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