84 lines
2.7 KiB
Rust
84 lines
2.7 KiB
Rust
//! Checks for useless borrowed references.
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//!
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//! This lint is **warn** by default
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use rustc::lint::*;
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use rustc::hir::{BindingAnnotation, MutImmutable, Pat, PatKind};
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use utils::{in_macro, snippet, span_lint_and_then};
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/// **What it does:** Checks for useless borrowed references.
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///
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/// **Why is this bad?** It is mostly useless and make the code look more
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/// complex than it
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/// actually is.
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///
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/// **Known problems:** It seems that the `&ref` pattern is sometimes useful.
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/// For instance in the following snippet:
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/// ```rust
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/// enum Animal {
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/// Cat(u64),
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/// Dog(u64),
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/// }
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///
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/// fn foo(a: &Animal, b: &Animal) {
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/// match (a, b) {
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/// (&Animal::Cat(v), k) | (k, &Animal::Cat(v)) => (), // lifetime
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/// mismatch error
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/// (&Animal::Dog(ref c), &Animal::Dog(_)) => ()
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/// }
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/// }
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/// ```
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/// There is a lifetime mismatch error for `k` (indeed a and b have distinct
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/// lifetime).
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/// This can be fixed by using the `&ref` pattern.
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/// However, the code can also be fixed by much cleaner ways
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///
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/// **Example:**
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/// ```rust
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/// let mut v = Vec::<String>::new();
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/// let _ = v.iter_mut().filter(|&ref a| a.is_empty());
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/// ```
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/// This closure takes a reference on something that has been matched as a
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/// reference and
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/// de-referenced.
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/// As such, it could just be |a| a.is_empty()
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declare_lint! {
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pub NEEDLESS_BORROWED_REFERENCE,
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Warn,
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"taking a needless borrowed reference"
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}
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#[derive(Copy, Clone)]
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pub struct NeedlessBorrowedRef;
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impl LintPass for NeedlessBorrowedRef {
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fn get_lints(&self) -> LintArray {
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lint_array!(NEEDLESS_BORROWED_REFERENCE)
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}
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}
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impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessBorrowedRef {
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fn check_pat(&mut self, cx: &LateContext<'a, 'tcx>, pat: &'tcx Pat) {
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if in_macro(pat.span) {
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// OK, simple enough, lints doesn't check in macro.
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return;
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}
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if_chain! {
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// Only lint immutable refs, because `&mut ref T` may be useful.
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if let PatKind::Ref(ref sub_pat, MutImmutable) = pat.node;
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// Check sub_pat got a `ref` keyword (excluding `ref mut`).
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if let PatKind::Binding(BindingAnnotation::Ref, _, spanned_name, ..) = sub_pat.node;
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then {
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span_lint_and_then(cx, NEEDLESS_BORROWED_REFERENCE, pat.span,
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"this pattern takes a reference on something that is being de-referenced",
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|db| {
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let hint = snippet(cx, spanned_name.span, "..").into_owned();
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db.span_suggestion(pat.span, "try removing the `&ref` part and just keep", hint);
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});
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}
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}
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}
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}
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