Auto merge of #111916 - fee1-dead-contrib:noop-method-call-warn, r=compiler-errors

make `noop_method_call` warn by default

r? `@compiler-errors`

compiler/rustc_lint/messages.ftl

@@ -410,8 +410,8 @@ lint_non_upper_case_global = {$sort} `{$name}` should have an upper case name
     .label = should have an UPPER_CASE name
 
 lint_noop_method_call = call to `.{$method}()` on a reference in this situation does nothing
-    .label = unnecessary method call
-    .note = the type `{$receiver_ty}` which `{$method}` is being called on is the same as the type returned from `{$method}`, so the method call does not do anything and can be removed
+    .suggestion = remove this redundant call
+    .note = the type `{$orig_ty}` does not implement `{$trait_}`, so calling `{$method}` on `&{$orig_ty}` copies the reference, which does not do anything and can be removed
 
 lint_only_cast_u8_to_char = only `u8` can be cast into `char`
     .suggestion = use a `char` literal instead

compiler/rustc_lint/src/lints.rs

@@ -1231,8 +1231,9 @@ pub enum NonUpperCaseGlobalSub {
 #[note]
 pub struct NoopMethodCallDiag<'a> {
     pub method: Symbol,
-    pub receiver_ty: Ty<'a>,
-    #[label]
+    pub orig_ty: Ty<'a>,
+    pub trait_: Symbol,
+    #[suggestion(code = "", applicability = "machine-applicable")]
     pub label: Span,
 }
 

compiler/rustc_lint/src/noop_method_call.rs

@@ -18,7 +18,6 @@ declare_lint! {
     ///
     /// ```rust
     /// # #![allow(unused)]
-    /// #![warn(noop_method_call)]
     /// struct Foo;
     /// let foo = &Foo;
     /// let clone: &Foo = foo.clone();
@@ -34,7 +33,7 @@ declare_lint! {
     /// 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,
+    Warn,
     "detects the use of well-known noop methods"
 }
 
@@ -86,10 +85,9 @@ impl<'tcx> LateLintPass<'tcx> for NoopMethodCall {
 
         let Some(trait_id) = cx.tcx.trait_of_item(did) else { return };
 
-        if !matches!(
-            cx.tcx.get_diagnostic_name(trait_id),
-            Some(sym::Borrow | sym::Clone | sym::Deref)
-        ) {
+        let Some(trait_) = cx.tcx.get_diagnostic_name(trait_id) else { return };
+
+        if !matches!(trait_, sym::Borrow | sym::Clone | sym::Deref) {
             return;
         };
 
@@ -114,11 +112,13 @@ impl<'tcx> LateLintPass<'tcx> for NoopMethodCall {
         let expr_span = expr.span;
         let span = expr_span.with_lo(receiver.span.hi());
 
+        let orig_ty = expr_ty.peel_refs();
+
         if receiver_ty == expr_ty {
             cx.emit_spanned_lint(
                 NOOP_METHOD_CALL,
                 span,
-                NoopMethodCallDiag { method: call.ident.name, receiver_ty, label: span },
+                NoopMethodCallDiag { method: call.ident.name, orig_ty, trait_, label: span },
             );
         } else {
             match name {

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -41,7 +41,6 @@ use rustc_span::{BytePos, DesugaringKind, ExpnKind, MacroKind, Span, DUMMY_SP};
 use rustc_target::spec::abi;
 use std::borrow::Cow;
 use std::iter;
-use std::ops::Deref;
 
 use super::InferCtxtPrivExt;
 use crate::infer::InferCtxtExt as _;
@@ -3577,7 +3576,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             // to an associated type (as seen from `trait_pred`) in the predicate. Like in
             // trait_pred `S: Sum<<Self as Iterator>::Item>` and predicate `i32: Sum<&()>`
             let mut type_diffs = vec![];
-            if let ObligationCauseCode::ExprBindingObligation(def_id, _, _, idx) = parent_code.deref()
+            if let ObligationCauseCode::ExprBindingObligation(def_id, _, _, idx) = parent_code
                 && let Some(node_args) = typeck_results.node_args_opt(call_hir_id)
                 && let where_clauses = self.tcx.predicates_of(def_id).instantiate(self.tcx, node_args)
                 && let Some(where_pred) = where_clauses.predicates.get(*idx)