Reword diagnostics about relaxed bounds in invalid contexts

Author: fmease

compiler/rustc_ast_lowering/src/lib.rs

@@ -60,7 +60,7 @@ use rustc_index::{Idx, IndexSlice, IndexVec};
 use rustc_macros::extension;
 use rustc_middle::span_bug;
 use rustc_middle::ty::{ResolverAstLowering, TyCtxt};
-use rustc_session::parse::{add_feature_diagnostics, feature_err};
+use rustc_session::parse::add_feature_diagnostics;
 use rustc_span::symbol::{Ident, Symbol, kw, sym};
 use rustc_span::{DUMMY_SP, DesugaringKind, Span};
 use smallvec::SmallVec;
@@ -2071,7 +2071,12 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         span: Span,
         rbp: RelaxedBoundPolicy<'_>,
     ) {
-        let err = |message| feature_err(&self.tcx.sess, sym::more_maybe_bounds, span, message);
+        // Even though feature `more_maybe_bounds` bypasses the given policy and (currently) enables
+        // relaxed bounds in every conceivable position[^1], we don't want to advertise it to the user
+        // (via a feature gate) since it's super internal. Besides this, it'd be quite distracting.
+        //
+        // [^1]: Strictly speaking, this is incorrect (at the very least for `Sized`) because it's
+        //       no longer fully consistent with default trait elaboration in HIR ty lowering.
 
         match rbp {
             RelaxedBoundPolicy::Allowed => return,
@@ -2093,11 +2098,17 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
 
                 match reason {
                     RelaxedBoundForbiddenReason::TraitObjectTy => {
-                        err("`?Trait` is not permitted in trait object types").emit();
+                        self.dcx().span_err(
+                            span,
+                            "relaxed bounds are not permitted in trait object types",
+                        );
                         return;
                     }
                     RelaxedBoundForbiddenReason::SuperTrait => {
-                        let mut diag = err("`?Trait` is not permitted in supertraits");
+                        let mut diag = self.dcx().struct_span_err(
+                            span,
+                            "relaxed bounds are not permitted in supertrait bounds",
+                        );
                         if let Some(def_id) = trait_ref.trait_def_id()
                             && self.tcx.is_lang_item(def_id, hir::LangItem::Sized)
                         {
@@ -2111,7 +2122,12 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             }
         }
 
-        err("`?Trait` bounds are only permitted at the point where a type parameter is declared")
+        self.dcx()
+            .struct_span_err(span, "this relaxed bound is not permitted here")
+            .with_note(
+                "in this context, relaxed bounds are only allowed on \
+                 type parameters defined by the closest item",
+            )
             .emit();
     }
 

tests/ui/associated-item/missing-associated_item_or_field_def_ids.rs

@@ -2,6 +2,6 @@
 
 fn main() -> dyn Iterator + ?Iterator::advance_by(usize) {
     //~^ ERROR expected trait, found associated function `Iterator::advance_by`
-    //~| ERROR `?Trait` is not permitted in trait object types
+    //~| ERROR relaxed bounds are not permitted in trait object types
     todo!()
 }

tests/ui/associated-item/missing-associated_item_or_field_def_ids.stderr

@@ -4,14 +4,12 @@ error[E0404]: expected trait, found associated function `Iterator::advance_by`
 LL | fn main() -> dyn Iterator + ?Iterator::advance_by(usize) {
    |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^ not a trait
 
-error[E0658]: `?Trait` is not permitted in trait object types
+error[E0658]: relaxed bounds are not permitted in trait object types
   --> $DIR/missing-associated_item_or_field_def_ids.rs:3:29
    |
 LL | fn main() -> dyn Iterator + ?Iterator::advance_by(usize) {
    |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = help: add `#![feature(more_maybe_bounds)]` to the crate attributes to enable
-   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error: aborting due to 2 previous errors
 

tests/ui/associated-types/avoid-getting-associated-items-of-undefined-trait.rs

@@ -7,5 +7,5 @@ trait Tr {
 fn main() {
     let _: dyn Tr + ?Foo<Assoc = ()>;
     //~^ ERROR: cannot find trait `Foo` in this scope
-    //~| ERROR: `?Trait` is not permitted in trait object types
+    //~| ERROR: relaxed bounds are not permitted in trait object types
 }

tests/ui/associated-types/avoid-getting-associated-items-of-undefined-trait.stderr

@@ -4,14 +4,12 @@ error[E0405]: cannot find trait `Foo` in this scope
 LL |     let _: dyn Tr + ?Foo<Assoc = ()>;
    |                      ^^^ not found in this scope
 
-error[E0658]: `?Trait` is not permitted in trait object types
+error[E0658]: relaxed bounds are not permitted in trait object types
   --> $DIR/avoid-getting-associated-items-of-undefined-trait.rs:8:21
    |
 LL |     let _: dyn Tr + ?Foo<Assoc = ()>;
    |                     ^^^^^^^^^^^^^^^^
    |
-   = help: add `#![feature(more_maybe_bounds)]` to the crate attributes to enable
-   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error: aborting due to 2 previous errors
 

tests/ui/feature-gates/feature-gate-more-maybe-bounds.rs

@@ -2,13 +2,11 @@
 
 trait Trait1 {}
 auto trait Trait2 {}
-trait Trait3: ?Trait1 {}
-//~^  ERROR `?Trait` is not permitted in supertraits
-trait Trait4 where Self: ?Trait1 {}
-//~^ ERROR ?Trait` bounds are only permitted at the point where a type parameter is declared
+trait Trait3: ?Trait1 {} //~ ERROR relaxed bounds are not permitted in supertrait bounds
+trait Trait4 where Self: ?Trait1 {} //~ ERROR this relaxed bound is not permitted here
 
 fn foo(_: Box<dyn Trait1 + ?Trait2>) {}
-//~^  ERROR `?Trait` is not permitted in trait object types
+//~^  ERROR relaxed bounds are not permitted in trait object types
 fn bar<T: ?Trait1 + ?Trait2>(_: T) {}
 //~^ ERROR type parameter has more than one relaxed default bound, only one is supported
 //~| ERROR relaxing a default bound only does something for `?Sized`; all other traits are not bound by default

tests/ui/feature-gates/feature-gate-more-maybe-bounds.stderr

@@ -1,65 +1,58 @@
-error[E0658]: `?Trait` is not permitted in supertraits
+error[E0658]: relaxed bounds are not permitted in supertrait bounds
   --> $DIR/feature-gate-more-maybe-bounds.rs:5:15
    |
 LL | trait Trait3: ?Trait1 {}
    |               ^^^^^^^
    |
-   = help: add `#![feature(more_maybe_bounds)]` to the crate attributes to enable
-   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error[E0658]: `?Trait` bounds are only permitted at the point where a type parameter is declared
-  --> $DIR/feature-gate-more-maybe-bounds.rs:7:26
+error[E0658]: this relaxed bound is not permitted here
+  --> $DIR/feature-gate-more-maybe-bounds.rs:6:26
    |
 LL | trait Trait4 where Self: ?Trait1 {}
    |                          ^^^^^^^
    |
-   = help: add `#![feature(more_maybe_bounds)]` to the crate attributes to enable
-   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = note: in this context, relaxed bounds are only allowed on type parameters defined by the closest item
 
-error[E0658]: `?Trait` is not permitted in trait object types
-  --> $DIR/feature-gate-more-maybe-bounds.rs:10:28
+error[E0658]: relaxed bounds are not permitted in trait object types
+  --> $DIR/feature-gate-more-maybe-bounds.rs:8:28
    |
 LL | fn foo(_: Box<dyn Trait1 + ?Trait2>) {}
    |                            ^^^^^^^
    |
-   = help: add `#![feature(more_maybe_bounds)]` to the crate attributes to enable
-   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error[E0203]: type parameter has more than one relaxed default bound, only one is supported
-  --> $DIR/feature-gate-more-maybe-bounds.rs:12:11
+  --> $DIR/feature-gate-more-maybe-bounds.rs:10:11
    |
 LL | fn bar<T: ?Trait1 + ?Trait2>(_: T) {}
    |           ^^^^^^^   ^^^^^^^
    |
-   = help: add `#![feature(more_maybe_bounds)]` to the crate attributes to enable
-   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error: relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
-  --> $DIR/feature-gate-more-maybe-bounds.rs:12:11
+  --> $DIR/feature-gate-more-maybe-bounds.rs:10:11
    |
 LL | fn bar<T: ?Trait1 + ?Trait2>(_: T) {}
    |           ^^^^^^^
 
 error: relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
-  --> $DIR/feature-gate-more-maybe-bounds.rs:12:21
+  --> $DIR/feature-gate-more-maybe-bounds.rs:10:21
    |
 LL | fn bar<T: ?Trait1 + ?Trait2>(_: T) {}
    |                     ^^^^^^^
 
 error[E0203]: type parameter has more than one relaxed default bound, only one is supported
-  --> $DIR/feature-gate-more-maybe-bounds.rs:19:11
+  --> $DIR/feature-gate-more-maybe-bounds.rs:17:11
    |
 LL | fn baz<T: ?Trait + ?Trait>(_ : T) {}
    |           ^^^^^^   ^^^^^^
 
 error: relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
-  --> $DIR/feature-gate-more-maybe-bounds.rs:19:11
+  --> $DIR/feature-gate-more-maybe-bounds.rs:17:11
    |
 LL | fn baz<T: ?Trait + ?Trait>(_ : T) {}
    |           ^^^^^^
 
 error: relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
-  --> $DIR/feature-gate-more-maybe-bounds.rs:19:20
+  --> $DIR/feature-gate-more-maybe-bounds.rs:17:20
    |
 LL | fn baz<T: ?Trait + ?Trait>(_ : T) {}
    |                    ^^^^^^

tests/ui/parser/trait-object-trait-parens.rs

@@ -6,17 +6,17 @@ fn f<T: (Copy) + (?Sized) + (for<'a> Trait<'a>)>() {}
 
 fn main() {
     let _: Box<(Obj) + (?Sized) + (for<'a> Trait<'a>)>;
-    //~^ ERROR `?Trait` is not permitted in trait object types
+    //~^ ERROR relaxed bounds are not permitted in trait object types
     //~| ERROR only auto traits can be used as additional traits
     //~| WARN trait objects without an explicit `dyn` are deprecated
     //~| WARN this is accepted in the current edition
     let _: Box<?Sized + (for<'a> Trait<'a>) + (Obj)>;
-    //~^ ERROR `?Trait` is not permitted in trait object types
+    //~^ ERROR relaxed bounds are not permitted in trait object types
     //~| ERROR only auto traits can be used as additional traits
     //~| WARN trait objects without an explicit `dyn` are deprecated
     //~| WARN this is accepted in the current edition
     let _: Box<for<'a> Trait<'a> + (Obj) + (?Sized)>;
-    //~^ ERROR `?Trait` is not permitted in trait object types
+    //~^ ERROR relaxed bounds are not permitted in trait object types
     //~| ERROR only auto traits can be used as additional traits
     //~| WARN trait objects without an explicit `dyn` are deprecated
     //~| WARN this is accepted in the current edition

tests/ui/parser/trait-object-trait-parens.stderr

@@ -1,29 +1,23 @@
-error[E0658]: `?Trait` is not permitted in trait object types
+error[E0658]: relaxed bounds are not permitted in trait object types
   --> $DIR/trait-object-trait-parens.rs:8:24
    |
 LL |     let _: Box<(Obj) + (?Sized) + (for<'a> Trait<'a>)>;
    |                        ^^^^^^^^
    |
-   = help: add `#![feature(more_maybe_bounds)]` to the crate attributes to enable
-   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error[E0658]: `?Trait` is not permitted in trait object types
+error[E0658]: relaxed bounds are not permitted in trait object types
   --> $DIR/trait-object-trait-parens.rs:13:16
    |
 LL |     let _: Box<?Sized + (for<'a> Trait<'a>) + (Obj)>;
    |                ^^^^^^
    |
-   = help: add `#![feature(more_maybe_bounds)]` to the crate attributes to enable
-   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error[E0658]: `?Trait` is not permitted in trait object types
+error[E0658]: relaxed bounds are not permitted in trait object types
   --> $DIR/trait-object-trait-parens.rs:18:44
    |
 LL |     let _: Box<for<'a> Trait<'a> + (Obj) + (?Sized)>;
    |                                            ^^^^^^^^
    |
-   = help: add `#![feature(more_maybe_bounds)]` to the crate attributes to enable
-   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 warning: trait objects without an explicit `dyn` are deprecated
   --> $DIR/trait-object-trait-parens.rs:8:16

tests/ui/sized-hierarchy/default-supertrait.rs

@@ -6,18 +6,18 @@ use std::marker::{MetaSized, PointeeSized};
 trait Sized_: Sized { }
 
 trait NegSized: ?Sized { }
-//~^ ERROR `?Trait` is not permitted in supertraits
+//~^ ERROR relaxed bounds are not permitted in supertrait bounds
 
 trait MetaSized_: MetaSized { }
 
 trait NegMetaSized: ?MetaSized { }
-//~^ ERROR `?Trait` is not permitted in supertraits
+//~^ ERROR relaxed bounds are not permitted in supertrait bounds
 
 
 trait PointeeSized_: PointeeSized { }
 
 trait NegPointeeSized: ?PointeeSized { }
-//~^ ERROR `?Trait` is not permitted in supertraits
+//~^ ERROR relaxed bounds are not permitted in supertrait bounds
 
 trait Bare {}