src/tools/clippy/clippy_lints/src/let_underscore.rs

   1 use if_chain::if_chain;
   2 use rustc_hir::{Local, PatKind};
   3 use rustc_lint::{LateContext, LateLintPass};
   4 use rustc_middle::lint::in_external_macro;
   5 use rustc_middle::ty::subst::GenericArgKind;
   6 use rustc_session::{declare_lint_pass, declare_tool_lint};
   7
   8 use crate::utils::{is_must_use_func_call, is_must_use_ty, match_type, paths, span_lint_and_help};
   9
  10 declare_clippy_lint! {
  11     /// **What it does:** Checks for `let _ = <expr>`
  12     /// where expr is #[must_use]
  13     ///
  14     /// **Why is this bad?** It's better to explicitly
  15     /// handle the value of a #[must_use] expr
  16     ///
  17     /// **Known problems:** None.
  18     ///
  19     /// **Example:**
  20     /// ```rust
  21     /// fn f() -> Result<u32, u32> {
  22     ///     Ok(0)
  23     /// }
  24     ///
  25     /// let _ = f();
  26     /// // is_ok() is marked #[must_use]
  27     /// let _ = f().is_ok();
  28     /// ```
  29     pub LET_UNDERSCORE_MUST_USE,
  30     restriction,
  31     "non-binding let on a `#[must_use]` expression"
  32 }
  33
  34 declare_clippy_lint! {
  35     /// **What it does:** Checks for `let _ = sync_lock`
  36     ///
  37     /// **Why is this bad?** This statement immediately drops the lock instead of
  38     /// extending its lifetime to the end of the scope, which is often not intended.
  39     /// To extend lock lifetime to the end of the scope, use an underscore-prefixed
  40     /// name instead (i.e. _lock). If you want to explicitly drop the lock,
  41     /// `std::mem::drop` conveys your intention better and is less error-prone.
  42     ///
  43     /// **Known problems:** None.
  44     ///
  45     /// **Example:**
  46     ///
  47     /// Bad:
  48     /// ```rust,ignore
  49     /// let _ = mutex.lock();
  50     /// ```
  51     ///
  52     /// Good:
  53     /// ```rust,ignore
  54     /// let _lock = mutex.lock();
  55     /// ```
  56     pub LET_UNDERSCORE_LOCK,
  57     correctness,
  58     "non-binding let on a synchronization lock"
  59 }
  60
  61 declare_lint_pass!(LetUnderscore => [LET_UNDERSCORE_MUST_USE, LET_UNDERSCORE_LOCK]);
  62
  63 const SYNC_GUARD_PATHS: [&[&str]; 3] = [
  64     &paths::MUTEX_GUARD,
  65     &paths::RWLOCK_READ_GUARD,
  66     &paths::RWLOCK_WRITE_GUARD,
  67 ];
  68
  69 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LetUnderscore {
  70     fn check_local(&mut self, cx: &LateContext<'_, '_>, local: &Local<'_>) {
  71         if in_external_macro(cx.tcx.sess, local.span) {
  72             return;
  73         }
  74
  75         if_chain! {
  76             if let PatKind::Wild = local.pat.kind;
  77             if let Some(ref init) = local.init;
  78             then {
  79                 let init_ty = cx.tables.expr_ty(init);
  80                 let contains_sync_guard = init_ty.walk().any(|inner| match inner.unpack() {
  81                     GenericArgKind::Type(inner_ty) => {
  82                         SYNC_GUARD_PATHS.iter().any(|path| match_type(cx, inner_ty, path))
  83                     },
  84
  85                     GenericArgKind::Lifetime(_) | GenericArgKind::Const(_) => false,
  86                 });
  87                 if contains_sync_guard {
  88                     span_lint_and_help(
  89                         cx,
  90                         LET_UNDERSCORE_LOCK,
  91                         local.span,
  92                         "non-binding let on a synchronization lock",
  93                         None,
  94                         "consider using an underscore-prefixed named \
  95                             binding or dropping explicitly with `std::mem::drop`"
  96                     )
  97                 } else if is_must_use_ty(cx, cx.tables.expr_ty(init)) {
  98                     span_lint_and_help(
  99                         cx,
 100                         LET_UNDERSCORE_MUST_USE,
 101                         local.span,
 102                         "non-binding let on an expression with `#[must_use]` type",
 103                         None,
 104                         "consider explicitly using expression value"
 105                     )
 106                 } else if is_must_use_func_call(cx, init) {
 107                     span_lint_and_help(
 108                         cx,
 109                         LET_UNDERSCORE_MUST_USE,
 110                         local.span,
 111                         "non-binding let on a result of a `#[must_use]` function",
 112                         None,
 113                         "consider explicitly using function result"
 114                     )
 115                 }
 116             }
 117         }
 118     }
 119 }