clippy_lints/src/needless_borrowed_ref.rs

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