clippy_lints/src/let_underscore.rs

   1 use if_chain::if_chain;
   2 use rustc::declare_lint_pass;
   3 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   4 use rustc_hir::*;
   5 use rustc_session::declare_tool_lint;
   6
   7 use crate::utils::{is_must_use_func_call, is_must_use_ty, span_help_and_lint};
   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_lint_pass!(LetUnderscore => [LET_UNDERSCORE_MUST_USE]);
  34
  35 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LetUnderscore {
  36     fn check_stmt(&mut self, cx: &LateContext<'_, '_>, stmt: &Stmt<'_>) {
  37         if_chain! {
  38             if let StmtKind::Local(ref local) = stmt.kind;
  39             if let PatKind::Wild = local.pat.kind;
  40             if let Some(ref init) = local.init;
  41             then {
  42                 if is_must_use_ty(cx, cx.tables.expr_ty(init)) {
  43                    span_help_and_lint(
  44                         cx,
  45                         LET_UNDERSCORE_MUST_USE,
  46                         stmt.span,
  47                         "non-binding let on an expression with `#[must_use]` type",
  48                         "consider explicitly using expression value"
  49                     )
  50                 } else if is_must_use_func_call(cx, init) {
  51                     span_help_and_lint(
  52                         cx,
  53                         LET_UNDERSCORE_MUST_USE,
  54                         stmt.span,
  55                         "non-binding let on a result of a `#[must_use]` function",
  56                         "consider explicitly using function result"
  57                     )
  58                 }
  59             }
  60         }
  61     }
  62 }