clippy_lints/src/infallible_destructuring_match.rs

   1 use super::utils::{get_arg_name, match_var, remove_blocks, snippet_with_applicability, span_lint_and_sugg};
   2 use if_chain::if_chain;
   3 use rustc::hir::*;
   4 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   5 use rustc::{declare_lint_pass, declare_tool_lint};
   6 use rustc_errors::Applicability;
   7
   8 declare_clippy_lint! {
   9     /// **What it does:** Checks for matches being used to destructure a single-variant enum
  10     /// or tuple struct where a `let` will suffice.
  11     ///
  12     /// **Why is this bad?** Just readability – `let` doesn't nest, whereas a `match` does.
  13     ///
  14     /// **Known problems:** None.
  15     ///
  16     /// **Example:**
  17     /// ```rust
  18     /// enum Wrapper {
  19     ///     Data(i32),
  20     /// }
  21     ///
  22     /// let wrapper = Wrapper::Data(42);
  23     ///
  24     /// let data = match wrapper {
  25     ///     Wrapper::Data(i) => i,
  26     /// };
  27     /// ```
  28     ///
  29     /// The correct use would be:
  30     /// ```rust
  31     /// enum Wrapper {
  32     ///     Data(i32),
  33     /// }
  34     ///
  35     /// let wrapper = Wrapper::Data(42);
  36     /// let Wrapper::Data(data) = wrapper;
  37     /// ```
  38     pub INFALLIBLE_DESTRUCTURING_MATCH,
  39     style,
  40     "a match statement with a single infallible arm instead of a `let`"
  41 }
  42
  43 declare_lint_pass!(InfallibleDestructingMatch => [INFALLIBLE_DESTRUCTURING_MATCH]);
  44
  45 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for InfallibleDestructingMatch {
  46     fn check_local(&mut self, cx: &LateContext<'a, 'tcx>, local: &'tcx Local) {
  47         if_chain! {
  48             if let Some(ref expr) = local.init;
  49             if let ExprKind::Match(ref target, ref arms, MatchSource::Normal) = expr.kind;
  50             if arms.len() == 1 && arms[0].guard.is_none();
  51             if let PatKind::TupleStruct(QPath::Resolved(None, ref variant_name), ref args, _) = arms[0].pat.kind;
  52             if args.len() == 1;
  53             if let Some(arg) = get_arg_name(&args[0]);
  54             let body = remove_blocks(&arms[0].body);
  55             if match_var(body, arg);
  56
  57             then {
  58                 let mut applicability = Applicability::MachineApplicable;
  59                 span_lint_and_sugg(
  60                     cx,
  61                     INFALLIBLE_DESTRUCTURING_MATCH,
  62                     local.span,
  63                     "you seem to be trying to use match to destructure a single infallible pattern. \
  64                      Consider using `let`",
  65                     "try this",
  66                     format!(
  67                         "let {}({}) = {};",
  68                         snippet_with_applicability(cx, variant_name.span, "..", &mut applicability),
  69                         snippet_with_applicability(cx, local.pat.span, "..", &mut applicability),
  70                         snippet_with_applicability(cx, target.span, "..", &mut applicability),
  71                     ),
  72                     applicability,
  73                 );
  74             }
  75         }
  76     }
  77 }