Rollup merge of rust-lang#108884 - compiler-errors:tweak-illegal-copy-impl-message, r=WaffleLapkin

Tweak illegal `Copy` impl message

The phrase "may not" can both mean "is not able to" and "possibly does not". Disambiguate this by just using "cannot".
`@Lokathor` expressed being annoyed by this [here](https://twitter.com/Lokathor/status/1633200313544089602?s=20).

Also drive-by fix for this extremely noisy message: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=6a37275bc810f7846bfe191845b7d11d.

r? diagnostics

Author: GuillaumeGomez

compiler/rustc_hir_analysis/locales/en-US.ftl

@@ -89,14 +89,14 @@ hir_analysis_missing_type_params =
     .note = because of the default `Self` reference, type parameters must be specified on object types
 
 hir_analysis_copy_impl_on_type_with_dtor =
-    the trait `Copy` may not be implemented for this type; the type has a destructor
+    the trait `Copy` cannot be implemented for this type; the type has a destructor
     .label = `Copy` not allowed on types with destructors
 
 hir_analysis_multiple_relaxed_default_bounds =
     type parameter has more than one relaxed default bound, only one is supported
 
 hir_analysis_copy_impl_on_non_adt =
-    the trait `Copy` may not be implemented for this type
+    the trait `Copy` cannot be implemented for this type
     .label = type is not a structure or enumeration
 
 hir_analysis_const_impl_for_non_const_trait =

compiler/rustc_hir_analysis/src/coherence/builtin.rs

@@ -2,6 +2,7 @@
 //! up data structures required by type-checking/codegen.
 
 use crate::errors::{CopyImplOnNonAdt, CopyImplOnTypeWithDtor, DropImplOnWrongItem};
+use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::{struct_span_err, MultiSpan};
 use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
@@ -86,15 +87,22 @@ fn visit_implementation_of_copy(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
                 tcx.sess,
                 span,
                 E0204,
-                "the trait `Copy` may not be implemented for this type"
+                "the trait `Copy` cannot be implemented for this type"
             );
 
             // We'll try to suggest constraining type parameters to fulfill the requirements of
             // their `Copy` implementation.
             let mut errors: BTreeMap<_, Vec<_>> = Default::default();
             let mut bounds = vec![];
 
+            let mut seen_tys = FxHashSet::default();
+
             for (field, ty, reason) in fields {
+                // Only report an error once per type.
+                if !seen_tys.insert(ty) {
+                    continue;
+                }
+
                 let field_span = tcx.def_span(field.did);
                 err.span_label(field_span, "this field does not implement `Copy`");
 

compiler/rustc_hir_typeck/locales/en-US.ftl

@@ -4,14 +4,14 @@ hir_typeck_field_multiply_specified_in_initializer =
     .previous_use_label = first use of `{$ident}`
 
 hir_typeck_copy_impl_on_type_with_dtor =
-    the trait `Copy` may not be implemented for this type; the type has a destructor
+    the trait `Copy` cannot be implemented for this type; the type has a destructor
     .label = `Copy` not allowed on types with destructors
 
 hir_typeck_multiple_relaxed_default_bounds =
     type parameter has more than one relaxed default bound, only one is supported
 
 hir_typeck_copy_impl_on_non_adt =
-    the trait `Copy` may not be implemented for this type
+    the trait `Copy` cannot be implemented for this type
     .label = type is not a structure or enumeration
 
 hir_typeck_trait_object_declared_with_no_traits =

library/core/src/marker.rs

@@ -324,7 +324,7 @@ pub trait StructuralEq {
 /// attempt to derive a `Copy` implementation, we'll get an error:
 ///
 /// ```text
-/// the trait `Copy` may not be implemented for this type; field `points` does not implement `Copy`
+/// the trait `Copy` cannot be implemented for this type; field `points` does not implement `Copy`
 /// ```
 ///
 /// Shared references (`&T`) are also `Copy`, so a type can be `Copy`, even when it holds

tests/ui/coherence/coherence-impls-copy.stderr

@@ -52,19 +52,19 @@ LL | impl Copy for [MyType] {}
    |
    = note: define and implement a trait or new type instead
 
-error[E0206]: the trait `Copy` may not be implemented for this type
+error[E0206]: the trait `Copy` cannot be implemented for this type
   --> $DIR/coherence-impls-copy.rs:21:15
    |
 LL | impl Copy for &'static mut MyType {}
    |               ^^^^^^^^^^^^^^^^^^^ type is not a structure or enumeration
 
-error[E0206]: the trait `Copy` may not be implemented for this type
+error[E0206]: the trait `Copy` cannot be implemented for this type
   --> $DIR/coherence-impls-copy.rs:25:15
    |
 LL | impl Copy for (MyType, MyType) {}
    |               ^^^^^^^^^^^^^^^^ type is not a structure or enumeration
 
-error[E0206]: the trait `Copy` may not be implemented for this type
+error[E0206]: the trait `Copy` cannot be implemented for this type
   --> $DIR/coherence-impls-copy.rs:30:15
    |
 LL | impl Copy for [MyType] {}

tests/ui/coherence/deep-bad-copy-reason.rs

@@ -31,7 +31,7 @@ impl<'tcx, T> Clone for List<'tcx, T> {
 }
 
 impl<'tcx, T> Copy for List<'tcx, T> {}
-//~^ ERROR the trait `Copy` may not be implemented for this type
+//~^ ERROR the trait `Copy` cannot be implemented for this type
 
 fn assert_is_copy<T: Copy>() {}
 

tests/ui/coherence/deep-bad-copy-reason.stderr

@@ -1,4 +1,4 @@
-error[E0204]: the trait `Copy` may not be implemented for this type
+error[E0204]: the trait `Copy` cannot be implemented for this type
   --> $DIR/deep-bad-copy-reason.rs:33:24
    |
 LL | pub struct List<'tcx, T>(Interned<'tcx, ListS<T>>);

tests/ui/error-codes/E0184.stderr

@@ -1,4 +1,4 @@
-error[E0184]: the trait `Copy` may not be implemented for this type; the type has a destructor
+error[E0184]: the trait `Copy` cannot be implemented for this type; the type has a destructor
   --> $DIR/E0184.rs:1:10
    |
 LL | #[derive(Copy)]

tests/ui/error-codes/E0206.rs

@@ -2,7 +2,7 @@
 struct Bar;
 
 impl Copy for &'static mut Bar { }
-//~^ ERROR the trait `Copy` may not be implemented for this type
+//~^ ERROR the trait `Copy` cannot be implemented for this type
 
 fn main() {
 }

tests/ui/error-codes/E0206.stderr

@@ -1,4 +1,4 @@
-error[E0206]: the trait `Copy` may not be implemented for this type
+error[E0206]: the trait `Copy` cannot be implemented for this type
   --> $DIR/E0206.rs:4:15
    |
 LL | impl Copy for &'static mut Bar { }