Rollup of 6 pull requests

RELEASES.md

@@ -1,5 +1,5 @@
-Version 1.90 (2025-09-18)
-==========================
+Version 1.90.0 (2025-09-18)
+===========================
 
 <a id="1.90-Language"></a>
 

compiler/rustc_ast_passes/messages.ftl

@@ -64,8 +64,6 @@ ast_passes_body_in_extern = incorrect `{$kind}` inside `extern` block
 
 ast_passes_bound_in_context = bounds on `type`s in {$ctx} have no effect
 
-ast_passes_c_variadic_associated_function = associated functions cannot have a C variable argument list
-
 ast_passes_c_variadic_bad_extern = `...` is not supported for `extern "{$abi}"` functions
     .label = `extern "{$abi}"` because of this
     .help = only `extern "C"` and `extern "C-unwind"` functions may have a C variable argument list

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -696,7 +696,7 @@ impl<'a> AstValidator<'a> {
 
         match fn_ctxt {
             FnCtxt::Foreign => return,
-            FnCtxt::Free => match sig.header.ext {
+            FnCtxt::Free | FnCtxt::Assoc(_) => match sig.header.ext {
                 Extern::Implicit(_) => {
                     if !matches!(sig.header.safety, Safety::Unsafe(_)) {
                         self.dcx().emit_err(errors::CVariadicMustBeUnsafe {
@@ -726,11 +726,6 @@ impl<'a> AstValidator<'a> {
                     self.dcx().emit_err(err);
                 }
             },
-            FnCtxt::Assoc(_) => {
-                // For now, C variable argument lists are unsupported in associated functions.
-                let err = errors::CVariadicAssociatedFunction { span: variadic_param.span };
-                self.dcx().emit_err(err);
-            }
         }
     }
 

compiler/rustc_ast_passes/src/errors.rs

@@ -318,13 +318,6 @@ pub(crate) struct ExternItemAscii {
     pub block: Span,
 }
 
-#[derive(Diagnostic)]
-#[diag(ast_passes_c_variadic_associated_function)]
-pub(crate) struct CVariadicAssociatedFunction {
-    #[primary_span]
-    pub span: Span,
-}
-
 #[derive(Diagnostic)]
 #[diag(ast_passes_c_variadic_no_extern)]
 #[help]

compiler/rustc_borrowck/src/handle_placeholders.rs

@@ -166,13 +166,9 @@ impl RegionTracker {
         }
     }
 
-    /// Determine if the tracked universes of the two SCCs are compatible.
-    pub(crate) fn universe_compatible_with(&self, other: Self) -> bool {
-        // HACK: We first check whether we can name the highest existential universe
-        // of `other`. This only exists to avoid errors in case that scc already
-        // depends on a placeholder it cannot name itself.
-        self.max_nameable_universe().can_name(other.max_nameable_universe())
-            || other.reachable_placeholders.can_be_named_by(self.max_nameable_universe())
+    /// Determine if we can name all the placeholders in `other`.
+    pub(crate) fn can_name_all_placeholders(&self, other: Self) -> bool {
+        other.reachable_placeholders.can_be_named_by(self.max_nameable_universe.0)
     }
 
     /// If this SCC reaches a placeholder it can't name, return it.

compiler/rustc_borrowck/src/region_infer/mod.rs

@@ -571,11 +571,15 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         }
     }
 
-    /// Returns `true` if all the elements in the value of `scc_b` are nameable
+    /// Returns `true` if all the placeholders in the value of `scc_b` are nameable
     /// in `scc_a`. Used during constraint propagation, and only once
     /// the value of `scc_b` has been computed.
-    fn universe_compatible(&self, scc_b: ConstraintSccIndex, scc_a: ConstraintSccIndex) -> bool {
-        self.scc_annotations[scc_a].universe_compatible_with(self.scc_annotations[scc_b])
+    fn can_name_all_placeholders(
+        &self,
+        scc_a: ConstraintSccIndex,
+        scc_b: ConstraintSccIndex,
+    ) -> bool {
+        self.scc_annotations[scc_a].can_name_all_placeholders(self.scc_annotations[scc_b])
     }
 
     /// Once regions have been propagated, this method is used to see
@@ -964,16 +968,22 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             return true;
         }
 
+        let fr_static = self.universal_regions().fr_static;
+
         // If we are checking that `'sup: 'sub`, and `'sub` contains
         // some placeholder that `'sup` cannot name, then this is only
         // true if `'sup` outlives static.
-        if !self.universe_compatible(sub_region_scc, sup_region_scc) {
+        //
+        // Avoid infinite recursion if `sub_region` is already `'static`
+        if sub_region != fr_static
+            && !self.can_name_all_placeholders(sup_region_scc, sub_region_scc)
+        {
             debug!(
                 "sub universe `{sub_region_scc:?}` is not nameable \
                 by super `{sup_region_scc:?}`, promoting to static",
             );
 
-            return self.eval_outlives(sup_region, self.universal_regions().fr_static);
+            return self.eval_outlives(sup_region, fr_static);
         }
 
         // Both the `sub_region` and `sup_region` consist of the union

compiler/rustc_borrowck/src/type_check/canonical.rs

@@ -230,8 +230,14 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
         location: impl NormalizeLocation,
     ) -> Ty<'tcx> {
         let tcx = self.tcx();
+        let body = self.body;
+
+        let cause = ObligationCause::misc(
+            location.to_locations().span(body),
+            body.source.def_id().expect_local(),
+        );
+
         if self.infcx.next_trait_solver() {
-            let body = self.body;
             let param_env = self.infcx.param_env;
             // FIXME: Make this into a real type op?
             self.fully_perform_op(
@@ -241,10 +247,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                     |ocx| {
                         let structurally_normalize = |ty| {
                             ocx.structurally_normalize_ty(
-                                &ObligationCause::misc(
-                                    location.to_locations().span(body),
-                                    body.source.def_id().expect_local(),
-                                ),
+                                &cause,
                                 param_env,
                                 ty,
                             )
@@ -253,6 +256,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
 
                         let tail = tcx.struct_tail_raw(
                             ty,
+                            &cause,
                             structurally_normalize,
                             || {},
                         );
@@ -265,7 +269,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             .unwrap_or_else(|guar| Ty::new_error(tcx, guar))
         } else {
             let mut normalize = |ty| self.normalize(ty, location);
-            let tail = tcx.struct_tail_raw(ty, &mut normalize, || {});
+            let tail = tcx.struct_tail_raw(ty, &cause, &mut normalize, || {});
             normalize(tail)
         }
     }

compiler/rustc_const_eval/src/const_eval/valtrees.rs

@@ -1,6 +1,7 @@
 use rustc_abi::{BackendRepr, FieldIdx, VariantIdx};
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_middle::mir::interpret::{EvalToValTreeResult, GlobalId, ValTreeCreationError};
+use rustc_middle::traits::ObligationCause;
 use rustc_middle::ty::layout::{LayoutCx, TyAndLayout};
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_middle::{bug, mir};
@@ -196,6 +197,7 @@ fn reconstruct_place_meta<'tcx>(
     // Traverse the type, and update `last_valtree` as we go.
     let tail = tcx.struct_tail_raw(
         layout.ty,
+        &ObligationCause::dummy(),
         |ty| ty,
         || {
             let branches = last_valtree.unwrap_branch();

compiler/rustc_hir_typeck/src/expectation.rs

@@ -1,3 +1,4 @@
+use rustc_middle::traits::ObligationCause;
 use rustc_middle::ty::{self, Ty};
 use rustc_span::Span;
 
@@ -74,8 +75,14 @@ impl<'a, 'tcx> Expectation<'tcx> {
     /// See the test case `test/ui/coerce-expect-unsized.rs` and #20169
     /// for examples of where this comes up,.
     pub(super) fn rvalue_hint(fcx: &FnCtxt<'a, 'tcx>, ty: Ty<'tcx>) -> Expectation<'tcx> {
+        let span = match ty.kind() {
+            ty::Adt(adt_def, _) => fcx.tcx.def_span(adt_def.did()),
+            _ => fcx.tcx.def_span(fcx.body_id),
+        };
+        let cause = ObligationCause::misc(span, fcx.body_id);
+
         // FIXME: This is not right, even in the old solver...
-        match fcx.tcx.struct_tail_raw(ty, |ty| ty, || {}).kind() {
+        match fcx.tcx.struct_tail_raw(ty, &cause, |ty| ty, || {}).kind() {
             ty::Slice(_) | ty::Str | ty::Dynamic(..) => ExpectRvalueLikeUnsized(ty),
             _ => ExpectHasType(ty),
         }

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -424,6 +424,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         if !ty.references_error() {
             let tail = self.tcx.struct_tail_raw(
                 ty,
+                &self.misc(span),
                 |ty| {
                     if self.next_trait_solver() {
                         self.try_structurally_resolve_type(span, ty)

compiler/rustc_middle/src/error.rs

@@ -71,6 +71,8 @@ pub enum TypeMismatchReason {
 #[diag(middle_recursion_limit_reached)]
 #[help]
 pub(crate) struct RecursionLimitReached<'tcx> {
+    #[primary_span]
+    pub span: Span,
     pub ty: Ty<'tcx>,
     pub suggested_limit: rustc_hir::limit::Limit,
 }

compiler/rustc_middle/src/traits/mod.rs

@@ -823,6 +823,9 @@ impl DynCompatibilityViolation {
             DynCompatibilityViolation::Method(name, MethodViolationCode::AsyncFn, _) => {
                 format!("method `{name}` is `async`").into()
             }
+            DynCompatibilityViolation::Method(name, MethodViolationCode::CVariadic, _) => {
+                format!("method `{name}` is C-variadic").into()
+            }
             DynCompatibilityViolation::Method(
                 name,
                 MethodViolationCode::WhereClauseReferencesSelf,
@@ -977,6 +980,9 @@ pub enum MethodViolationCode {
     /// e.g., `fn foo<A>()`
     Generic,
 
+    /// e.g., `fn (mut ap: ...)`
+    CVariadic,
+
     /// the method's receiver (`self` argument) can't be dispatched on
     UndispatchableReceiver(Option<Span>),
 }

compiler/rustc_middle/src/ty/layout.rs

@@ -22,6 +22,7 @@ use {rustc_abi as abi, rustc_hir as hir};
 
 use crate::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use crate::query::TyCtxtAt;
+use crate::traits::ObligationCause;
 use crate::ty::normalize_erasing_regions::NormalizationError;
 use crate::ty::{self, CoroutineArgsExt, Ty, TyCtxt, TypeVisitableExt};
 
@@ -384,6 +385,7 @@ impl<'tcx> SizeSkeleton<'tcx> {
 
                 let tail = tcx.struct_tail_raw(
                     pointee,
+                    &ObligationCause::dummy(),
                     |ty| match tcx.try_normalize_erasing_regions(typing_env, ty) {
                         Ok(ty) => ty,
                         Err(e) => Ty::new_error_with_message(

compiler/rustc_middle/src/ty/sty.rs

@@ -23,6 +23,7 @@ use ty::util::IntTypeExt;
 
 use super::GenericParamDefKind;
 use crate::infer::canonical::Canonical;
+use crate::traits::ObligationCause;
 use crate::ty::InferTy::*;
 use crate::ty::{
     self, AdtDef, BoundRegionKind, Discr, GenericArg, GenericArgs, GenericArgsRef, List, ParamEnv,
@@ -1638,7 +1639,7 @@ impl<'tcx> Ty<'tcx> {
         tcx: TyCtxt<'tcx>,
         normalize: impl FnMut(Ty<'tcx>) -> Ty<'tcx>,
     ) -> Result<Ty<'tcx>, Ty<'tcx>> {
-        let tail = tcx.struct_tail_raw(self, normalize, || {});
+        let tail = tcx.struct_tail_raw(self, &ObligationCause::dummy(), normalize, || {});
         match tail.kind() {
             // Sized types
             ty::Infer(ty::IntVar(_) | ty::FloatVar(_))

compiler/rustc_middle/src/ty/util.rs

@@ -24,6 +24,7 @@ use super::TypingEnv;
 use crate::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use crate::mir;
 use crate::query::Providers;
+use crate::traits::ObligationCause;
 use crate::ty::layout::{FloatExt, IntegerExt};
 use crate::ty::{
     self, Asyncness, FallibleTypeFolder, GenericArgKind, GenericArgsRef, Ty, TyCtxt, TypeFoldable,
@@ -216,7 +217,12 @@ impl<'tcx> TyCtxt<'tcx> {
         typing_env: ty::TypingEnv<'tcx>,
     ) -> Ty<'tcx> {
         let tcx = self;
-        tcx.struct_tail_raw(ty, |ty| tcx.normalize_erasing_regions(typing_env, ty), || {})
+        tcx.struct_tail_raw(
+            ty,
+            &ObligationCause::dummy(),
+            |ty| tcx.normalize_erasing_regions(typing_env, ty),
+            || {},
+        )
     }
 
     /// Returns true if a type has metadata.
@@ -248,6 +254,7 @@ impl<'tcx> TyCtxt<'tcx> {
     pub fn struct_tail_raw(
         self,
         mut ty: Ty<'tcx>,
+        cause: &ObligationCause<'tcx>,
         mut normalize: impl FnMut(Ty<'tcx>) -> Ty<'tcx>,
         // This is currently used to allow us to walk a ValTree
         // in lockstep with the type in order to get the ValTree branch that
@@ -261,9 +268,11 @@ impl<'tcx> TyCtxt<'tcx> {
                     Limit(0) => Limit(2),
                     limit => limit * 2,
                 };
-                let reported = self
-                    .dcx()
-                    .emit_err(crate::error::RecursionLimitReached { ty, suggested_limit });
+                let reported = self.dcx().emit_err(crate::error::RecursionLimitReached {
+                    span: cause.span,
+                    ty,
+                    suggested_limit,
+                });
                 return Ty::new_error(self, reported);
             }
             match *ty.kind() {

compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs

@@ -426,6 +426,9 @@ fn virtual_call_violations_for_method<'tcx>(
     if let Some(code) = contains_illegal_impl_trait_in_trait(tcx, method.def_id, sig.output()) {
         errors.push(code);
     }
+    if sig.skip_binder().c_variadic {
+        errors.push(MethodViolationCode::CVariadic);
+    }
 
     // We can't monomorphize things like `fn foo<A>(...)`.
     let own_counts = tcx.generics_of(method.def_id).own_counts();

compiler/rustc_trait_selection/src/traits/project.rs

@@ -1057,6 +1057,7 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
                     Some(LangItem::PointeeTrait) => {
                         let tail = selcx.tcx().struct_tail_raw(
                             self_ty,
+                            &obligation.cause,
                             |ty| {
                                 // We throw away any obligations we get from this, since we normalize
                                 // and confirm these obligations once again during confirmation

compiler/rustc_ty_utils/src/layout.rs

@@ -10,6 +10,7 @@ use rustc_hashes::Hash64;
 use rustc_index::IndexVec;
 use rustc_middle::bug;
 use rustc_middle::query::Providers;
+use rustc_middle::traits::ObligationCause;
 use rustc_middle::ty::layout::{
     FloatExt, HasTyCtxt, IntegerExt, LayoutCx, LayoutError, LayoutOf, TyAndLayout,
 };
@@ -390,30 +391,31 @@ fn layout_of_uncached<'tcx>(
 
             let metadata = if let Some(metadata_def_id) = tcx.lang_items().metadata_type() {
                 let pointee_metadata = Ty::new_projection(tcx, metadata_def_id, [pointee]);
-                let metadata_ty =
-                    match tcx.try_normalize_erasing_regions(cx.typing_env, pointee_metadata) {
-                        Ok(metadata_ty) => metadata_ty,
-                        Err(mut err) => {
-                            // Usually `<Ty as Pointee>::Metadata` can't be normalized because
-                            // its struct tail cannot be normalized either, so try to get a
-                            // more descriptive layout error here, which will lead to less confusing
-                            // diagnostics.
-                            //
-                            // We use the raw struct tail function here to get the first tail
-                            // that is an alias, which is likely the cause of the normalization
-                            // error.
-                            match tcx.try_normalize_erasing_regions(
-                                cx.typing_env,
-                                tcx.struct_tail_raw(pointee, |ty| ty, || {}),
-                            ) {
-                                Ok(_) => {}
-                                Err(better_err) => {
-                                    err = better_err;
-                                }
+                let metadata_ty = match tcx
+                    .try_normalize_erasing_regions(cx.typing_env, pointee_metadata)
+                {
+                    Ok(metadata_ty) => metadata_ty,
+                    Err(mut err) => {
+                        // Usually `<Ty as Pointee>::Metadata` can't be normalized because
+                        // its struct tail cannot be normalized either, so try to get a
+                        // more descriptive layout error here, which will lead to less confusing
+                        // diagnostics.
+                        //
+                        // We use the raw struct tail function here to get the first tail
+                        // that is an alias, which is likely the cause of the normalization
+                        // error.
+                        match tcx.try_normalize_erasing_regions(
+                            cx.typing_env,
+                            tcx.struct_tail_raw(pointee, &ObligationCause::dummy(), |ty| ty, || {}),
+                        ) {
+                            Ok(_) => {}
+                            Err(better_err) => {
+                                err = better_err;
                             }
-                            return Err(error(cx, LayoutError::NormalizationFailure(pointee, err)));
                         }
-                    };
+                        return Err(error(cx, LayoutError::NormalizationFailure(pointee, err)));
+                    }
+                };
 
                 let metadata_layout = cx.layout_of(metadata_ty)?;
                 // If the metadata is a 1-zst, then the pointer is thin.

compiler/rustc_ty_utils/src/ty.rs

@@ -353,6 +353,7 @@ fn impl_self_is_guaranteed_unsized<'tcx>(tcx: TyCtxt<'tcx>, impl_def_id: DefId)
 
     let tail = tcx.struct_tail_raw(
         tcx.type_of(impl_def_id).instantiate_identity(),
+        &cause,
         |ty| {
             ocx.structurally_normalize_ty(&cause, param_env, ty).unwrap_or_else(|_| {
                 Ty::new_error_with_message(