compiler/rustc_error_codes/src/error_codes/E0203.md

@@ -1,8 +1,10 @@
+#### Note: this error code is no longer emitted by the compiler.
+
 Having multiple relaxed default bounds is unsupported.
 
-Erroneous code example:
+Previously erroneous code example:
 
-```compile_fail,E0203
+```compile_fail
 struct Bad<T: ?Sized + ?Send>{
     inner: T
 }

compiler/rustc_error_codes/src/lib.rs

@@ -575,6 +575,7 @@ E0797: 0797,
 //  E0190, // deprecated: can only cast a &-pointer to an &-object
 //  E0194, // merged into E0403
 //  E0196, // cannot determine a type for this closure
+//  E0203  // replaced with a `allow_maybe_polarity` feature gate
 //  E0209, // builtin traits can only be implemented on structs or enums
 //  E0213, // associated types are not accepted in this context
 //  E0215, // angle-bracket notation is not stable with `Fn`

compiler/rustc_hir_analysis/src/astconv/bounds.rs

@@ -4,8 +4,9 @@ use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::ty::{self as ty, Ty};
+use rustc_session::parse::feature_err;
 use rustc_span::symbol::Ident;
-use rustc_span::{ErrorGuaranteed, Span};
+use rustc_span::{sym, ErrorGuaranteed, Span};
 use rustc_trait_selection::traits;
 use smallvec::SmallVec;
 
@@ -67,9 +68,13 @@ impl<'tcx> dyn AstConv<'tcx> + '_ {
         }
 
         if unbounds.len() > 1 && !tcx.features().allow_maybe_polarity {
-            tcx.dcx().emit_err(errors::MultipleRelaxedDefaultBounds {
-                spans: unbounds.iter().map(|ptr| ptr.span).collect(),
-            });
+            feature_err(
+                tcx.sess,
+                sym::allow_maybe_polarity,
+                unbounds.iter().map(|ptr| ptr.span).collect::<Vec<Span>>(),
+                "type parameter has more than one relaxed default bound, only one is supported",
+            )
+            .emit();
         }
 
         let mut seen_sized_unbound = false;

tests/ui/traits/maybe-polarity-pass.rs

@@ -1,8 +1,8 @@
 //@ check-pass
 
+#![feature(allow_maybe_polarity)]
 #![feature(auto_traits)]
 #![feature(negative_impls)]
-#![feature(allow_maybe_polarity)]
 
 trait Trait1 {}
 auto trait Trait2 {}

tests/ui/unsized/maybe-bounds-where.stderr

@@ -34,11 +34,14 @@ warning: relaxing a default bound only does something for `?Sized`; all other tr
 LL | struct S4<T>(T) where for<'a> T: ?Trait<'a>;
    |                                  ^^^^^^^^^^
 
-error[E0203]: type parameter has more than one relaxed default bound, only one is supported
+error[E0658]: type parameter has more than one relaxed default bound, only one is supported
   --> $DIR/maybe-bounds-where.rs:16:33
    |
 LL | struct S5<T>(*const T) where T: ?Trait<'static> + ?Sized;
    |                                 ^^^^^^^^^^^^^^^   ^^^^^^
+   |
+   = help: add `#![feature(allow_maybe_polarity)]` 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: relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
   --> $DIR/maybe-bounds-where.rs:16:33
@@ -48,4 +51,4 @@ LL | struct S5<T>(*const T) where T: ?Trait<'static> + ?Sized;
 
 error: aborting due to 6 previous errors; 2 warnings emitted
 
-For more information about this error, try `rustc --explain E0203`.
+For more information about this error, try `rustc --explain E0658`.