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_tool_lint, lint_array};
   6 use rustc_errors::Applicability;
   7
   8 /// **What it does:** Checks for matches being used to destructure a single-variant enum
   9 /// or tuple struct where a `let` will suffice.
  10 ///
  11 /// **Why is this bad?** Just readability – `let` doesn't nest, whereas a `match` does.
  12 ///
  13 /// **Known problems:** None.
  14 ///
  15 /// **Example:**
  16 /// ```rust
  17 /// enum Wrapper {
  18 ///     Data(i32),
  19 /// }
  20 ///
  21 /// let wrapper = Wrapper::Data(42);
  22 ///
  23 /// let data = match wrapper {
  24 ///     Wrapper::Data(i) => i,
  25 /// };
  26 /// ```
  27 ///
  28 /// The correct use would be:
  29 /// ```rust
  30 /// enum Wrapper {
  31 ///     Data(i32),
  32 /// }
  33 ///
  34 /// let wrapper = Wrapper::Data(42);
  35 /// let Wrapper::Data(data) = wrapper;
  36 /// ```
  37 declare_clippy_lint! {
  38     pub INFALLIBLE_DESTRUCTURING_MATCH,
  39     style,
  40     "a match statement with a single infallible arm instead of a `let`"
  41 }
  42
  43 #[derive(Copy, Clone, Default)]
  44 pub struct Pass;
  45
  46 impl LintPass for Pass {
  47     fn get_lints(&self) -> LintArray {
  48         lint_array!(INFALLIBLE_DESTRUCTURING_MATCH)
  49     }
  50
  51     fn name(&self) -> &'static str {
  52         "InfallibleDestructingMatch"
  53     }
  54 }
  55
  56 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
  57     fn check_local(&mut self, cx: &LateContext<'a, 'tcx>, local: &'tcx Local) {
  58         if_chain! {
  59             if let Some(ref expr) = local.init;
  60             if let ExprKind::Match(ref target, ref arms, MatchSource::Normal) = expr.node;
  61             if arms.len() == 1 && arms[0].pats.len() == 1 && arms[0].guard.is_none();
  62             if let PatKind::TupleStruct(QPath::Resolved(None, ref variant_name), ref args, _) = arms[0].pats[0].node;
  63             if args.len() == 1;
  64             if let Some(arg) = get_arg_name(&args[0]);
  65             let body = remove_blocks(&arms[0].body);
  66             if match_var(body, arg);
  67
  68             then {
  69                 let mut applicability = Applicability::MachineApplicable;
  70                 span_lint_and_sugg(
  71                     cx,
  72                     INFALLIBLE_DESTRUCTURING_MATCH,
  73                     local.span,
  74                     "you seem to be trying to use match to destructure a single infallible pattern. \
  75                      Consider using `let`",
  76                     "try this",
  77                     format!(
  78                         "let {}({}) = {};",
  79                         snippet_with_applicability(cx, variant_name.span, "..", &mut applicability),
  80                         snippet_with_applicability(cx, local.pat.span, "..", &mut applicability),
  81                         snippet_with_applicability(cx, target.span, "..", &mut applicability),
  82                     ),
  83                     applicability,
  84                 );
  85             }
  86         }
  87     }
  88 }