make invalid_type_param_default lint show up in cargo future-compat reports

and remove the feature gate that silenced the lint

Author: RalfJung

compiler/rustc_feature/src/removed.rs

@@ -75,6 +75,9 @@ declare_features! (
     /// Allows the use of `#[derive(Anything)]` as sugar for `#[derive_Anything]`.
     (removed, custom_derive, "1.32.0", Some(29644),
      Some("subsumed by `#[proc_macro_derive]`")),
+    /// Allows default type parameters to influence type inference.
+    (removed, default_type_parameter_fallback, "CURRENT_RUSTC_VERSION", Some(27336),
+     Some("never properly implemented; requires significant design work")),
     /// Allows using `#[doc(keyword = "...")]`.
     (removed, doc_keyword, "1.28.0", Some(51315),
      Some("merged into `#![feature(rustdoc_internals)]`")),

compiler/rustc_feature/src/unstable.rs

@@ -428,8 +428,6 @@ declare_features! (
     (unstable, custom_test_frameworks, "1.30.0", Some(50297)),
     /// Allows declarative macros 2.0 (`macro`).
     (unstable, decl_macro, "1.17.0", Some(39412)),
-    /// Allows default type parameters to influence type inference.
-    (unstable, default_type_parameter_fallback, "1.3.0", Some(27336)),
     /// Allows using `#[deprecated_safe]` to deprecate the safeness of a function or trait
     (unstable, deprecated_safe, "1.61.0", Some(94978)),
     /// Allows having using `suggestion` in the `#[deprecated]` attribute.

compiler/rustc_hir_analysis/src/collect/generics_of.rs

@@ -323,8 +323,6 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
             if default.is_some() {
                 match allow_defaults {
                     Defaults::Allowed => {}
-                    Defaults::FutureCompatDisallowed
-                        if tcx.features().default_type_parameter_fallback => {}
                     Defaults::FutureCompatDisallowed => {
                         tcx.node_span_lint(
                             lint::builtin::INVALID_TYPE_PARAM_DEFAULT,

compiler/rustc_lint_defs/src/builtin.rs

@@ -1241,7 +1241,7 @@ declare_lint! {
     Deny,
     "type parameter default erroneously allowed in invalid location",
     @future_incompatible = FutureIncompatibleInfo {
-        reason: FutureIncompatibilityReason::FutureReleaseErrorDontReportInDeps,
+        reason: FutureIncompatibilityReason::FutureReleaseErrorReportInDeps,
         reference: "issue #36887 <https://github.com/rust-lang/rust/issues/36887>",
     };
 }

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

@@ -0,0 +1,43 @@
+error: defaults for type parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions
+  --> $DIR/default_type_parameter_in_fn_or_impl.rs:3:8
+   |
+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/default_type_parameter_in_fn_or_impl.rs:8:6
+   |
+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
+
+Future incompatibility report: Future breakage diagnostic:
+error: defaults for type parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions
+  --> $DIR/default_type_parameter_in_fn_or_impl.rs:3:8
+   |
+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
+
+Future breakage diagnostic:
+error: defaults for type parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions
+  --> $DIR/default_type_parameter_in_fn_or_impl.rs:8:6
+   |
+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>
+   = note: `#[deny(invalid_type_param_default)]` on by default
+