1 use crate::context::LintContext;
2 use crate::rustc_middle::ty::TypeVisitable;
3 use crate::LateContext;
4 use crate::LateLintPass;
5 use rustc_errors::fluent;
6 use rustc_hir::def::DefKind;
7 use rustc_hir::{Expr, ExprKind};
9 use rustc_span::symbol::sym;
12 /// The `noop_method_call` lint detects specific calls to noop methods
13 /// such as a calling `<&T as Clone>::clone` where `T: !Clone`.
18 /// # #![allow(unused)]
19 /// #![warn(noop_method_call)]
22 /// let clone: &Foo = foo.clone();
29 /// Some method calls are noops meaning that they do nothing. Usually such methods
30 /// are the result of blanket implementations that happen to create some method invocations
31 /// that end up not doing anything. For instance, `Clone` is implemented on all `&T`, but
32 /// calling `clone` on a `&T` where `T` does not implement clone, actually doesn't do anything
33 /// as references are copy. This lint detects these calls and warns the user about them.
36 "detects the use of well-known noop methods"
39 declare_lint_pass!(NoopMethodCall => [NOOP_METHOD_CALL]);
41 impl<'tcx> LateLintPass<'tcx> for NoopMethodCall {
42 fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
43 // We only care about method calls.
44 let ExprKind::MethodCall(call, receiver, ..) = &expr.kind else {
47 // We only care about method calls corresponding to the `Clone`, `Deref` and `Borrow`
48 // traits and ignore any other method call.
49 let (trait_id, did) = match cx.typeck_results().type_dependent_def(expr.hir_id) {
50 // Verify we are dealing with a method/associated function.
51 Some((DefKind::AssocFn, did)) => match cx.tcx.trait_of_item(did) {
52 // Check that we're dealing with a trait method for one of the traits we care about.
55 cx.tcx.get_diagnostic_name(trait_id),
56 Some(sym::Borrow | sym::Clone | sym::Deref)
65 let substs = cx.typeck_results().node_substs(expr.hir_id);
66 if substs.needs_subst() {
67 // We can't resolve on types that require monomorphization, so we don't handle them if
68 // we need to perform substitution.
71 let param_env = cx.tcx.param_env(trait_id);
72 // Resolve the trait method instance.
73 let Ok(Some(i)) = ty::Instance::resolve(cx.tcx, param_env, did, substs) else {
76 // (Re)check that it implements the noop diagnostic.
77 let Some(name) = cx.tcx.get_diagnostic_name(i.def_id()) else { return };
80 sym::noop_method_borrow | sym::noop_method_clone | sym::noop_method_deref
84 let receiver_ty = cx.typeck_results().expr_ty(receiver);
85 let expr_ty = cx.typeck_results().expr_ty_adjusted(expr);
86 if receiver_ty != expr_ty {
87 // This lint will only trigger if the receiver type and resulting expression \
88 // type are the same, implying that the method call is unnecessary.
91 let expr_span = expr.span;
92 let span = expr_span.with_lo(receiver.span.hi());
93 cx.struct_span_lint(NOOP_METHOD_CALL, span, fluent::lint::noop_method_call, |lint| {
94 lint.set_arg("method", call.ident.name)
95 .set_arg("receiver_ty", receiver_ty)
96 .span_label(span, fluent::lint::label)
97 .note(fluent::lint::note)