rust-clippy/clippy_lints/src/needless_borrow.rs

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//! Checks for needless address of operations (`&`)
//!
//! This lint is **warn** by default
use rustc::lint::*;
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use rustc::hir::{ExprAddrOf, Expr, MutImmutable, Pat, PatKind, BindingMode};
use rustc::ty;
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use rustc::ty::adjustment::{Adjustment, Adjust};
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use utils::{span_lint, in_macro};
/// **What it does:** Checks for address of operations (`&`) that are going to
/// be dereferenced immediately by the compiler.
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///
/// **Why is this bad?** Suggests that the receiver of the expression borrows
/// the expression.
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///
/// **Known problems:** None.
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///
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/// **Example:**
/// ```rust
/// let x: &i32 = &&&&&&5;
/// ```
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declare_lint! {
pub NEEDLESS_BORROW,
Warn,
"taking a reference that is going to be automatically dereferenced"
}
#[derive(Copy,Clone)]
pub struct NeedlessBorrow;
impl LintPass for NeedlessBorrow {
fn get_lints(&self) -> LintArray {
lint_array!(NEEDLESS_BORROW)
}
}
impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessBorrow {
fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr) {
if in_macro(e.span) {
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return;
}
if let ExprAddrOf(MutImmutable, ref inner) = e.node {
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if let ty::TyRef(..) = cx.tables.expr_ty(inner).sty {
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for adj3 in cx.tables.expr_adjustments(e).windows(3) {
if let [
Adjustment { kind: Adjust::Deref(_), .. },
Adjustment { kind: Adjust::Deref(_), .. },
Adjustment { kind: Adjust::Borrow(_), .. }
] = *adj3 {
span_lint(cx,
NEEDLESS_BORROW,
e.span,
"this expression borrows a reference that is immediately dereferenced by the \
compiler");
}
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}
}
}
}
fn check_pat(&mut self, cx: &LateContext<'a, 'tcx>, pat: &'tcx Pat) {
if in_macro(pat.span) {
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return;
}
if_let_chain! {[
let PatKind::Binding(BindingMode::BindByRef(MutImmutable), _, _, _) = pat.node,
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let ty::TyRef(_, ref tam) = cx.tables.pat_ty(pat).sty,
tam.mutbl == MutImmutable,
let ty::TyRef(_, ref tam) = tam.ty.sty,
// only lint immutable refs, because borrowed `&mut T` cannot be moved out
tam.mutbl == MutImmutable,
], {
span_lint(cx, NEEDLESS_BORROW, pat.span, "this pattern creates a reference to a reference")
}}
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}
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}