clippy_lints/src/let_underscore.rs

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