src/tools/clippy/clippy_lints/src/option_if_let_else.rs

   1 use crate::utils;
   2 use crate::utils::eager_or_lazy;
   3 use crate::utils::sugg::Sugg;
   4 use crate::utils::{is_type_diagnostic_item, paths, span_lint_and_sugg};
   5 use if_chain::if_chain;
   6
   7 use rustc_errors::Applicability;
   8 use rustc_hir::{Arm, BindingAnnotation, Block, Expr, ExprKind, MatchSource, Mutability, PatKind, UnOp};
   9 use rustc_lint::{LateContext, LateLintPass};
  10 use rustc_session::{declare_lint_pass, declare_tool_lint};
  11 use rustc_span::sym;
  12
  13 declare_clippy_lint! {
  14     /// **What it does:**
  15     /// Lints usage of `if let Some(v) = ... { y } else { x }` which is more
  16     /// idiomatically done with `Option::map_or` (if the else bit is a pure
  17     /// expression) or `Option::map_or_else` (if the else bit is an impure
  18     /// expression).
  19     ///
  20     /// **Why is this bad?**
  21     /// Using the dedicated functions of the Option type is clearer and
  22     /// more concise than an `if let` expression.
  23     ///
  24     /// **Known problems:**
  25     /// This lint uses a deliberately conservative metric for checking
  26     /// if the inside of either body contains breaks or continues which will
  27     /// cause it to not suggest a fix if either block contains a loop with
  28     /// continues or breaks contained within the loop.
  29     ///
  30     /// **Example:**
  31     ///
  32     /// ```rust
  33     /// # let optional: Option<u32> = Some(0);
  34     /// # fn do_complicated_function() -> u32 { 5 };
  35     /// let _ = if let Some(foo) = optional {
  36     ///     foo
  37     /// } else {
  38     ///     5
  39     /// };
  40     /// let _ = if let Some(foo) = optional {
  41     ///     foo
  42     /// } else {
  43     ///     let y = do_complicated_function();
  44     ///     y*y
  45     /// };
  46     /// ```
  47     ///
  48     /// should be
  49     ///
  50     /// ```rust
  51     /// # let optional: Option<u32> = Some(0);
  52     /// # fn do_complicated_function() -> u32 { 5 };
  53     /// let _ = optional.map_or(5, |foo| foo);
  54     /// let _ = optional.map_or_else(||{
  55     ///     let y = do_complicated_function();
  56     ///     y*y
  57     /// }, |foo| foo);
  58     /// ```
  59     pub OPTION_IF_LET_ELSE,
  60     pedantic,
  61     "reimplementation of Option::map_or"
  62 }
  63
  64 declare_lint_pass!(OptionIfLetElse => [OPTION_IF_LET_ELSE]);
  65
  66 /// Returns true iff the given expression is the result of calling `Result::ok`
  67 fn is_result_ok(cx: &LateContext<'_>, expr: &'_ Expr<'_>) -> bool {
  68     if let ExprKind::MethodCall(ref path, _, &[ref receiver], _) = &expr.kind {
  69         path.ident.name.as_str() == "ok"
  70             && is_type_diagnostic_item(cx, &cx.typeck_results().expr_ty(&receiver), sym::result_type)
  71     } else {
  72         false
  73     }
  74 }
  75
  76 /// A struct containing information about occurrences of the
  77 /// `if let Some(..) = .. else` construct that this lint detects.
  78 struct OptionIfLetElseOccurence {
  79     option: String,
  80     method_sugg: String,
  81     some_expr: String,
  82     none_expr: String,
  83     wrap_braces: bool,
  84 }
  85
  86 /// Extracts the body of a given arm. If the arm contains only an expression,
  87 /// then it returns the expression. Otherwise, it returns the entire block
  88 fn extract_body_from_arm<'a>(arm: &'a Arm<'a>) -> Option<&'a Expr<'a>> {
  89     if let ExprKind::Block(
  90         Block {
  91             stmts: statements,
  92             expr: Some(expr),
  93             ..
  94         },
  95         _,
  96     ) = &arm.body.kind
  97     {
  98         if let [] = statements {
  99             Some(&expr)
 100         } else {
 101             Some(&arm.body)
 102         }
 103     } else {
 104         None
 105     }
 106 }
 107
 108 /// If this is the else body of an if/else expression, then we need to wrap
 109 /// it in curly braces. Otherwise, we don't.
 110 fn should_wrap_in_braces(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
 111     utils::get_enclosing_block(cx, expr.hir_id).map_or(false, |parent| {
 112         let mut should_wrap = false;
 113
 114         if let Some(Expr {
 115             kind:
 116                 ExprKind::Match(
 117                     _,
 118                     arms,
 119                     MatchSource::IfLetDesugar {
 120                         contains_else_clause: true,
 121                     },
 122                 ),
 123             ..
 124         }) = parent.expr
 125         {
 126             should_wrap = expr.hir_id == arms[1].body.hir_id;
 127         } else if let Some(Expr {
 128             kind: ExprKind::If(_, _, Some(else_clause)),
 129             ..
 130         }) = parent.expr
 131         {
 132             should_wrap = expr.hir_id == else_clause.hir_id;
 133         }
 134
 135         should_wrap
 136     })
 137 }
 138
 139 fn format_option_in_sugg(cx: &LateContext<'_>, cond_expr: &Expr<'_>, as_ref: bool, as_mut: bool) -> String {
 140     format!(
 141         "{}{}",
 142         Sugg::hir(cx, cond_expr, "..").maybe_par(),
 143         if as_mut {
 144             ".as_mut()"
 145         } else if as_ref {
 146             ".as_ref()"
 147         } else {
 148             ""
 149         }
 150     )
 151 }
 152
 153 /// If this expression is the option if let/else construct we're detecting, then
 154 /// this function returns an `OptionIfLetElseOccurence` struct with details if
 155 /// this construct is found, or None if this construct is not found.
 156 fn detect_option_if_let_else<'tcx>(
 157     cx: &'_ LateContext<'tcx>,
 158     expr: &'_ Expr<'tcx>,
 159 ) -> Option<OptionIfLetElseOccurence> {
 160     if_chain! {
 161         if !utils::in_macro(expr.span); // Don't lint macros, because it behaves weirdly
 162         if let ExprKind::Match(cond_expr, arms, MatchSource::IfLetDesugar{contains_else_clause: true}) = &expr.kind;
 163         if arms.len() == 2;
 164         if !is_result_ok(cx, cond_expr); // Don't lint on Result::ok because a different lint does it already
 165         if let PatKind::TupleStruct(struct_qpath, &[inner_pat], _) = &arms[0].pat.kind;
 166         if utils::match_qpath(struct_qpath, &paths::OPTION_SOME);
 167         if let PatKind::Binding(bind_annotation, _, id, _) = &inner_pat.kind;
 168         if !utils::usage::contains_return_break_continue_macro(arms[0].body);
 169         if !utils::usage::contains_return_break_continue_macro(arms[1].body);
 170         then {
 171             let capture_mut = if bind_annotation == &BindingAnnotation::Mutable { "mut " } else { "" };
 172             let some_body = extract_body_from_arm(&arms[0])?;
 173             let none_body = extract_body_from_arm(&arms[1])?;
 174             let method_sugg = if eager_or_lazy::is_eagerness_candidate(cx, none_body) { "map_or" } else { "map_or_else" };
 175             let capture_name = id.name.to_ident_string();
 176             let wrap_braces = should_wrap_in_braces(cx, expr);
 177             let (as_ref, as_mut) = match &cond_expr.kind {
 178                 ExprKind::AddrOf(_, Mutability::Not, _) => (true, false),
 179                 ExprKind::AddrOf(_, Mutability::Mut, _) => (false, true),
 180                 _ => (bind_annotation == &BindingAnnotation::Ref, bind_annotation == &BindingAnnotation::RefMut),
 181             };
 182             let cond_expr = match &cond_expr.kind {
 183                 // Pointer dereferencing happens automatically, so we can omit it in the suggestion
 184                 ExprKind::Unary(UnOp::Deref, expr) | ExprKind::AddrOf(_, _, expr) => expr,
 185                 _ => cond_expr,
 186             };
 187             Some(OptionIfLetElseOccurence {
 188                 option: format_option_in_sugg(cx, cond_expr, as_ref, as_mut),
 189                 method_sugg: method_sugg.to_string(),
 190                 some_expr: format!("|{}{}| {}", capture_mut, capture_name, Sugg::hir(cx, some_body, "..")),
 191                 none_expr: format!("{}{}", if method_sugg == "map_or" { "" } else { "|| " }, Sugg::hir(cx, none_body, "..")),
 192                 wrap_braces,
 193             })
 194         } else {
 195             None
 196         }
 197     }
 198 }
 199
 200 impl<'tcx> LateLintPass<'tcx> for OptionIfLetElse {
 201     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &Expr<'tcx>) {
 202         if let Some(detection) = detect_option_if_let_else(cx, expr) {
 203             span_lint_and_sugg(
 204                 cx,
 205                 OPTION_IF_LET_ELSE,
 206                 expr.span,
 207                 format!("use Option::{} instead of an if let/else", detection.method_sugg).as_str(),
 208                 "try",
 209                 format!(
 210                     "{}{}.{}({}, {}){}",
 211                     if detection.wrap_braces { "{ " } else { "" },
 212                     detection.option,
 213                     detection.method_sugg,
 214                     detection.none_expr,
 215                     detection.some_expr,
 216                     if detection.wrap_braces { " }" } else { "" },
 217                 ),
 218                 Applicability::MaybeIncorrect,
 219             );
 220         }
 221     }
 222 }