1 files changed, 103 insertions, 0 deletions
diff --git a/compiler/rustc_lint/src/noop_method_call.rs b/compiler/rustc_lint/src/noop_method_call.rs
new file mode 100644
index 000000000..11a752ff0
--- /dev/null
+++ b/compiler/rustc_lint/src/noop_method_call.rs
@@ -0,0 +1,103 @@
+use crate::context::LintContext;
+use crate::rustc_middle::ty::TypeVisitable;
+use crate::LateContext;
+use crate::LateLintPass;
+use rustc_errors::fluent;
+use rustc_hir::def::DefKind;
+use rustc_hir::{Expr, ExprKind};
+use rustc_middle::ty;
+use rustc_span::symbol::sym;
+
+declare_lint! {
+    /// The `noop_method_call` lint detects specific calls to noop methods
+    /// such as a calling `<&T as Clone>::clone` where `T: !Clone`.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// # #![allow(unused)]
+    /// #![warn(noop_method_call)]
+    /// struct Foo;
+    /// let foo = &Foo;
+    /// let clone: &Foo = foo.clone();
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Some method calls are noops meaning that they do nothing. Usually such methods
+    /// are the result of blanket implementations that happen to create some method invocations
+    /// that end up not doing anything. For instance, `Clone` is implemented on all `&T`, but
+    /// calling `clone` on a `&T` where `T` does not implement clone, actually doesn't do anything
+    /// as references are copy. This lint detects these calls and warns the user about them.
+    pub NOOP_METHOD_CALL,
+    Allow,
+    "detects the use of well-known noop methods"
+}
+
+declare_lint_pass!(NoopMethodCall => [NOOP_METHOD_CALL]);
+
+impl<'tcx> LateLintPass<'tcx> for NoopMethodCall {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+        // We only care about method calls.
+        let ExprKind::MethodCall(call, elements, _) = &expr.kind else {
+            return
+        };
+        // We only care about method calls corresponding to the `Clone`, `Deref` and `Borrow`
+        // traits and ignore any other method call.
+        let (trait_id, did) = match cx.typeck_results().type_dependent_def(expr.hir_id) {
+            // Verify we are dealing with a method/associated function.
+            Some((DefKind::AssocFn, did)) => match cx.tcx.trait_of_item(did) {
+                // Check that we're dealing with a trait method for one of the traits we care about.
+                Some(trait_id)
+                    if matches!(
+                        cx.tcx.get_diagnostic_name(trait_id),
+                        Some(sym::Borrow | sym::Clone | sym::Deref)
+                    ) =>
+                {
+                    (trait_id, did)
+                }
+                _ => return,
+            },
+            _ => return,
+        };
+        let substs = cx.typeck_results().node_substs(expr.hir_id);
+        if substs.needs_subst() {
+            // We can't resolve on types that require monomorphization, so we don't handle them if
+            // we need to perform substitution.
+            return;
+        }
+        let param_env = cx.tcx.param_env(trait_id);
+        // Resolve the trait method instance.
+        let Ok(Some(i)) = ty::Instance::resolve(cx.tcx, param_env, did, substs) else {
+            return
+        };
+        // (Re)check that it implements the noop diagnostic.
+        let Some(name) = cx.tcx.get_diagnostic_name(i.def_id()) else { return };
+        if !matches!(
+            name,
+            sym::noop_method_borrow | sym::noop_method_clone | sym::noop_method_deref
+        ) {
+            return;
+        }
+        let receiver = &elements[0];
+        let receiver_ty = cx.typeck_results().expr_ty(receiver);
+        let expr_ty = cx.typeck_results().expr_ty_adjusted(expr);
+        if receiver_ty != expr_ty {
+            // This lint will only trigger if the receiver type and resulting expression \
+            // type are the same, implying that the method call is unnecessary.
+            return;
+        }
+        let expr_span = expr.span;
+        let span = expr_span.with_lo(receiver.span.hi());
+        cx.struct_span_lint(NOOP_METHOD_CALL, span, |lint| {
+            lint.build(fluent::lint::noop_method_call)
+                .set_arg("method", call.ident.name)
+                .set_arg("receiver_ty", receiver_ty)
+                .span_label(span, fluent::lint::label)
+                .note(fluent::lint::note)
+                .emit();
+        });
+    }
+}