1 //! Checks for needless address of operations (`&`)
3 //! This lint is **warn** by default
5 use clippy_utils::diagnostics::span_lint_and_then;
6 use clippy_utils::source::{snippet_opt, snippet_with_applicability, snippet_with_context};
7 use clippy_utils::{get_parent_expr, in_macro, path_to_local};
8 use if_chain::if_chain;
9 use rustc_ast::util::parser::PREC_POSTFIX;
10 use rustc_data_structures::fx::FxIndexMap;
11 use rustc_errors::Applicability;
12 use rustc_hir::{BindingAnnotation, Body, BodyId, BorrowKind, Expr, ExprKind, HirId, Mutability, Pat, PatKind, UnOp};
13 use rustc_lint::{LateContext, LateLintPass};
15 use rustc_middle::ty::adjustment::{Adjust, Adjustment};
16 use rustc_session::{declare_tool_lint, impl_lint_pass};
19 declare_clippy_lint! {
21 /// Checks for address of operations (`&`) that are going to
22 /// be dereferenced immediately by the compiler.
24 /// ### Why is this bad?
25 /// Suggests that the receiver of the expression borrows
31 /// let x: &i32 = &&&&&&5;
38 "taking a reference that is going to be automatically dereferenced"
41 declare_clippy_lint! {
43 /// Checks for `ref` bindings which create a reference to a reference.
45 /// ### Why is this bad?
46 /// The address-of operator at the use site is clearer about the need for a reference.
52 /// if let Some(ref x) = x {
58 /// if let Some(x) = x {
62 pub REF_BINDING_TO_REFERENCE,
64 "`ref` binding to a reference"
67 impl_lint_pass!(NeedlessBorrow => [NEEDLESS_BORROW, REF_BINDING_TO_REFERENCE]);
69 pub struct NeedlessBorrow {
70 /// The body the first local was found in. Used to emit lints when the traversal of the body has
71 /// been finished. Note we can't lint at the end of every body as they can be nested within each
73 current_body: Option<BodyId>,
74 /// The list of locals currently being checked by the lint.
75 /// If the value is `None`, then the binding has been seen as a ref pattern, but is not linted.
76 /// This is needed for or patterns where one of the branches can be linted, but another can not
79 /// e.g. `m!(x) | Foo::Bar(ref x)`
80 ref_locals: FxIndexMap<HirId, Option<RefPat>>,
84 /// Whether every usage of the binding is dereferenced.
86 /// The spans of all the ref bindings for this local.
88 /// The applicability of this suggestion.
90 /// All the replacements which need to be made.
91 replacements: Vec<(Span, String)>,
94 impl<'tcx> LateLintPass<'tcx> for NeedlessBorrow {
95 fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
96 if let Some(local) = path_to_local(e) {
97 self.check_local_usage(cx, e, local);
100 if e.span.from_expansion() {
103 if let ExprKind::AddrOf(BorrowKind::Ref, Mutability::Not, inner) = e.kind {
104 if let ty::Ref(_, ty, _) = cx.typeck_results().expr_ty(inner).kind() {
105 for adj3 in cx.typeck_results().expr_adjustments(e).windows(3) {
107 kind: Adjust::Deref(_), ..
109 kind: Adjust::Deref(_), ..
111 kind: Adjust::Borrow(_),
120 "this expression borrows a reference (`&{}`) that is immediately dereferenced \
125 if let Some(snippet) = snippet_opt(cx, inner.span) {
126 diag.span_suggestion(
130 Applicability::MachineApplicable,
141 fn check_pat(&mut self, cx: &LateContext<'tcx>, pat: &'tcx Pat<'_>) {
142 if let PatKind::Binding(BindingAnnotation::Ref, id, name, _) = pat.kind {
143 if let Some(opt_prev_pat) = self.ref_locals.get_mut(&id) {
144 // This binding id has been seen before. Add this pattern to the list of changes.
145 if let Some(prev_pat) = opt_prev_pat {
146 if in_macro(pat.span) {
147 // Doesn't match the context of the previous pattern. Can't lint here.
148 *opt_prev_pat = None;
150 prev_pat.spans.push(pat.span);
151 prev_pat.replacements.push((
153 snippet_with_context(cx, name.span, pat.span.ctxt(), "..", &mut prev_pat.app)
163 if !in_macro(pat.span);
164 if let ty::Ref(_, tam, _) = *cx.typeck_results().pat_ty(pat).kind();
165 // only lint immutable refs, because borrowed `&mut T` cannot be moved out
166 if let ty::Ref(_, _, Mutability::Not) = *tam.kind();
168 let mut app = Applicability::MachineApplicable;
169 let snip = snippet_with_context(cx, name.span, pat.span.ctxt(), "..", &mut app).0;
170 self.current_body = self.current_body.or(cx.enclosing_body);
171 self.ref_locals.insert(
175 spans: vec![pat.span],
177 replacements: vec![(pat.span, snip.into())],
185 fn check_body_post(&mut self, cx: &LateContext<'tcx>, body: &'tcx Body<'_>) {
186 if Some(body.id()) == self.current_body {
187 for pat in self.ref_locals.drain(..).filter_map(|(_, x)| x) {
188 let replacements = pat.replacements;
192 if pat.always_deref {
195 REF_BINDING_TO_REFERENCE
198 "this pattern creates a reference to a reference",
200 diag.multipart_suggestion("try this", replacements, app);
204 self.current_body = None;
208 impl NeedlessBorrow {
209 fn check_local_usage(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>, local: HirId) {
210 if let Some(outer_pat) = self.ref_locals.get_mut(&local) {
211 if let Some(pat) = outer_pat {
212 // Check for auto-deref
214 cx.typeck_results().expr_adjustments(e),
217 kind: Adjust::Deref(_),
221 kind: Adjust::Deref(_),
227 match get_parent_expr(cx, e) {
228 // Field accesses are the same no matter the number of references.
230 kind: ExprKind::Field(..),
235 kind: ExprKind::Unary(UnOp::Deref, _),
237 }) if !in_macro(span) => {
238 // Remove explicit deref.
239 let snip = snippet_with_context(cx, e.span, span.ctxt(), "..", &mut pat.app).0;
240 pat.replacements.push((span, snip.into()));
242 Some(parent) if !in_macro(parent.span) => {
243 // Double reference might be needed at this point.
244 if parent.precedence().order() == PREC_POSTFIX {
245 // Parentheses would be needed here, don't lint.
248 pat.always_deref = false;
249 let snip = snippet_with_context(cx, e.span, parent.span.ctxt(), "..", &mut pat.app).0;
250 pat.replacements.push((e.span, format!("&{}", snip)));
253 _ if !in_macro(e.span) => {
254 // Double reference might be needed at this point.
255 pat.always_deref = false;
256 let snip = snippet_with_applicability(cx, e.span, "..", &mut pat.app);
257 pat.replacements.push((e.span, format!("&{}", snip)));
259 // Edge case for macros. The span of the identifier will usually match the context of the
260 // binding, but not if the identifier was created in a macro. e.g. `concat_idents` and proc
262 _ => *outer_pat = None,