1 //! Checks for useless borrowed references.
3 //! This lint is **warn** by default
5 use crate::utils::{in_macro, snippet, span_lint_and_then};
6 use if_chain::if_chain;
7 use rustc::hir::{BindingAnnotation, MutImmutable, Pat, PatKind};
8 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
9 use rustc::{declare_tool_lint, lint_array};
10 use rustc_errors::Applicability;
12 /// **What it does:** Checks for useless borrowed references.
14 /// **Why is this bad?** It is mostly useless and make the code look more
18 /// **Known problems:** It seems that the `&ref` pattern is sometimes useful.
19 /// For instance in the following snippet:
26 /// fn foo(a: &Animal, b: &Animal) {
28 /// (&Animal::Cat(v), k) | (k, &Animal::Cat(v)) => (), // lifetime
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 declare_clippy_lint! {
49 pub NEEDLESS_BORROWED_REFERENCE,
51 "taking a needless borrowed reference"
54 #[derive(Copy, Clone)]
55 pub struct NeedlessBorrowedRef;
57 impl LintPass for NeedlessBorrowedRef {
58 fn get_lints(&self) -> LintArray {
59 lint_array!(NEEDLESS_BORROWED_REFERENCE)
63 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessBorrowedRef {
64 fn check_pat(&mut self, cx: &LateContext<'a, 'tcx>, pat: &'tcx Pat) {
65 if in_macro(pat.span) {
66 // OK, simple enough, lints doesn't check in macro.
71 // Only lint immutable refs, because `&mut ref T` may be useful.
72 if let PatKind::Ref(ref sub_pat, MutImmutable) = pat.node;
74 // Check sub_pat got a `ref` keyword (excluding `ref mut`).
75 if let PatKind::Binding(BindingAnnotation::Ref, _, spanned_name, ..) = sub_pat.node;
77 span_lint_and_then(cx, NEEDLESS_BORROWED_REFERENCE, pat.span,
78 "this pattern takes a reference on something that is being de-referenced",
80 let hint = snippet(cx, spanned_name.span, "..").into_owned();
81 db.span_suggestion_with_applicability(
83 "try removing the `&ref` part and just keep",
85 Applicability::MachineApplicable, // snippet