clippy_lints/src/needless_borrowed_ref.rs

   1 use clippy_utils::diagnostics::span_lint_and_then;
   2 use clippy_utils::source::snippet_with_applicability;
   3 use if_chain::if_chain;
   4 use rustc_errors::Applicability;
   5 use rustc_hir::{BindingAnnotation, Mutability, Node, Pat, PatKind};
   6 use rustc_lint::{LateContext, LateLintPass};
   7 use rustc_session::{declare_lint_pass, declare_tool_lint};
   8
   9 declare_clippy_lint! {
  10     /// ### What it does
  11     /// Checks for bindings that destructure a reference and borrow the inner
  12     /// value with `&ref`.
  13     ///
  14     /// ### Why is this bad?
  15     /// This pattern has no effect in almost all cases.
  16     ///
  17     /// ### Known problems
  18     /// In some cases, `&ref` is needed to avoid a lifetime mismatch error.
  19     /// Example:
  20     /// ```rust
  21     /// fn foo(a: &Option<String>, b: &Option<String>) {
  22     ///     match (a, b) {
  23     ///         (None, &ref c) | (&ref c, None) => (),
  24     ///         (&Some(ref c), _) => (),
  25     ///     };
  26     /// }
  27     /// ```
  28     ///
  29     /// ### Example
  30     /// Bad:
  31     /// ```rust
  32     /// let mut v = Vec::<String>::new();
  33     /// let _ = v.iter_mut().filter(|&ref a| a.is_empty());
  34     /// ```
  35     ///
  36     /// Good:
  37     /// ```rust
  38     /// let mut v = Vec::<String>::new();
  39     /// let _ = v.iter_mut().filter(|a| a.is_empty());
  40     /// ```
  41     #[clippy::version = "pre 1.29.0"]
  42     pub NEEDLESS_BORROWED_REFERENCE,
  43     complexity,
  44     "destructuring a reference and borrowing the inner value"
  45 }
  46
  47 declare_lint_pass!(NeedlessBorrowedRef => [NEEDLESS_BORROWED_REFERENCE]);
  48
  49 impl<'tcx> LateLintPass<'tcx> for NeedlessBorrowedRef {
  50     fn check_pat(&mut self, cx: &LateContext<'tcx>, pat: &'tcx Pat<'_>) {
  51         if pat.span.from_expansion() {
  52             // OK, simple enough, lints doesn't check in macro.
  53             return;
  54         }
  55
  56         if_chain! {
  57             // Only lint immutable refs, because `&mut ref T` may be useful.
  58             if let PatKind::Ref(sub_pat, Mutability::Not) = pat.kind;
  59
  60             // Check sub_pat got a `ref` keyword (excluding `ref mut`).
  61             if let PatKind::Binding(BindingAnnotation::Ref, .., spanned_name, _) = sub_pat.kind;
  62             let parent_id = cx.tcx.hir().get_parent_node(pat.hir_id);
  63             if let Some(parent_node) = cx.tcx.hir().find(parent_id);
  64             then {
  65                 // do not recurse within patterns, as they may have other references
  66                 // XXXManishearth we can relax this constraint if we only check patterns
  67                 // with a single ref pattern inside them
  68                 if let Node::Pat(_) = parent_node {
  69                     return;
  70                 }
  71                 let mut applicability = Applicability::MachineApplicable;
  72                 span_lint_and_then(cx, NEEDLESS_BORROWED_REFERENCE, pat.span,
  73                                    "this pattern takes a reference on something that is being de-referenced",
  74                                    |diag| {
  75                                        let hint = snippet_with_applicability(cx, spanned_name.span, "..", &mut applicability).into_owned();
  76                                        diag.span_suggestion(
  77                                            pat.span,
  78                                            "try removing the `&ref` part and just keep",
  79                                            hint,
  80                                            applicability,
  81                                        );
  82                                    });
  83             }
  84         }
  85     }
  86 }