clippy_lints/src/let_underscore.rs

   1 use if_chain::if_chain;
   2 use rustc::lint::{LateContext, LateLintPass};
   3 use rustc_hir::*;
   4 use rustc_session::{declare_lint_pass, declare_tool_lint};
   5
   6 use crate::utils::{is_must_use_func_call, is_must_use_ty, span_help_and_lint};
   7
   8 declare_clippy_lint! {
   9     /// **What it does:** Checks for `let _ = <expr>`
  10     /// where expr is #[must_use]
  11     ///
  12     /// **Why is this bad?** It's better to explicitly
  13     /// handle the value of a #[must_use] expr
  14     ///
  15     /// **Known problems:** None.
  16     ///
  17     /// **Example:**
  18     /// ```rust
  19     /// fn f() -> Result<u32, u32> {
  20     ///     Ok(0)
  21     /// }
  22     ///
  23     /// let _ = f();
  24     /// // is_ok() is marked #[must_use]
  25     /// let _ = f().is_ok();
  26     /// ```
  27     pub LET_UNDERSCORE_MUST_USE,
  28     restriction,
  29     "non-binding let on a `#[must_use]` expression"
  30 }
  31
  32 declare_lint_pass!(LetUnderscore => [LET_UNDERSCORE_MUST_USE]);
  33
  34 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LetUnderscore {
  35     fn check_stmt(&mut self, cx: &LateContext<'_, '_>, stmt: &Stmt<'_>) {
  36         if_chain! {
  37             if let StmtKind::Local(ref local) = stmt.kind;
  38             if let PatKind::Wild = local.pat.kind;
  39             if let Some(ref init) = local.init;
  40             then {
  41                 if is_must_use_ty(cx, cx.tables.expr_ty(init)) {
  42                    span_help_and_lint(
  43                         cx,
  44                         LET_UNDERSCORE_MUST_USE,
  45                         stmt.span,
  46                         "non-binding let on an expression with `#[must_use]` type",
  47                         "consider explicitly using expression value"
  48                     )
  49                 } else if is_must_use_func_call(cx, init) {
  50                     span_help_and_lint(
  51                         cx,
  52                         LET_UNDERSCORE_MUST_USE,
  53                         stmt.span,
  54                         "non-binding let on a result of a `#[must_use]` function",
  55                         "consider explicitly using function result"
  56                     )
  57                 }
  58             }
  59         }
  60     }
  61 }