clippy_lints/src/let_underscore.rs

   1 use if_chain::if_chain;
   2 use rustc::lint::in_external_macro;
   3 use rustc_hir::*;
   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, 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 in_external_macro(cx.tcx.sess, stmt.span) {
  38             return;
  39         }
  40
  41         if_chain! {
  42             if let StmtKind::Local(ref local) = stmt.kind;
  43             if let PatKind::Wild = local.pat.kind;
  44             if let Some(ref init) = local.init;
  45             then {
  46                 if is_must_use_ty(cx, cx.tables.expr_ty(init)) {
  47                    span_help_and_lint(
  48                         cx,
  49                         LET_UNDERSCORE_MUST_USE,
  50                         stmt.span,
  51                         "non-binding let on an expression with `#[must_use]` type",
  52                         "consider explicitly using expression value"
  53                     )
  54                 } else if is_must_use_func_call(cx, init) {
  55                     span_help_and_lint(
  56                         cx,
  57                         LET_UNDERSCORE_MUST_USE,
  58                         stmt.span,
  59                         "non-binding let on a result of a `#[must_use]` function",
  60                         "consider explicitly using function result"
  61                     )
  62                 }
  63             }
  64         }
  65     }
  66 }