1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
//! Checks for useless borrowed references.
//!
//! This lint is **warn** by default
use rustc::lint::*;
use rustc::hir::{MutImmutable, Pat, PatKind, BindingAnnotation};
use utils::{span_lint_and_then, in_macro, snippet};
/// **What it does:** Checks for useless borrowed references.
///
/// **Why is this bad?** It is mostly useless and make the code look more complex than it
/// actually is.
///
/// **Known problems:** It seems that the `&ref` pattern is sometimes useful.
/// For instance in the following snippet:
/// ```rust
/// enum Animal {
/// Cat(u64),
/// Dog(u64),
/// }
///
/// fn foo(a: &Animal, b: &Animal) {
/// match (a, b) {
/// (&Animal::Cat(v), k) | (k, &Animal::Cat(v)) => (), // lifetime mismatch error
/// (&Animal::Dog(ref c), &Animal::Dog(_)) => ()
/// }
/// }
/// ```
/// There is a lifetime mismatch error for `k` (indeed a and b have distinct lifetime).
/// This can be fixed by using the `&ref` pattern.
/// However, the code can also be fixed by much cleaner ways
///
/// **Example:**
/// ```rust
/// let mut v = Vec::<String>::new();
/// let _ = v.iter_mut().filter(|&ref a| a.is_empty());
/// ```
/// This clojure takes a reference on something that has been matched as a
/// reference and
/// de-referenced.
/// As such, it could just be |a| a.is_empty()
declare_lint! {
pub NEEDLESS_BORROWED_REFERENCE,
Warn,
"taking a needless borrowed reference"
}
#[derive(Copy, Clone)]
pub struct NeedlessBorrowedRef;
impl LintPass for NeedlessBorrowedRef {
fn get_lints(&self) -> LintArray {
lint_array!(NEEDLESS_BORROWED_REFERENCE)
}
}
impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessBorrowedRef {
fn check_pat(&mut self, cx: &LateContext<'a, 'tcx>, pat: &'tcx Pat) {
if in_macro(pat.span) {
// OK, simple enough, lints doesn't check in macro.
return;
}
if_let_chain! {[
// Only lint immutable refs, because `&mut ref T` may be useful.
let PatKind::Ref(ref sub_pat, MutImmutable) = pat.node,
// Check sub_pat got a `ref` keyword (excluding `ref mut`).
let PatKind::Binding(BindingAnnotation::Ref, _, spanned_name, ..) = sub_pat.node,
], {
span_lint_and_then(cx, NEEDLESS_BORROWED_REFERENCE, pat.span,
"this pattern takes a reference on something that is being de-referenced",
|db| {
let hint = snippet(cx, spanned_name.span, "..").into_owned();
db.span_suggestion(pat.span, "try removing the `&ref` part and just keep", hint);
});
}}
}
}