[experiment] Deny noop_method_call

compiler/rustc_lint/messages.ftl

@@ -405,7 +405,7 @@ lint_non_upper_case_global = {$sort} `{$name}` 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
+    .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

@@ -1206,7 +1206,8 @@ pub enum NonUpperCaseGlobalSub {
 #[note]
 pub struct NoopMethodCallDiag<'a> {
     pub method: Symbol,
-    pub receiver_ty: Ty<'a>,
+    pub orig_ty: Ty<'a>,
+    pub trait_: Symbol,
     #[label]
     pub label: Span,
 }

compiler/rustc_lint/src/noop_method_call.rs

@@ -16,7 +16,6 @@ declare_lint! {
     ///
     /// ```rust
     /// # #![allow(unused)]
-    /// #![warn(noop_method_call)]
     /// struct Foo;
     /// let foo = &Foo;
     /// let clone: &Foo = foo.clone();
@@ -32,7 +31,7 @@ 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,
+    Deny,
     "detects the use of well-known noop methods"
 }
 
@@ -76,21 +75,16 @@ impl<'tcx> LateLintPass<'tcx> for NoopMethodCall {
 
         // We only care about method calls corresponding to the `Clone`, `Deref` and `Borrow`
         // traits and ignore any other method call.
-        let 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)
-                    ) =>
-                {
-                    did
-                }
-                _ => return,
-            },
-            _ => return,
+        //
+        // Verify we are dealing with a method/associated function, and check that we're dealing
+        // with a trait method for one of the traits we care about.
+        let (did, trait_) = if let Some((DefKind::AssocFn, did)) = cx.typeck_results().type_dependent_def(expr.hir_id)
+            && let Some(trait_id) = cx.tcx.trait_of_item(did)
+            && let Some(trait_ @ (sym::Borrow | sym::Clone | sym::Deref)) = cx.tcx.get_diagnostic_name(trait_id)
+        {
+            (did, trait_)
+        } else {
+            return;
         };
         let substs = cx
             .tcx
@@ -122,11 +116,13 @@ 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 {
             cx.emit_spanned_lint(

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

@@ -40,7 +40,6 @@ 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 _;
@@ -3466,7 +3465,7 @@ 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_substs) = typeck_results.node_substs_opt(call_hir_id)
                 && let where_clauses = self.tcx.predicates_of(def_id).instantiate(self.tcx, node_substs)
                 && let Some(where_pred) = where_clauses.predicates.get(*idx)

library/core/benches/iter.rs

@@ -421,6 +421,7 @@ 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::uninlined_format_args

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::uninlined_format_args

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

@@ -1,73 +1,73 @@
 error: explicit `deref` method call
-  --> $DIR/explicit_deref_methods.rs:36:19
+  --> $DIR/explicit_deref_methods.rs:37:19
    |
 LL |     let b: &str = a.deref();
    |                   ^^^^^^^^^ help: try this: `&*a`
    |
    = note: `-D clippy::explicit-deref-methods` implied by `-D warnings`
 
 error: explicit `deref_mut` method call
-  --> $DIR/explicit_deref_methods.rs:38:23
+  --> $DIR/explicit_deref_methods.rs:39:23
    |
 LL |     let b: &mut str = a.deref_mut();
    |                       ^^^^^^^^^^^^^ help: try this: `&mut **a`
 
 error: explicit `deref` method call
-  --> $DIR/explicit_deref_methods.rs:41:39
+  --> $DIR/explicit_deref_methods.rs:42:39
    |
 LL |     let b: String = format!("{}, {}", a.deref(), a.deref());
    |                                       ^^^^^^^^^ help: try this: `&*a`
 
 error: explicit `deref` method call
-  --> $DIR/explicit_deref_methods.rs:41:50
+  --> $DIR/explicit_deref_methods.rs:42:50
    |
 LL |     let b: String = format!("{}, {}", a.deref(), a.deref());
    |                                                  ^^^^^^^^^ help: try this: `&*a`
 
 error: explicit `deref` method call
-  --> $DIR/explicit_deref_methods.rs:43:20
+  --> $DIR/explicit_deref_methods.rs:44:20
    |
 LL |     println!("{}", a.deref());
    |                    ^^^^^^^^^ help: try this: `&*a`
 
 error: explicit `deref` method call
-  --> $DIR/explicit_deref_methods.rs:46:11
+  --> $DIR/explicit_deref_methods.rs:47:11
    |
 LL |     match a.deref() {
    |           ^^^^^^^^^ help: try this: `&*a`
 
 error: explicit `deref` method call
-  --> $DIR/explicit_deref_methods.rs:50:28
+  --> $DIR/explicit_deref_methods.rs:51:28
    |
 LL |     let b: String = concat(a.deref());
    |                            ^^^^^^^^^ help: try this: `&*a`
 
 error: explicit `deref` method call
-  --> $DIR/explicit_deref_methods.rs:52:13
+  --> $DIR/explicit_deref_methods.rs:53:13
    |
 LL |     let b = just_return(a).deref();
    |             ^^^^^^^^^^^^^^^^^^^^^^ help: try this: `just_return(a)`
 
 error: explicit `deref` method call
-  --> $DIR/explicit_deref_methods.rs:54:28
+  --> $DIR/explicit_deref_methods.rs:55:28
    |
 LL |     let b: String = concat(just_return(a).deref());
    |                            ^^^^^^^^^^^^^^^^^^^^^^ help: try this: `just_return(a)`
 
 error: explicit `deref` method call
-  --> $DIR/explicit_deref_methods.rs:56:19
+  --> $DIR/explicit_deref_methods.rs:57:19
    |
 LL |     let b: &str = a.deref().deref();
    |                   ^^^^^^^^^^^^^^^^^ help: try this: `&**a`
 
 error: explicit `deref` method call
-  --> $DIR/explicit_deref_methods.rs:59:13
+  --> $DIR/explicit_deref_methods.rs:60:13
    |
 LL |     let b = opt_a.unwrap().deref();
    |             ^^^^^^^^^^^^^^^^^^^^^^ help: try this: `&*opt_a.unwrap()`
 
 error: explicit `deref` method call
-  --> $DIR/explicit_deref_methods.rs:85:31
+  --> $DIR/explicit_deref_methods.rs:86:31
    |
 LL |     let b: &str = expr_deref!(a.deref());
    |                               ^^^^^^^^^ help: try this: `&*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.stderr

@@ -4,7 +4,7 @@ warning: call to `.clone()` on a reference in this situation does nothing
 LL |     let non_clone_type_ref_clone: &PlainType<u32> = non_clone_type_ref.clone();
    |                                                                       ^^^^^^^^ unnecessary method call
    |
-   = note: the type `&PlainType<u32>` which `clone` is being called on is the same as the type returned from `clone`, so the method call does not do anything and can be removed
+   = note: the type `PlainType<u32>` does not implement `Clone`, so calling `clone` on `&PlainType<u32>` copies the reference, which does not do anything and can be removed
 note: the lint level is defined here
   --> $DIR/noop-method-call.rs:4:9
    |
@@ -25,7 +25,7 @@ warning: call to `.deref()` on a reference in this situation does nothing
 LL |     let non_deref_type_deref: &PlainType<u32> = non_deref_type.deref();
    |                                                               ^^^^^^^^ unnecessary method call
    |
-   = note: the type `&PlainType<u32>` which `deref` is being called on is the same as the type returned from `deref`, so the method call does not do anything and can be removed
+   = note: the type `PlainType<u32>` does not implement `Deref`, so calling `deref` on `&PlainType<u32>` copies the reference, which does not do anything and can be removed
 
 warning: using `.deref()` on a double reference, which returns `&PlainType<u32>` instead of dereferencing the inner type
   --> $DIR/noop-method-call.rs:31:63
@@ -39,7 +39,7 @@ warning: call to `.borrow()` on a reference in this situation does nothing
 LL |     let non_borrow_type_borrow: &PlainType<u32> = non_borrow_type.borrow();
    |                                                                  ^^^^^^^^^ unnecessary method call
    |
-   = note: the type `&PlainType<u32>` which `borrow` is being called on is the same as the type returned from `borrow`, so the method call does not do anything and can be removed
+   = note: the type `PlainType<u32>` does not implement `Borrow`, so calling `borrow` on `&PlainType<u32>` copies the reference, which does not do anything and can be removed
 
 warning: using `.clone()` on a double reference, which returns `&str` instead of cloning the inner type
   --> $DIR/noop-method-call.rs:43:44
@@ -53,15 +53,15 @@ warning: call to `.clone()` on a reference in this situation does nothing
 LL |     non_clone_type.clone();
    |                   ^^^^^^^^ unnecessary method call
    |
-   = note: the type `&PlainType<T>` which `clone` is being called on is the same as the type returned from `clone`, so the method call does not do anything and can be removed
+   = note: the type `PlainType<T>` does not implement `Clone`, so calling `clone` on `&PlainType<T>` copies the reference, which does not do anything and can be removed
 
 warning: call to `.clone()` on a reference in this situation does nothing
   --> $DIR/noop-method-call.rs:53:19
    |
 LL |     non_clone_type.clone();
    |                   ^^^^^^^^ unnecessary method call
    |
-   = note: the type `&PlainType<u32>` which `clone` is being called on is the same as the type returned from `clone`, so the method call does not do anything and can be removed
+   = note: the type `PlainType<u32>` does not implement `Clone`, so calling `clone` on `&PlainType<u32>` copies the reference, which does not do anything and can be removed
 
 warning: 8 warnings emitted
 

tests/ui/lint/suspicious-double-ref-op.stderr

@@ -16,7 +16,7 @@ error: call to `.clone()` on a reference in this situation does nothing
 LL |     let _ = &mut encoded.clone();
    |                         ^^^^^^^^ unnecessary method call
    |
-   = note: the type `&[u8]` which `clone` is being called on is the same as the type returned from `clone`, so the method call does not do anything and can be removed
+   = note: the type `[u8]` does not implement `Clone`, so calling `clone` on `&[u8]` copies the reference, which does not do anything and can be removed
 note: the lint level is defined here
   --> $DIR/suspicious-double-ref-op.rs:2:35
    |
@@ -29,7 +29,7 @@ error: call to `.clone()` on a reference in this situation does nothing
 LL |     let _ = &encoded.clone();
    |                     ^^^^^^^^ unnecessary method call
    |
-   = note: the type `&[u8]` which `clone` is being called on is the same as the type returned from `clone`, so the method call does not do anything and can be removed
+   = note: the type `[u8]` does not implement `Clone`, so calling `clone` on `&[u8]` copies the reference, which does not do anything and can be removed
 
 error: aborting due to 3 previous errors
 

tests/ui/underscore-imports/cycle.rs

@@ -2,6 +2,8 @@
 
 // check-pass
 
+#![allow(noop_method_call)]
+
 mod x {
     pub use crate::y::*;
     pub use std::ops::Deref as _;

tests/ui/underscore-imports/hygiene.rs

@@ -3,6 +3,7 @@
 // check-pass
 
 #![feature(decl_macro)]
+#![allow(noop_method_call)]
 
 mod x {
     pub use std::ops::Deref as _;