Rollup merge of #157305 - oli-obk:relaxed-bounds-maybe, r=fmease

Eagerly decide whether relaxed bounds are allowed or not

r? @davidtwco or @fmease

Previously introduced in #142693 by @fmease

I am trying to resolve https://github.com/rust-lang/rust/blob/d7986943e496ccb07987f4ad83c6f7033d725861/compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs#L196, but then got into a rabbit hole somehwere.

I think this PR further unblocks resolving that FIXME, because it makes it easier to reason about whether we should be reporting an error about duplicate bounds or about relaxed bounds in general

Author: JonathanBrouwer

compiler/rustc_ast_lowering/src/item.rs

@@ -1992,7 +1992,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         bounds: &[GenericBound],
         colon_span: Option<Span>,
         parent_span: Span,
-        rbp: RelaxedBoundPolicy<'_>,
+        rbp: RelaxedBoundPolicy,
         itctx: ImplTraitContext,
         origin: PredicateOrigin,
     ) -> Option<hir::WherePredicate<'hir>> {
@@ -2067,10 +2067,15 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 bounded_ty,
                 bounds,
             }) => {
-                let rbp = if bound_generic_params.is_empty() {
-                    RelaxedBoundPolicy::AllowedIfOnTyParam(bounded_ty.id, params)
+                let rbp = if bound_generic_params.is_empty()
+                    && let Some(res) =
+                        self.get_partial_res(bounded_ty.id).and_then(|r| r.full_res())
+                    && let Res::Def(DefKind::TyParam, def_id) = res
+                    && params.iter().any(|p| def_id == self.local_def_id(p.id).to_def_id())
+                {
+                    RelaxedBoundPolicy::Allowed
                 } else {
-                    RelaxedBoundPolicy::Forbidden(RelaxedBoundForbiddenReason::LateBoundVarsInScope)
+                    RelaxedBoundPolicy::Forbidden(RelaxedBoundForbiddenReason::WhereBound)
                 };
                 hir::WherePredicateKind::BoundPredicate(hir::WhereBoundPredicate {
                     bound_generic_params: self.lower_generic_params(

compiler/rustc_ast_lowering/src/lib.rs

@@ -313,9 +313,8 @@ impl<'tcx> ResolverAstLowering<'tcx> {
 /// Relaxed bounds should only be allowed in places where we later
 /// (namely during HIR ty lowering) perform *sized elaboration*.
 #[derive(Clone, Copy, Debug)]
-enum RelaxedBoundPolicy<'a> {
+enum RelaxedBoundPolicy {
     Allowed,
-    AllowedIfOnTyParam(NodeId, &'a [ast::GenericParam]),
     Forbidden(RelaxedBoundForbiddenReason),
 }
 
@@ -325,7 +324,9 @@ enum RelaxedBoundForbiddenReason {
     SuperTrait,
     TraitAlias,
     AssocTyBounds,
-    LateBoundVarsInScope,
+    /// We do not allow where bounds doing relaxed bounds,
+    /// except if it's for generic parameters of the current item.
+    WhereBound,
 }
 
 /// Context of `impl Trait` in code, which determines whether it is allowed in an HIR subtree,
@@ -1950,7 +1951,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
     fn lower_param_bound(
         &mut self,
         tpb: &GenericBound,
-        rbp: RelaxedBoundPolicy<'_>,
+        rbp: RelaxedBoundPolicy,
         itctx: ImplTraitContext,
     ) -> hir::GenericBound<'hir> {
         match tpb {
@@ -2188,7 +2189,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
     fn lower_poly_trait_ref(
         &mut self,
         PolyTraitRef { bound_generic_params, modifiers, trait_ref, span, parens: _ }: &PolyTraitRef,
-        rbp: RelaxedBoundPolicy<'_>,
+        rbp: RelaxedBoundPolicy,
         itctx: ImplTraitContext,
     ) -> hir::PolyTraitRef<'hir> {
         let bound_generic_params =
@@ -2212,7 +2213,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         &self,
         trait_ref: hir::TraitRef<'_>,
         span: Span,
-        rbp: RelaxedBoundPolicy<'_>,
+        rbp: RelaxedBoundPolicy,
     ) {
         // Even though feature `more_maybe_bounds` enables the user to relax all default bounds
         // other than `Sized` in a lot more positions (thereby bypassing the given policy), we don't
@@ -2225,14 +2226,6 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
         match rbp {
             RelaxedBoundPolicy::Allowed => return,
-            RelaxedBoundPolicy::AllowedIfOnTyParam(id, params) => {
-                if let Some(res) = self.get_partial_res(id).and_then(|r| r.full_res())
-                    && let Res::Def(DefKind::TyParam, def_id) = res
-                    && params.iter().any(|p| def_id == self.local_def_id(p.id).to_def_id())
-                {
-                    return;
-                }
-            }
             RelaxedBoundPolicy::Forbidden(reason) => {
                 let gate = |context, subject| {
                     let extended = self.tcx.features().more_maybe_bounds();
@@ -2272,7 +2265,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         return;
                     }
                     RelaxedBoundForbiddenReason::AssocTyBounds
-                    | RelaxedBoundForbiddenReason::LateBoundVarsInScope => {}
+                    | RelaxedBoundForbiddenReason::WhereBound => {}
                 };
             }
         }
@@ -2294,7 +2287,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
     fn lower_param_bounds(
         &mut self,
         bounds: &[GenericBound],
-        rbp: RelaxedBoundPolicy<'_>,
+        rbp: RelaxedBoundPolicy,
         itctx: ImplTraitContext,
     ) -> hir::GenericBounds<'hir> {
         self.arena.alloc_from_iter(self.lower_param_bounds_mut(bounds, rbp, itctx))
@@ -2303,7 +2296,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
     fn lower_param_bounds_mut(
         &mut self,
         bounds: &[GenericBound],
-        rbp: RelaxedBoundPolicy<'_>,
+        rbp: RelaxedBoundPolicy,
         itctx: ImplTraitContext,
     ) -> impl Iterator<Item = hir::GenericBound<'hir>> {
         bounds.iter().map(move |bound| self.lower_param_bound(bound, rbp, itctx))