir::BinOpKind::Ge => hir::BinOpKind::Le,
 434                     _ => return binop,
 435                 },
 436             )
 437         }
 438
 439         fn check_limits(
 440             cx: &LateContext<'_>,
 441             binop: hir::BinOp,
 442             l: &hir::Expr<'_>,
 443             r: &hir::Expr<'_>,
 444         ) -> bool {
 445             let (lit, expr, swap) = match (&l.kind, &r.kind) {
 446                 (&hir::ExprKind::Lit(_), _) => (l, r, true),
 447                 (_, &hir::ExprKind::Lit(_)) => (r, l, false),
 448                 _ => return true,
 449             };
 450             // Normalize the binop so that the literal is always on the RHS in
 451             // the comparison
 452             let norm_binop = if swap { rev_binop(binop) } else { binop };
 453             match cx.typeck_results().node_type(expr.hir_id).kind {
 454                 ty::Int(int_ty) => {
 455                     let (min, max) = int_ty_range(int_ty);
 456                     let lit_val: i128 = match lit.kind {
 457                         hir::ExprKind::Lit(ref li) => match li.node {
 458                             ast::LitKind::Int(
 459                                 v,
 460                                 ast::LitIntType::Signed(_) | ast::LitIntType::Unsuffixed,
 461                             ) => v as i128,
 462                             _ => return true,
 463                         },
 464                         _ => bug!(),
 465                     };
 466                     is_valid(norm_binop, lit_val, min, max)
 467                 }
 468                 ty::Uint(uint_ty) => {
 469                     let (min, max): (u128, u128) = uint_ty_range(uint_ty);
 470                     let lit_val: u128 = match lit.kind {
 471                         hir::ExprKind::Lit(ref li) => match li.node {
 472                             ast::LitKind::Int(v, _) => v,
 473                             _ => return true,
 474                         },
 475                         _ => bug!(),
 476                     };
 477                     is_valid(norm_binop, lit_val, min, max)
 478                 }
 479                 _ => true,
 480             }
 481         }
 482
 483         fn is_comparison(binop: hir::BinOp) -> bool {
 484             match binop.node {
 485                 hir::BinOpKind::Eq
 486                 | hir::BinOpKind::Lt
 487                 | hir::BinOpKind::Le
 488                 | hir::BinOpKind::Ne
 489                 | hir::BinOpKind::Ge
 490                 | hir::BinOpKind::Gt => true,
 491                 _ => false,
 492             }
 493         }
 494     }
 495 }
 496
 497 declare_lint! {
 498     IMPROPER_CTYPES,
 499     Warn,
 500     "proper use of libc types in foreign modules"
 501 }
 502
 503 declare_lint_pass!(ImproperCTypesDeclarations => [IMPROPER_CTYPES]);
 504
 505 declare_lint! {
 506     IMPROPER_CTYPES_DEFINITIONS,
 507     Warn,
 508     "proper use of libc types in foreign item definitions"
 509 }
 510
 511 declare_lint_pass!(ImproperCTypesDefinitions => [IMPROPER_CTYPES_DEFINITIONS]);
 512
 513 #[derive(Clone, Copy)]
 514 crate enum CItemKind {
 515     Declaration,
 516     Definition,
 517 }
 518
 519 struct ImproperCTypesVisitor<'a, 'tcx> {
 520     cx: &'a LateContext<'tcx>,
 521     mode: CItemKind,
 522 }
 523
 524 enum FfiResult<'tcx> {
 525     FfiSafe,
 526     FfiPhantom(Ty<'tcx>),
 527     FfiUnsafe { ty: Ty<'tcx>, reason: String, help: Option<String> },
 528 }
 529
 530 /// Is type known to be non-null?
 531 fn ty_is_known_nonnull<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>, mode: CItemKind) -> bool {
 532     let tcx = cx.tcx;
 533     match ty.kind {
 534         ty::FnPtr(_) => true,
 535         ty::Ref(..) => true,
 536         ty::Adt(def, _) if def.is_box() && matches!(mode, CItemKind::Definition) => true,
 537         ty::Adt(def, substs) if def.repr.transparent() && !def.is_union() => {
 538             let guaranteed_nonnull_optimization = tcx
 539                 .get_attrs(def.did)
 540                 .iter()
 541                 .any(|a| tcx.sess.check_name(a, sym::rustc_nonnull_optimization_guaranteed));
 542
 543             if guaranteed_nonnull_optimization {
 544                 return true;
 545             }
 546             for variant in &def.variants {
 547                 if let Some(field) = variant.transparent_newtype_field(tcx) {
 548                     if ty_is_known_nonnull(cx, field.ty(tcx, substs), mode) {
 549                         return true;
 550                     }
 551                 }
 552             }
 553
 554             false
 555         }
 556         _ => false,
 557     }
 558 }
 559
 560 /// Given a non-null scalar (or transparent) type `ty`, return the nullable version of that type.
 561 /// If the type passed in was not scalar, returns None.
 562 fn get_nullable_type<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
 563     let tcx = cx.tcx;
 564     Some(match ty.kind {
 565         ty::Adt(field_def, field_substs) => {
 566             let inner_field_ty = {
 567                 let first_non_zst_ty =
 568                     field_def.variants.iter().filter_map(|v| v.transparent_newtype_field(tcx));
 569                 debug_assert_eq!(
 570                     first_non_zst_ty.clone().count(),
 571                     1,
 572                     "Wrong number of fields for transparent type"
 573                 );
 574                 first_non_zst_ty
 575                     .last()
 576                     .expect("No non-zst fields in transparent type.")
 577                     .ty(tcx, field_substs)
 578             };
 579             return get_nullable_type(cx, inner_field_ty);
 580         }
 581         ty::Int(ty) => tcx.mk_mach_int(ty),
 582         ty::Uint(ty) => tcx.mk_mach_uint(ty),
 583         ty::RawPtr(ty_mut) => tcx.mk_ptr(ty_mut),
 584         // As these types are always non-null, the nullable equivalent of
 585         // Option<T> of these types are their raw pointer counterparts.
 586         ty::Ref(_region, ty, mutbl) => tcx.mk_ptr(ty::TypeAndMut { ty, mutbl }),
 587         ty::FnPtr(..) => {
 588             // There is no nullable equivalent for Rust's function pointers -- you
 589             // must use an Option<fn(..) -> _> to represent it.
 590             ty
 591         }
 592
 593         // We should only ever reach this case if ty_is_known_nonnull is extended
 594         // to other types.
 595         ref unhandled => {
 596             debug!(
 597                 "get_nullable_type: Unhandled scalar kind: {:?} while checking {:?}",
 598                 unhandled, ty
 599             );
 600             return None;
 601         }
 602     })
 603 }
 604
 605 /// Check if this enum can be safely exported based on the "nullable pointer optimization". If it
 606 /// can, return the the type that `ty` can be safely converted to, otherwise return `None`.
 607 /// Currently restricted to function pointers, boxes, references, `core::num::NonZero*`,
 608 /// `core::ptr::NonNull`, and `#[repr(transparent)]` newtypes.
 609 /// FIXME: This duplicates code in codegen.
 610 crate fn repr_nullable_ptr<'tcx>(
 611     cx: &LateContext<'tcx>,
 612     ty: Ty<'tcx>,
 613     ckind: CItemKind,
 614 ) -> Option<Ty<'tcx>> {
 615     debug!("is_repr_nullable_ptr(cx, ty = {:?})", ty);
 616     if let ty::Adt(ty_def, substs) = ty.kind {
 617         if ty_def.variants.len() != 2 {
 618             return None;
 619         }
 620
 621         let get_variant_fields = |index| &ty_def.variants[VariantIdx::new(index)].fields;
 622         let variant_fields = [get_variant_fields(0), get_variant_fields(1)];
 623         let fields = if variant_fields[0].is_empty() {
 624             &variant_fields[1]
 625         } else if variant_fields[1].is_empty() {
 626             &variant_fields[0]
 627         } else {
 628             return None;
 629         };
 630
 631         if fields.len() != 1 {
 632             return None;
 633         }
 634
 635         let field_ty = fields[0].ty(cx.tcx, substs);
 636         if !ty_is_known_nonnull(cx, field_ty, ckind) {
 637             return None;
 638         }
 639
 640         // At this point, the field's type is known to be nonnull and the parent enum is Option-like.
 641         // If the computed size for the field and the enum are different, the nonnull optimization isn't
 642         // being applied (and we've got a problem somewhere).
 643         let compute_size_skeleton = |t| SizeSkeleton::compute(t, cx.tcx, cx.param_env).unwrap();
 644         if !compute_size_skeleton(ty).same_size(compute_size_skeleton(field_ty)) {
 645             bug!("improper_ctypes: Option nonnull optimization not applied?");
 646         }
 647
 648         // Return the nullable type this Option-like enum can be safely represented with.
 649         let field_ty_abi = &cx.layout_of(field_ty).unwrap().abi;
 650         if let Abi::Scalar(field_ty_scalar) = field_ty_abi {
 651             match (field_ty_scalar.valid_range.start(), field_ty_scalar.valid_range.end()) {
 652                 (0, _) => unreachable!("Non-null optimisation extended to a non-zero value."),
 653                 (1, _) => {
 654                     return Some(get_nullable_type(cx, field_ty).unwrap());
 655                 }
 656                 (start, end) => unreachable!("Unhandled start and end range: ({}, {})", start, end),
 657             };
 658         }
 659     }
 660     None
 661 }
 662
 663 impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
 664     /// Check if the type is array and emit an unsafe type lint.
 665     fn check_for_array_ty(&mut self, sp: Span, ty: Ty<'tcx>) -> bool {
 666         if let ty::Array(..) = ty.kind {
 667             self.emit_ffi_unsafe_type_lint(
 668                 ty,
 669                 sp,
 670                 "passing raw arrays by value is not FFI-safe",
 671                 Some("consider passing a pointer to the array"),
 672             );
 673             true
 674         } else {
 675             false
 676         }
 677     }
 678
 679     /// Checks if the given field's type is "ffi-safe".
 680     fn check_field_type_for_ffi(
 681         &self,
 682         cache: &mut FxHashSet<Ty<'tcx>>,
 683         field: &ty::FieldDef,
 684         substs: SubstsRef<'tcx>,
 685     ) -> FfiResult<'tcx> {
 686         let field_ty = field.ty(self.cx.tcx, substs);
 687         if field_ty.has_opaque_types() {
 688             self.check_type_for_ffi(cache, field_ty)
 689         } else {
 690             let field_ty = self.cx.tcx.normalize_erasing_regions(self.cx.param_env, field_ty);
 691             self.check_type_for_ffi(cache, field_ty)
 692         }
 693     }
 694
 695     /// Checks if the given `VariantDef`'s field types are "ffi-safe".
 696     fn check_variant_for_ffi(
 697         &self,
 698         cache: &mut FxHashSet<Ty<'tcx>>,
 699         ty: Ty<'tcx>,
 700         def: &ty::AdtDef,
 701         variant: &ty::VariantDef,
 702         substs: SubstsRef<'tcx>,
 703     ) -> FfiResult<'tcx> {
 704         use FfiResult::*;
 705
 706         if def.repr.transparent() {
 707             // Can assume that only one field is not a ZST, so only check
 708             // that field's type for FFI-safety.
 709             if let Some(field) = variant.transparent_newtype_field(self.cx.tcx) {
 710                 self.check_field_type_for_ffi(cache, field, substs)
 711             } else {
 712                 bug!("malformed transparent type");
 713             }
 714         } else {
 715             // We can't completely trust repr(C) markings; make sure the fields are
 716             // actually safe.
 717             let mut all_phantom = !variant.fields.is_empty();
 718             for field in &variant.fields {
 719                 match self.check_field_type_for_ffi(cache, &field, substs) {
 720                     FfiSafe => {
 721                         all_phantom = false;
 722                     }
 723                     FfiPhantom(..) if def.is_enum() => {
 724                         return FfiUnsafe {
 725                             ty,
 726                             reason: "this enum contains a PhantomData field".into(),
 727                             help: None,
 728                         };
 729                     }
 730                     FfiPhantom(..) => {}
 731                     r => return r,
 732                 }
 733             }
 734
 735             if all_phantom { FfiPhantom(ty) } else { FfiSafe }
 736         }
 737     }
 738
 739     /// Checks if the given type is "ffi-safe" (has a stable, well-defined
 740     /// representation which can be exported to C code).
 741     fn check_type_for_ffi(&self, cache: &mut FxHashSet<Ty<'tcx>>, ty: Ty<'tcx>) -> FfiResult<'tcx> {
 742         use FfiResult::*;
 743
 744         let tcx = self.cx.tcx;
 745
 746         // Protect against infinite recursion, for example
 747         // `struct S(*mut S);`.
 748         // FIXME: A recursion limit is necessary as well, for irregular
 749         // recursive types.
 750         if !cache.insert(ty) {
 751             return FfiSafe;
 752         }
 753
 754         match ty.kind {
 755             ty::Adt(def, _) if def.is_box() && matches!(self.mode, CItemKind::Definition) => {
 756                 FfiSafe
 757             }
 758
 759             ty::Adt(def, substs) => {
 760                 if def.is_phantom_data() {
 761                     return FfiPhantom(ty);
 762                 }
 763                 match def.adt_kind() {
 764                     AdtKind::Struct | AdtKind::Union => {
 765                         let kind = if def.is_struct() { "struct" } else { "union" };
 766
 767                         if !def.repr.c() && !def.repr.transparent() {
 768                             return FfiUnsafe {
 769                                 ty,
 770                                 reason: format!("this {} has unspecified layout", kind),
 771                                 help: Some(format!(
 772                                     "consider adding a `#[repr(C)]` or \
 773                                              `#[repr(transparent)]` attribute to this {}",
 774                                     kind
 775                                 )),
 776                             };
 777                         }
 778
 779                         let is_non_exhaustive =
 780                             def.non_enum_variant().is_field_list_non_exhaustive();
 781                         if is_non_exhaustive && !def.did.is_local() {
 782                             return FfiUnsafe {
 783                                 ty,
 784                                 reason: format!("this {} is non-exhaustive", kind),
 785                                 help: None,
 786                             };
 787                         }
 788
 789                         if def.non_enum_variant().fields.is_empty() {
 790                             return FfiUnsafe {
 791                                 ty,
 792                                 reason: format!("this {} has no fields", kind),
 793                                 help: Some(format!("consider adding a member to this {}", kind)),
 794                             };
 795                         }
 796
 797                         self.check_variant_for_ffi(cache, ty, def, def.non_enum_variant(), substs)
 798                     }
 799                     AdtKind::Enum => {
 800                         if def.variants.is_empty() {
 801                             // Empty enums are okay... although sort of useless.
 802                             return FfiSafe;
 803                         }
 804
 805                         // Check for a repr() attribute to specify the size of the
 806                         // discriminant.
 807                         if !def.repr.c() && !def.repr.transparent() && def.repr.int.is_none() {
 808                             // Special-case types like `Option<extern fn()>`.
 809                             if repr_nullable_ptr(self.cx, ty, self.mode).is_none() {
 810                                 return FfiUnsafe {
 811                                     ty,
 812                                     reason: "enum has no representation hint".into(),
 813                                     help: Some(
 814                                         "consider adding a `#[repr(C)]`, \
 815                                                 `#[repr(transparent)]`, or integer `#[repr(...)]` \
 816                                                 attribute to this enum"
 817                                             .into(),
 818                                     ),
 819                                 };
 820                             }
 821                         }
 822
 823                         if def.is_variant_list_non_exhaustive() && !def.did.is_local() {
 824                             return FfiUnsafe {
 825                                 ty,
 826                                 reason: "this enum is non-exhaustive".into(),
 827                                 help: None,
 828                             };
 829                         }
 830
 831                         // Check the contained variants.
 832                         for variant in &def.variants {
 833                             let is_non_exhaustive = variant.is_field_list_non_exhaustive();
 834                             if is_non_exhaustive && !variant.def_id.is_local() {
 835                                 return FfiUnsafe {
 836                                     ty,
 837                                     reason: "this enum has non-exhaustive variants".into(),
 838                                     help: None,
 839                                 };
 840                             }
 841
 842                             match self.check_variant_for_ffi(cache, ty, def, variant, substs) {
 843                                 FfiSafe => (),
 844                                 r => return r,
 845                             }
 846                         }
 847
 848                         FfiSafe
 849                     }
 850                 }
 851             }
 852
 853             ty::Char => FfiUnsafe {
 854                 ty,
 855                 reason: "the `char` type has no C equivalent".into(),
 856                 help: Some("consider using `u32` or `libc::wchar_t` instead".into()),
 857             },
 858
 859             ty::Int(ast::IntTy::I128) | ty::Uint(ast::UintTy::U128) => FfiUnsafe {
 860                 ty,
 861                 reason: "128-bit integers don't currently have a known stable ABI".into(),
 862                 help: None,
 863             },
 864
 865             // Primitive types with a stable representation.
 866             ty::Bool | ty::Int(..) | ty::Uint(..) | ty::Float(..) | ty::Never => FfiSafe,
 867
 868             ty::Slice(_) => FfiUnsafe {
 869                 ty,
 870                 reason: "slices have no C equivalent".into(),
 871                 help: Some("consider using a raw pointer instead".into()),
 872             },
 873
 874             ty::Dynamic(..) => {
 875                 FfiUnsafe { ty, reason: "trait objects have no C equivalent".into(), help: None }
 876             }
 877
 878             ty::Str => FfiUnsafe {
 879                 ty,
 880                 reason: "string slices have no C equivalent".into(),
 881                 help: Some("consider using `*const u8` and a length instead".into()),
 882             },
 883
 884             ty::Tuple(..) => FfiUnsafe {
 885                 ty,
 886                 reason: "tuples have unspecified layout".into(),
 887                 help: Some("consider using a struct instead".into()),
 888             },
 889
 890             ty::RawPtr(ty::TypeAndMut { ty, .. }) | ty::Ref(_, ty, _)
 891                 if {
 892                     matches!(self.mode, CItemKind::Definition)
 893                         && ty.is_sized(self.cx.tcx.at(DUMMY_SP), self.cx.param_env)
 894                 } =>
 895             {
 896                 FfiSafe
 897             }
 898
 899             ty::RawPtr(ty::TypeAndMut { ty, .. }) | ty::Ref(_, ty, _) => {
 900                 self.check_type_for_ffi(cache, ty)
 901             }
 902
 903             ty::Array(inner_ty, _) => self.check_type_for_ffi(cache, inner_ty),
 904
 905             ty::FnPtr(sig) => {
 906                 if self.is_internal_abi(sig.abi()) {
 907                     return FfiUnsafe {
 908                         ty,
 909                         reason: "this function pointer has Rust-specific calling convention".into(),
 910                         help: Some(
 911                             "consider using an `extern fn(...) -> ...` \
 912                                     function pointer instead"
 913                                 .into(),
 914                         ),
 915                     };
 916                 }
 917
 918                 let sig = tcx.erase_late_bound_regions(&sig);
 919                 if !sig.output().is_unit() {
 920                     let r = self.check_type_for_ffi(cache, sig.output());
 921                     match r {
 922                         FfiSafe => {}
 923                         _ => {
 924                             return r;
 925                         }
 926                     }
 927                 }
 928                 for arg in sig.inputs() {
 929                     let r = self.check_type_for_ffi(cache, arg);
 930                     match r {
 931                         FfiSafe => {}
 932                         _ => {
 933                             return r;
 934                         }
 935                     }
 936                 }
 937                 FfiSafe
 938             }
 939
 940             ty::Foreign(..) => FfiSafe,
 941
 942             // While opaque types are checked for earlier, if a projection in a struct field
 943             // normalizes to an opaque type, then it will reach this branch.
 944             ty::Opaque(..) => {
 945                 FfiUnsafe { ty, reason: "opaque types have no C equivalent".into(), help: None }
 946             }
 947
 948             // `extern "C" fn` functions can have type parameters, which may or may not be FFI-safe,
 949             //  so they are currently ignored for the purposes of this lint.
 950             ty::Param(..) | ty::Projection(..) if matches!(self.mode, CItemKind::Definition) => {
 951                 FfiSafe
 952             }
 953
 954             ty::Param(..)
 955             | ty::Projection(..)
 956             | ty::Infer(..)
 957             | ty::Bound(..)
 958             | ty::Error(_)
 959             | ty::Closure(..)
 960             | ty::Generator(..)
 961             | ty::GeneratorWitness(..)
 962             | ty::Placeholder(..)
 963             | ty::FnDef(..) => bug!("unexpected type in foreign function: {:?}", ty),
 964         }
 965     }
 966
 967     fn emit_ffi_unsafe_type_lint(
 968         &mut self,
 969         ty: Ty<'tcx>,
 970         sp: Span,
 971         note: &str,
 972         help: Option<&str>,
 973     ) {
 974         let lint = match self.mode {
 975             CItemKind::Declaration => IMPROPER_CTYPES,
 976             CItemKind::Definition => IMPROPER_CTYPES_DEFINITIONS,
 977         };
 978
 979         self.cx.struct_span_lint(lint, sp, |lint| {
 980             let item_description = match self.mode {
 981                 CItemKind::Declaration => "block",
 982                 CItemKind::Definition => "fn",
 983             };
 984             let mut diag = lint.build(&format!(
 985                 "`extern` {} uses type `{}`, which is not FFI-safe",
 986                 item_description, ty
 987             ));
 988             diag.span_label(sp, "not FFI-safe");
 989             if let Some(help) = help {
 990                 diag.help(help);
 991             }
 992             diag.note(note);
 993             if let ty::Adt(def, _) = ty.kind {
 994                 if let Some(sp) = self.cx.tcx.hir().span_if_local(def.did) {
 995                     diag.span_note(sp, "the type is defined here");
 996                 }
 997             }
 998             diag.emit();
 999         });
1000     }
1001
1002     fn check_for_opaque_ty(&mut self, sp: Span, ty: Ty<'tcx>) -> bool {
1003         struct ProhibitOpaqueTypes<'a, 'tcx> {
1004             cx: &'a LateContext<'tcx>,
1005             ty: Option<Ty<'tcx>>,
1006         };
1007
1008         impl<'a, 'tcx> ty::fold::TypeVisitor<'tcx> for ProhibitOpaqueTypes<'a, 'tcx> {
1009             fn visit_ty(&mut self, ty: Ty<'tcx>) -> bool {
1010                 match ty.kind {
1011                     ty::Opaque(..) => {
1012                         self.ty = Some(ty);
1013                         true
1014                     }
1015                     // Consider opaque types within projections FFI-safe if they do not normalize
1016                     // to more opaque types.
1017                     ty::Projection(..) => {
1018                         let ty = self.cx.tcx.normalize_erasing_regions(self.cx.param_env, ty);
1019
1020                         // If `ty` is a opaque type directly then `super_visit_with` won't invoke
1021                         // this function again.
1022                         if ty.has_opaque_types() { self.visit_ty(ty) } else { false }
1023                     }
1024                     _ => ty.super_visit_with(self),
1025                 }
1026             }
1027         }
1028
1029         let mut visitor = ProhibitOpaqueTypes { cx: self.cx, ty: None };
1030         ty.visit_with(&mut visitor);
1031         if let Some(ty) = visitor.ty {
1032             self.emit_ffi_unsafe_type_lint(ty, sp, "opaque types have no C equivalent", None);
1033             true
1034         } else {
1035             false
1036         }
1037     }
1038
1039     fn check_type_for_ffi_and_report_errors(
1040         &mut self,
1041         sp: Span,
1042         ty: Ty<'tcx>,
1043         is_static: bool,
1044         is_return_type: bool,
1045     ) {
1046         // We have to check for opaque types before `normalize_erasing_regions`,
1047         // which will replace opaque types with their underlying concrete type.
1048         if self.check_for_opaque_ty(sp, ty) {
1049             // We've already emitted an error due to an opaque type.
1050             return;
1051         }
1052
1053         // it is only OK to use this function because extern fns cannot have
1054         // any generic types right now:
1055         let ty = self.cx.tcx.normalize_erasing_regions(self.cx.param_env, ty);
1056
1057         // C doesn't really support passing arrays by value - the only way to pass an array by value
1058         // is through a struct. So, first test that the top level isn't an array, and then
1059         // recursively check the types inside.
1060         if !is_static && self.check_for_array_ty(sp, ty) {
1061             return;
1062         }
1063
1064         // Don't report FFI errors for unit return types. This check exists here, and not in
1065         // `check_foreign_fn` (where it would make more sense) so that normalization has definitely
1066         // happened.
1067         if is_return_type && ty.is_unit() {
1068             return;
1069         }
1070
1071         match self.check_type_for_ffi(&mut FxHashSet::default(), ty) {
1072             FfiResult::FfiSafe => {}
1073             FfiResult::FfiPhantom(ty) => {
1074                 self.emit_ffi_unsafe_type_lint(ty, sp, "composed only of `PhantomData`", None);
1075             }
1076             // If `ty` is a `repr(transparent)` newtype, and the non-zero-sized type is a generic
1077             // argument, which after substitution, is `()`, then this branch can be hit.
1078             FfiResult::FfiUnsafe { ty, .. } if is_return_type && ty.is_unit() => {}
1079             FfiResult::FfiUnsafe { ty, reason, help } => {
1080                 self.emit_ffi_unsafe_type_lint(ty, sp, &reason, help.as_deref());
1081             }
1082         }
1083     }
1084
1085     fn check_foreign_fn(&mut self, id: hir::HirId, decl: &hir::FnDecl<'_>) {
1086         let def_id = self.cx.tcx.hir().local_def_id(id);
1087         let sig = self.cx.tcx.fn_sig(def_id);
1088         let sig = self.cx.tcx.erase_late_bound_regions(&sig);
1089
1090         for (input_ty, input_hir) in sig.inputs().iter().zip(decl.inputs) {
1091             self.check_type_for_ffi_and_report_errors(input_hir.span, input_ty, false, false);
1092         }
1093
1094         if let hir::FnRetTy::Return(ref ret_hir) = decl.output {
1095             let ret_ty = sig.output();
1096             self.check_type_for_ffi_and_report_errors(ret_hir.span, ret_ty, false, true);
1097         }
1098     }
1099
1100     fn check_foreign_static(&mut self, id: hir::HirId, span: Span) {
1101         let def_id = self.cx.tcx.hir().local_def_id(id);
1102         let ty = self.cx.tcx.type_of(def_id);
1103         self.check_type_for_ffi_and_report_errors(span, ty, true, false);
1104     }
1105
1106     fn is_internal_abi(&self, abi: SpecAbi) -> bool {
1107         if let SpecAbi::Rust
1108         | SpecAbi::RustCall
1109         | SpecAbi::RustIntrinsic
1110         | SpecAbi::PlatformIntrinsic = abi
1111         {
1112             true
1113         } else {
1114             false
1115         }
1116     }
1117 }
1118
1119 impl<'tcx> LateLintPass<'tcx> for ImproperCTypesDeclarations {
1120     fn check_foreign_item(&mut self, cx: &LateContext<'_>, it: &hir::ForeignItem<'_>) {
1121         let mut vis = ImproperCTypesVisitor { cx, mode: CItemKind::Declaration };
1122         let abi = cx.tcx.hir().get_foreign_abi(it.hir_id);
1123
1124         if !vis.is_internal_abi(abi) {
1125             match it.kind {
1126                 hir::ForeignItemKind::Fn(ref decl, _, _) => {
1127                     vis.check_foreign_fn(it.hir_id, decl);
1128                 }
1129                 hir::ForeignItemKind::Static(ref ty, _) => {
1130                     vis.check_foreign_static(it.hir_id, ty.span);
1131                 }
1132                 hir::ForeignItemKind::Type => (),
1133             }
1134         }
1135     }
1136 }
1137
1138 impl<'tcx> LateLintPass<'tcx> for ImproperCTypesDefinitions {
1139     fn check_fn(
1140         &mut self,
1141         cx: &LateContext<'tcx>,
1142         kind: hir::intravisit::FnKind<'tcx>,
1143         decl: &'tcx hir::FnDecl<'_>,
1144         _: &'tcx hir::Body<'_>,
1145         _: Span,
1146         hir_id: hir::HirId,
1147     ) {
1148         use hir::intravisit::FnKind;
1149
1150         let abi = match kind {
1151             FnKind::ItemFn(_, _, header, ..) => header.abi,
1152             FnKind::Method(_, sig, ..) => sig.header.abi,
1153             _ => return,
1154         };
1155
1156         let mut vis = ImproperCTypesVisitor { cx, mode: CItemKind::Definition };
1157         if !vis.is_internal_abi(abi) {
1158             vis.check_foreign_fn(hir_id, decl);
1159         }
1160     }
1161 }
1162
1163 declare_lint_pass!(VariantSizeDifferences => [VARIANT_SIZE_DIFFERENCES]);
1164
1165 impl<'tcx> LateLintPass<'tcx> for VariantSizeDifferences {
1166     fn check_item(&mut self, cx: &LateContext<'_>, it: &hir::Item<'_>) {
1167         if let hir::ItemKind::Enum(ref enum_definition, _) = it.kind {
1168             let item_def_id = cx.tcx.hir().local_def_id(it.hir_id);
1169             let t = cx.tcx.type_of(item_def_id);
1170             let ty = cx.tcx.erase_regions(&t);
1171             let layout = match cx.layout_of(ty) {
1172                 Ok(layout) => layout,
1173                 Err(
1174                     ty::layout::LayoutError::Unknown(_) | ty::layout::LayoutError::SizeOverflow(_),
1175                 ) => return,
1176             };
1177             let (variants, tag) = match layout.variants {
1178                 Variants::Multiple {
1179                     tag_encoding: TagEncoding::Direct,
1180                     ref tag,
1181                     ref variants,
1182                     ..
1183                 } => (variants, tag),
1184                 _ => return,
1185             };
1186
1187             let tag_size = tag.value.size(&cx.tcx).bytes();
1188
1189             debug!(
1190                 "enum `{}` is {} bytes large with layout:\n{:#?}",
1191                 t,
1192                 layout.size.bytes(),
1193                 layout
1194             );
1195
1196             let (largest, slargest, largest_index) = enum_definition
1197                 .variants
1198                 .iter()
1199                 .zip(variants)
1200                 .map(|(variant, variant_layout)| {
1201                     // Subtract the size of the enum tag.
1202                     let bytes = variant_layout.size.bytes().saturating_sub(tag_size);
1203
1204                     debug!("- variant `{}` is {} bytes large", variant.ident, bytes);
1205                     bytes
1206                 })
1207                 .enumerate()
1208                 .fold((0, 0, 0), |(l, s, li), (idx, size)| {
1209                     if size > l {
1210                         (size, l, idx)
1211                     } else if size > s {
1212                         (l, size, li)
1213                     } else {
1214                         (l, s, li)
1215                     }
1216                 });
1217
1218             // We only warn if the largest variant is at least thrice as large as
1219             // the second-largest.
1220             if largest > slargest * 3 && slargest > 0 {
1221                 cx.struct_span_lint(
1222                     VARIANT_SIZE_DIFFERENCES,
1223                     enum_definition.variants[largest_index].span,
1224                     |lint| {
1225                         lint.build(&format!(
1226                             "enum variant is more than three times \
1227                                           larger ({} bytes) than the next largest",
1228                             largest
1229                         ))
1230                         .emit()
1231                     },
1232                 );
1233             }
1234         }
1235     }
1236 }