rust-clippy/clippy_lints/src/needless_borrow.rs
2020-06-01 10:39:52 +02:00

128 lines
4.6 KiB
Rust

//! Checks for needless address of operations (`&`)
//!
//! This lint is **warn** by default
use crate::utils::{snippet_opt, span_lint_and_then};
use if_chain::if_chain;
use rustc_errors::Applicability;
use rustc_hir::{BindingAnnotation, BorrowKind, Expr, ExprKind, HirId, Item, Mutability, Pat, PatKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty;
use rustc_middle::ty::adjustment::{Adjust, Adjustment};
use rustc_session::{declare_tool_lint, impl_lint_pass};
declare_clippy_lint! {
/// **What it does:** Checks for address of operations (`&`) that are going to
/// be dereferenced immediately by the compiler.
///
/// **Why is this bad?** Suggests that the receiver of the expression borrows
/// the expression.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// // Bad
/// let x: &i32 = &&&&&&5;
///
/// // Good
/// let x: &i32 = &5;
/// ```
pub NEEDLESS_BORROW,
nursery,
"taking a reference that is going to be automatically dereferenced"
}
#[derive(Default)]
pub struct NeedlessBorrow {
derived_item: Option<HirId>,
}
impl_lint_pass!(NeedlessBorrow => [NEEDLESS_BORROW]);
impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessBorrow {
fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr<'_>) {
if e.span.from_expansion() || self.derived_item.is_some() {
return;
}
if let ExprKind::AddrOf(BorrowKind::Ref, Mutability::Not, ref inner) = e.kind {
if let ty::Ref(..) = cx.tables.expr_ty(inner).kind {
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_and_then(
cx,
NEEDLESS_BORROW,
e.span,
"this expression borrows a reference that is immediately dereferenced \
by the compiler",
|diag| {
if let Some(snippet) = snippet_opt(cx, inner.span) {
diag.span_suggestion(
e.span,
"change this to",
snippet,
Applicability::MachineApplicable,
);
}
},
);
}
}
}
}
}
fn check_pat(&mut self, cx: &LateContext<'a, 'tcx>, pat: &'tcx Pat<'_>) {
if pat.span.from_expansion() || self.derived_item.is_some() {
return;
}
if_chain! {
if let PatKind::Binding(BindingAnnotation::Ref, .., name, _) = pat.kind;
if let ty::Ref(_, tam, mutbl) = cx.tables.pat_ty(pat).kind;
if mutbl == Mutability::Not;
if let ty::Ref(_, _, mutbl) = tam.kind;
// only lint immutable refs, because borrowed `&mut T` cannot be moved out
if mutbl == Mutability::Not;
then {
span_lint_and_then(
cx,
NEEDLESS_BORROW,
pat.span,
"this pattern creates a reference to a reference",
|diag| {
if let Some(snippet) = snippet_opt(cx, name.span) {
diag.span_suggestion(
pat.span,
"change this to",
snippet,
Applicability::MachineApplicable,
);
}
}
)
}
}
}
fn check_item(&mut self, _: &LateContext<'a, 'tcx>, item: &'tcx Item<'_>) {
if item.attrs.iter().any(|a| a.check_name(sym!(automatically_derived))) {
debug_assert!(self.derived_item.is_none());
self.derived_item = Some(item.hir_id);
}
}
fn check_item_post(&mut self, _: &LateContext<'a, 'tcx>, item: &'tcx Item<'_>) {
if let Some(id) = self.derived_item {
if item.hir_id == id {
self.derived_item = None;
}
}
}
}