Rollup merge of #127655 - RalfJung:invalid_type_param_default, r=compiler-errors

turn `invalid_type_param_default` into a `FutureReleaseErrorReportInDeps`

`````@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.

However, turns out that outright removing it right now would lead to [tons of crater regressions](#127655 (comment)), so for now this PR just makes this future-compat lint show up in cargo's reports, so people are warned when they use a dependency that is affected by this.

Fixes #27336 by removing the feature gate (so there's no way to silence the lint even on nightly)
CC #36887

Author: matthiaskrgr

compiler/rustc_feature/src/removed.rs

@@ -82,6 +82,9 @@ 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

@@ -431,8 +431,6 @@ 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

@@ -338,8 +338,6 @@ 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

@@ -1267,7 +1267,7 @@ 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

@@ -433,3 +433,25 @@ error: aborting due to 50 previous errors
 
 Some errors have detailed explanations: E0053, E0118, E0283, E0562, E0599, E0658, E0666.
 For more information about an error, try `rustc --explain E0053`.
+Future incompatibility report: Future breakage diagnostic:
+error: defaults for type parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions
+  --> $DIR/where-allowed.rs:239:7
+   |
+LL | impl <T = impl Debug> 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/where-allowed.rs:246:36
+   |
+LL | fn in_method_generic_param_default<T = impl Debug>(_: 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
+

tests/ui/impl-trait/where-allowed.stderr

@@ -1,6 +1,6 @@
-#![feature(default_type_parameter_fallback)]
-
 fn avg<T=T::Item>(_: T) {}
 //~^ ERROR generic parameters with a default cannot use forward declared identifiers
+//~| ERROR defaults for type parameters
+//~| WARN previously accepted
 
 fn main() {}

tests/ui/issues/issue-26812.rs

@@ -1,9 +1,30 @@
 error[E0128]: generic parameters with a default cannot use forward declared identifiers
-  --> $DIR/issue-26812.rs:3:10
+  --> $DIR/issue-26812.rs:1:10
    |
 LL | fn avg<T=T::Item>(_: T) {}
    |          ^^^^^^^ defaulted generic parameters cannot be forward declared
 
-error: aborting due to 1 previous error
+error: defaults for type parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions
+  --> $DIR/issue-26812.rs:1:8
+   |
+LL | fn avg<T=T::Item>(_: 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: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0128`.
+Future incompatibility report: Future breakage diagnostic:
+error: defaults for type parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions
+  --> $DIR/issue-26812.rs:1:8
+   |
+LL | fn avg<T=T::Item>(_: 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
+

tests/ui/issues/issue-26812.stderr

@@ -72,3 +72,14 @@ error: aborting due to 8 previous errors
 
 Some errors have detailed explanations: E0106, E0214, E0308, E0770.
 For more information about an error, try `rustc --explain E0106`.
+Future incompatibility report: Future breakage diagnostic:
+error: defaults for type parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions
+  --> $DIR/unusual-rib-combinations.rs:15:6
+   |
+LL | fn c<T = u8()>() {}
+   |      ^^^^^^^^
+   |
+   = 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
+

tests/ui/lifetimes/unusual-rib-combinations.stderr

@@ -12,3 +12,13 @@ LL |     foo::<T, U>();
 error: aborting due to 1 previous error
 
 For more information about this error, try `rustc --explain E0282`.
+Future incompatibility report: Future breakage diagnostic:
+warning: defaults for type parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions
+  --> $DIR/unbounded-type-param-in-fn-with-assoc-type.rs:3:11
+   |
+LL | fn foo<T, U = u64>() -> (T, U) {
+   |           ^^^^^^^
+   |
+   = 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>
+