make noop_method_call warn by default

compiler/rustc_lint/messages.ftl

@@ -412,8 +412,8 @@ 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 _;
@@ -3580,7 +3579,7 @@ 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)

library/core/benches/iter.rs

@@ -473,6 +473,7 @@ fn bench_next_chunk_copied(b: &mut Bencher) {
 
 /// Exercises the TrustedRandomAccess specialization in ArrayChunks
 #[bench]
+#[allow(noop_method_call)]
 fn bench_next_chunk_trusted_random_access(b: &mut Bencher) {
     let v = vec![1u8; 1024];
 

src/tools/clippy/tests/ui/explicit_deref_methods.fixed

@@ -4,6 +4,7 @@
 #![allow(
     clippy::borrow_deref_ref,
     suspicious_double_ref_op,
+    noop_method_call,
     clippy::explicit_auto_deref,
     clippy::needless_borrow,
     clippy::no_effect,

src/tools/clippy/tests/ui/explicit_deref_methods.rs

@@ -4,6 +4,7 @@
 #![allow(
     clippy::borrow_deref_ref,
     suspicious_double_ref_op,
+    noop_method_call,
     clippy::explicit_auto_deref,
     clippy::needless_borrow,
     clippy::no_effect,

src/tools/clippy/tests/ui/explicit_deref_methods.stderr

@@ -1,73 +1,73 @@
 error: explicit `deref` method call
-  --> $DIR/explicit_deref_methods.rs:54:19
+  --> $DIR/explicit_deref_methods.rs:55:19
    |
 LL |     let b: &str = a.deref();
    |                   ^^^^^^^^^ help: try: `&*a`
    |
    = note: `-D clippy::explicit-deref-methods` implied by `-D warnings`
 
 error: explicit `deref_mut` method call
-  --> $DIR/explicit_deref_methods.rs:56:23
+  --> $DIR/explicit_deref_methods.rs:57:23
    |
 LL |     let b: &mut str = a.deref_mut();
    |                       ^^^^^^^^^^^^^ help: try: `&mut **a`
 
 error: explicit `deref` method call
-  --> $DIR/explicit_deref_methods.rs:59:39
+  --> $DIR/explicit_deref_methods.rs:60:39
    |
 LL |     let b: String = format!("{}, {}", a.deref(), a.deref());
    |                                       ^^^^^^^^^ help: try: `&*a`
 
 error: explicit `deref` method call
-  --> $DIR/explicit_deref_methods.rs:59:50
+  --> $DIR/explicit_deref_methods.rs:60:50
    |
 LL |     let b: String = format!("{}, {}", a.deref(), a.deref());
    |                                                  ^^^^^^^^^ help: try: `&*a`
 
 error: explicit `deref` method call
-  --> $DIR/explicit_deref_methods.rs:61:20
+  --> $DIR/explicit_deref_methods.rs:62:20
    |
 LL |     println!("{}", a.deref());
    |                    ^^^^^^^^^ help: try: `&*a`
 
 error: explicit `deref` method call
-  --> $DIR/explicit_deref_methods.rs:64:11
+  --> $DIR/explicit_deref_methods.rs:65:11
    |
 LL |     match a.deref() {
    |           ^^^^^^^^^ help: try: `&*a`
 
 error: explicit `deref` method call
-  --> $DIR/explicit_deref_methods.rs:68:28
+  --> $DIR/explicit_deref_methods.rs:69:28
    |
 LL |     let b: String = concat(a.deref());
    |                            ^^^^^^^^^ help: try: `&*a`
 
 error: explicit `deref` method call
-  --> $DIR/explicit_deref_methods.rs:70:13
+  --> $DIR/explicit_deref_methods.rs:71:13
    |
 LL |     let b = just_return(a).deref();
    |             ^^^^^^^^^^^^^^^^^^^^^^ help: try: `just_return(a)`
 
 error: explicit `deref` method call
-  --> $DIR/explicit_deref_methods.rs:72:28
+  --> $DIR/explicit_deref_methods.rs:73:28
    |
 LL |     let b: String = concat(just_return(a).deref());
    |                            ^^^^^^^^^^^^^^^^^^^^^^ help: try: `just_return(a)`
 
 error: explicit `deref` method call
-  --> $DIR/explicit_deref_methods.rs:74:19
+  --> $DIR/explicit_deref_methods.rs:75:19
    |
 LL |     let b: &str = a.deref().deref();
    |                   ^^^^^^^^^^^^^^^^^ help: try: `&**a`
 
 error: explicit `deref` method call
-  --> $DIR/explicit_deref_methods.rs:77:13
+  --> $DIR/explicit_deref_methods.rs:78:13
    |
 LL |     let b = opt_a.unwrap().deref();
    |             ^^^^^^^^^^^^^^^^^^^^^^ help: try: `&*opt_a.unwrap()`
 
 error: explicit `deref` method call
-  --> $DIR/explicit_deref_methods.rs:114:31
+  --> $DIR/explicit_deref_methods.rs:115:31
    |
 LL |     let b: &str = expr_deref!(a.deref());
    |                               ^^^^^^^^^ help: try: `&*a`

src/tools/compiletest/src/lib.rs

@@ -1119,7 +1119,7 @@ fn check_overlapping_tests(found_paths: &BTreeSet<PathBuf>) {
     for path in found_paths {
         for ancestor in path.ancestors().skip(1) {
             if found_paths.contains(ancestor) {
-                collisions.push((path, ancestor.clone()));
+                collisions.push((path, ancestor));
             }
         }
     }

tests/ui/issues/issue-11820.rs

@@ -1,6 +1,8 @@
 // run-pass
 // pretty-expanded FIXME #23616
 
+#![allow(noop_method_call)]
+
 struct NoClone;
 
 fn main() {

tests/ui/lint/noop-method-call.fixed

@@ -0,0 +1,51 @@
+// check-pass
+// run-rustfix
+
+#![allow(unused)]
+
+use std::borrow::Borrow;
+use std::ops::Deref;
+
+struct PlainType<T>(T);
+
+#[derive(Clone)]
+struct CloneType<T>(T);
+
+fn check(mut encoded: &[u8]) {
+    let _ = &mut encoded;
+    //~^ WARN call to `.clone()` on a reference in this situation does nothing
+    let _ = &encoded;
+    //~^ WARN call to `.clone()` on a reference in this situation does nothing
+}
+
+fn main() {
+    let non_clone_type_ref = &PlainType(1u32);
+    let non_clone_type_ref_clone: &PlainType<u32> = non_clone_type_ref;
+    //~^ WARN call to `.clone()` on a reference in this situation does nothing
+
+    let clone_type_ref = &CloneType(1u32);
+    let clone_type_ref_clone: CloneType<u32> = clone_type_ref.clone();
+
+
+    let non_deref_type = &PlainType(1u32);
+    let non_deref_type_deref: &PlainType<u32> = non_deref_type;
+    //~^ WARN call to `.deref()` on a reference in this situation does nothing
+
+    let non_borrow_type = &PlainType(1u32);
+    let non_borrow_type_borrow: &PlainType<u32> = non_borrow_type;
+    //~^ WARN call to `.borrow()` on a reference in this situation does nothing
+
+    // Borrowing a &&T does not warn since it has collapsed the double reference
+    let non_borrow_type = &&PlainType(1u32);
+    let non_borrow_type_borrow: &PlainType<u32> = non_borrow_type.borrow();
+}
+
+fn generic<T>(non_clone_type: &PlainType<T>) {
+    non_clone_type;
+    //~^ WARN call to `.clone()` on a reference in this situation does nothing
+}
+
+fn non_generic(non_clone_type: &PlainType<u32>) {
+    non_clone_type;
+    //~^ WARN call to `.clone()` on a reference in this situation does nothing
+}

tests/ui/lint/noop-method-call.rs

@@ -1,7 +1,7 @@
 // check-pass
+// run-rustfix
 
 #![allow(unused)]
-#![warn(noop_method_call)]
 
 use std::borrow::Borrow;
 use std::ops::Deref;
@@ -11,45 +11,41 @@ struct PlainType<T>(T);
 #[derive(Clone)]
 struct CloneType<T>(T);
 
+fn check(mut encoded: &[u8]) {
+    let _ = &mut encoded.clone();
+    //~^ WARN call to `.clone()` on a reference in this situation does nothing
+    let _ = &encoded.clone();
+    //~^ WARN call to `.clone()` on a reference in this situation does nothing
+}
+
 fn main() {
     let non_clone_type_ref = &PlainType(1u32);
     let non_clone_type_ref_clone: &PlainType<u32> = non_clone_type_ref.clone();
-    //~^ WARNING call to `.clone()` on a reference in this situation does nothing
+    //~^ WARN call to `.clone()` on a reference in this situation does nothing
 
     let clone_type_ref = &CloneType(1u32);
     let clone_type_ref_clone: CloneType<u32> = clone_type_ref.clone();
 
-    let clone_type_ref = &&CloneType(1u32);
-    let clone_type_ref_clone: &CloneType<u32> = clone_type_ref.clone();
-    //~^ WARNING using `.clone()` on a double reference, which returns `&CloneType<u32>`
 
     let non_deref_type = &PlainType(1u32);
     let non_deref_type_deref: &PlainType<u32> = non_deref_type.deref();
-    //~^ WARNING call to `.deref()` on a reference in this situation does nothing
-
-    let non_deref_type = &&PlainType(1u32);
-    let non_deref_type_deref: &PlainType<u32> = non_deref_type.deref();
-    //~^ WARNING using `.deref()` on a double reference, which returns `&PlainType<u32>`
+    //~^ WARN call to `.deref()` on a reference in this situation does nothing
 
     let non_borrow_type = &PlainType(1u32);
     let non_borrow_type_borrow: &PlainType<u32> = non_borrow_type.borrow();
-    //~^ WARNING call to `.borrow()` on a reference in this situation does nothing
+    //~^ WARN call to `.borrow()` on a reference in this situation does nothing
 
     // Borrowing a &&T does not warn since it has collapsed the double reference
     let non_borrow_type = &&PlainType(1u32);
     let non_borrow_type_borrow: &PlainType<u32> = non_borrow_type.borrow();
-
-    let xs = ["a", "b", "c"];
-    let _v: Vec<&str> = xs.iter().map(|x| x.clone()).collect(); // could use `*x` instead
-    //~^ WARNING using `.clone()` on a double reference, which returns `&str`
 }
 
 fn generic<T>(non_clone_type: &PlainType<T>) {
     non_clone_type.clone();
-    //~^ WARNING call to `.clone()` on a reference in this situation does nothing
+    //~^ WARN call to `.clone()` on a reference in this situation does nothing
 }
 
 fn non_generic(non_clone_type: &PlainType<u32>) {
     non_clone_type.clone();
-    //~^ WARNING call to `.clone()` on a reference in this situation does nothing
+    //~^ WARN call to `.clone()` on a reference in this situation does nothing
 }