Rollup merge of #137913 - compiler-errors:struct-field-default-generic, r=BoxyUwU

Allow struct field default values to reference struct's generics

Right now, the default field value feature (https://github.com/rust-lang/rust/issues/132162) lowers anon consts whose types may reference ADT params that the const doesn't inherit.

This PR fixes this, so that these defaults can reference ADTs' generics, and sets the `generics_of` parenting up correctly.

There doesn't seem to be a good reason not to support this, since the anon const has a well-defined type from the field, and the anon const doesn't interact with the type system like generic parameter defaults do.

r? `````@boxyuwu````` or reassign

I could also make this into an error if this seems problematic (https://github.com/rust-lang/rust/compare/master...compiler-errors:rust:default-field-value-implicit-param?expand=1)...... but I'd rather make this work and register an open question on the tracking issue about validating that this is well-vetted.

Fixes #137896

1 files changed, 3 insertions, 1 deletions

diff --git a/compiler/rustc_resolve/src/late.rs b/compiler/rustc_resolve/src/late.rs
index 1e86b9721d4..29b606b7a59 100644
--- a/compiler/rustc_resolve/src/late.rs
+++ b/compiler/rustc_resolve/src/late.rs
@@ -78,6 +78,7 @@ struct IsNeverPattern;
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 enum AnonConstKind {
     EnumDiscriminant,
+    FieldDefaultValue,
     InlineConst,
     ConstArg(IsRepeatExpr),
 }
@@ -1406,7 +1407,7 @@ impl<'ra: 'ast, 'ast, 'tcx> Visitor<'ast> for LateResolutionVisitor<'_, 'ast, 'r
         visit_opt!(self, visit_ident, ident);
         try_visit!(self.visit_ty(ty));
         if let Some(v) = &default {
-            self.resolve_anon_const(v, AnonConstKind::ConstArg(IsRepeatExpr::No));
+            self.resolve_anon_const(v, AnonConstKind::FieldDefaultValue);
         }
     }
 }
@@ -4659,6 +4660,7 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
             AnonConstKind::EnumDiscriminant => {
                 ConstantHasGenerics::No(NoConstantGenericsReason::IsEnumDiscriminant)
             }
+            AnonConstKind::FieldDefaultValue => ConstantHasGenerics::Yes,
             AnonConstKind::InlineConst => ConstantHasGenerics::Yes,
             AnonConstKind::ConstArg(_) => {
                 if self.r.tcx.features().generic_const_exprs() || is_trivial_const_arg {