Auto merge of #127655 - RalfJung:invalid_type_param_default, r=<try>

WIP: make invalid_type_param_default a hard error

Cc #36887
`@rust-lang/types` I assume the plan is still to disallow this? It has been a future-compat lint for a long time, seems ripe to go for hard error. This should be cratered. (But I can also make it `FutureReleaseErrorReportInDeps` first if you prefer.)

compiler/rustc_hir_analysis/src/collect/generics_of.rs

@@ -230,6 +230,7 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
     enum Defaults {
         Allowed,
         // See #36887
+        #[allow(dead_code)] // FIXME remove if the PR moves forward
         FutureCompatDisallowed,
         Deny,
     }
@@ -262,7 +263,7 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
                 | ItemKind::OpaqueTy(..)
                 | ItemKind::Union(..) => (None, Defaults::Allowed),
                 ItemKind::Const(..) => (None, Defaults::Deny),
-                _ => (None, Defaults::FutureCompatDisallowed),
+                _ => (None, Defaults::Deny),
             }
         }
 
@@ -274,7 +275,7 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
             (None, Defaults::Deny)
         }
 
-        _ => (None, Defaults::FutureCompatDisallowed),
+        _ => (None, Defaults::Deny),
     };
 
     let has_self = opt_self.is_some();

tests/ui/feature-gates/feature-gate-default_type_parameter_fallback.rs

@@ -2,11 +2,9 @@
 
 fn avg<T=i32>(_: T) {}
 //~^ ERROR defaults for type parameters are only allowed
-//~| WARN this was previously accepted
 
 struct S<T>(T);
 impl<T=i32> S<T> {}
 //~^ ERROR defaults for type parameters are only allowed
-//~| WARN this was previously accepted
 
 fn main() {}

tests/ui/feature-gates/feature-gate-default_type_parameter_fallback.stderr

@@ -3,19 +3,12 @@ error: defaults for type parameters are only allowed in `struct`, `enum`, `type`
    |
 LL | fn avg<T=i32>(_: T) {}
    |        ^^^^^
-   |
-   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #36887 <https://github.com/rust-lang/rust/issues/36887>
-   = note: `#[deny(invalid_type_param_default)]` on by default
 
 error: defaults for type parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions
-  --> $DIR/feature-gate-default_type_parameter_fallback.rs:8:6
+  --> $DIR/feature-gate-default_type_parameter_fallback.rs:7:6
    |
 LL | impl<T=i32> S<T> {}
    |      ^^^^^
-   |
-   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #36887 <https://github.com/rust-lang/rust/issues/36887>
 
 error: aborting due to 2 previous errors