1 //! Checks for useless borrowed references.
3 //! This lint is **warn** by default
5 use crate::utils::{snippet_with_applicability, span_lint_and_then};
6 use if_chain::if_chain;
7 use rustc_errors::Applicability;
8 use rustc_hir::{BindingAnnotation, Mutability, Node, Pat, PatKind};
9 use rustc_lint::{LateContext, LateLintPass};
10 use rustc_session::{declare_lint_pass, declare_tool_lint};
12 declare_clippy_lint! {
13 /// **What it does:** Checks for useless borrowed references.
15 /// **Why is this bad?** It is mostly useless and make the code look more
19 /// **Known problems:** It seems that the `&ref` pattern is sometimes useful.
20 /// For instance in the following snippet:
27 /// fn foo(a: &Animal, b: &Animal) {
29 /// (&Animal::Cat(v), k) | (k, &Animal::Cat(v)) => (), // lifetime mismatch error
30 /// (&Animal::Dog(ref c), &Animal::Dog(_)) => ()
34 /// There is a lifetime mismatch error for `k` (indeed a and b have distinct
36 /// This can be fixed by using the `&ref` pattern.
37 /// However, the code can also be fixed by much cleaner ways
41 /// let mut v = Vec::<String>::new();
42 /// let _ = v.iter_mut().filter(|&ref a| a.is_empty());
44 /// This closure takes a reference on something that has been matched as a
47 /// As such, it could just be |a| a.is_empty()
48 pub NEEDLESS_BORROWED_REFERENCE,
50 "taking a needless borrowed reference"
53 declare_lint_pass!(NeedlessBorrowedRef => [NEEDLESS_BORROWED_REFERENCE]);
55 impl<'tcx> LateLintPass<'tcx> for NeedlessBorrowedRef {
56 fn check_pat(&mut self, cx: &LateContext<'tcx>, pat: &'tcx Pat<'_>) {
57 if pat.span.from_expansion() {
58 // OK, simple enough, lints doesn't check in macro.
63 // Only lint immutable refs, because `&mut ref T` may be useful.
64 if let PatKind::Ref(ref sub_pat, Mutability::Not) = pat.kind;
66 // Check sub_pat got a `ref` keyword (excluding `ref mut`).
67 if let PatKind::Binding(BindingAnnotation::Ref, .., spanned_name, _) = sub_pat.kind;
68 let parent_id = cx.tcx.hir().get_parent_node(pat.hir_id);
69 if let Some(parent_node) = cx.tcx.hir().find(parent_id);
71 // do not recurse within patterns, as they may have other references
72 // XXXManishearth we can relax this constraint if we only check patterns
73 // with a single ref pattern inside them
74 if let Node::Pat(_) = parent_node {
77 let mut applicability = Applicability::MachineApplicable;
78 span_lint_and_then(cx, NEEDLESS_BORROWED_REFERENCE, pat.span,
79 "this pattern takes a reference on something that is being de-referenced",
81 let hint = snippet_with_applicability(cx, spanned_name.span, "..", &mut applicability).into_owned();
84 "try removing the `&ref` part and just keep",