Auto merge of rust-lang#136303 - lcnr:elaborate-skip-bad-projections, r=<try>

elaborate: avoid projections with unconstrained bound regions

Should only do this for where-bounds and item bounds. Gonna rewrite later I think.

This means we'll no longer need to support unconstrained regions in implied bounds, see lcnr/random-rust-snippets#15. This fixes
```rust
trait Super {
    type SAssoc;
}
trait Trait<'a>: Super<SAssoc = <Self as Trait<'a>>::TAssoc> {
    type TAssoc;
}

fn test<'a, T: 'a>() {}

fn unconstrained_lt<'arg, T: for<'a> Trait<'a>>(x: &'arg <T as Super>::SAssoc) {
    test::<'arg, <T as Super>::SAssoc>();
}
```
but breaks
```rust
trait Super {
    type SAssoc;
}
trait Trait<'a>: Super<SAssoc = <Self as Trait<'a>>::TAssoc> {
    type TAssoc;
}

fn test<'a, T: 'a>() {}

fn unconstrained_lt<'arg, T: for<'a> Trait<'a, TAssoc = usize>>(x: <T as Super>::SAssoc) -> usize {
    x
}
```

r? `@compiler-errors`

Author: bors

compiler/rustc_ast/src/lib.rs

@@ -44,6 +44,8 @@ pub mod token;
 pub mod tokenstream;
 pub mod visit;
 
+pub use rustc_ast_ir::Limit;
+
 pub use self::ast::*;
 pub use self::ast_traits::{AstDeref, AstNodeWrapper, HasAttrs, HasNodeId, HasTokens};
 

compiler/rustc_ast_ir/src/lib.rs

@@ -5,6 +5,9 @@
 #![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
+use std::fmt;
+use std::ops::{Div, Mul};
+
 #[cfg(feature = "nightly")]
 use rustc_macros::{Decodable, Encodable, HashStable_NoContext};
 
@@ -86,3 +89,51 @@ pub enum Pinnedness {
     Not,
     Pinned,
 }
+
+/// New-type wrapper around `usize` for representing limits. Ensures that comparisons against
+/// limits are consistent throughout the compiler.
+#[derive(Clone, Copy, Debug)]
+#[cfg_attr(feature = "nightly", derive(Encodable, Decodable, HashStable_NoContext))]
+pub struct Limit(pub usize);
+
+impl Limit {
+    /// Create a new limit from a `usize`.
+    pub fn new(value: usize) -> Self {
+        Limit(value)
+    }
+
+    /// Check that `value` is within the limit. Ensures that the same comparisons are used
+    /// throughout the compiler, as mismatches can cause ICEs, see #72540.
+    #[inline]
+    pub fn value_within_limit(&self, value: usize) -> bool {
+        value <= self.0
+    }
+}
+
+impl From<usize> for Limit {
+    fn from(value: usize) -> Self {
+        Self::new(value)
+    }
+}
+
+impl fmt::Display for Limit {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.0.fmt(f)
+    }
+}
+
+impl Div<usize> for Limit {
+    type Output = Limit;
+
+    fn div(self, rhs: usize) -> Self::Output {
+        Limit::new(self.0 / rhs)
+    }
+}
+
+impl Mul<usize> for Limit {
+    type Output = Limit;
+
+    fn mul(self, rhs: usize) -> Self::Output {
+        Limit::new(self.0 * rhs)
+    }
+}

compiler/rustc_errors/src/diagnostic_impls.rs

@@ -6,6 +6,7 @@ use std::path::{Path, PathBuf};
 use std::process::ExitStatus;
 
 use rustc_abi::TargetDataLayoutErrors;
+use rustc_ast::Limit;
 use rustc_ast::util::parser::ExprPrecedence;
 use rustc_ast_pretty::pprust;
 use rustc_macros::Subdiagnostic;
@@ -190,6 +191,12 @@ impl IntoDiagArg for PathBuf {
     }
 }
 
+impl IntoDiagArg for Limit {
+    fn into_diag_arg(self) -> rustc_errors::DiagArgValue {
+        self.to_string().into_diag_arg()
+    }
+}
+
 impl IntoDiagArg for PanicStrategy {
     fn into_diag_arg(self) -> DiagArgValue {
         DiagArgValue::Str(Cow::Owned(self.desc().to_string()))

compiler/rustc_hir_analysis/src/collect.rs

@@ -42,6 +42,7 @@ use rustc_span::{DUMMY_SP, Ident, Span, Symbol, kw, sym};
 use rustc_trait_selection::error_reporting::traits::suggestions::NextTypeParamName;
 use rustc_trait_selection::infer::InferCtxtExt;
 use rustc_trait_selection::traits::ObligationCtxt;
+use rustc_type_ir::visit::collect_referenced_late_bound_regions;
 use tracing::{debug, instrument};
 
 use crate::check::intrinsic::intrinsic_operation_unsafety;
@@ -635,11 +636,10 @@ fn get_new_lifetime_name<'tcx>(
     poly_trait_ref: ty::PolyTraitRef<'tcx>,
     generics: &hir::Generics<'tcx>,
 ) -> String {
-    let existing_lifetimes = tcx
-        .collect_referenced_late_bound_regions(poly_trait_ref)
+    let existing_lifetimes = collect_referenced_late_bound_regions(tcx, poly_trait_ref)
         .into_iter()
         .filter_map(|lt| {
-            if let ty::BoundRegionKind::Named(_, name) = lt {
+            if let ty::BoundRegionKind::Named(_, name) = lt.kind {
                 Some(name.as_str().to_string())
             } else {
                 None

compiler/rustc_hir_analysis/src/constrained_generic_params.rs

@@ -3,7 +3,7 @@ use rustc_middle::bug;
 use rustc_middle::ty::visit::{TypeSuperVisitable, TypeVisitor};
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_span::Span;
-use rustc_type_ir::fold::TypeFoldable;
+use rustc_type_ir::fold::{TypeFoldable, expand_weak_alias_tys};
 use tracing::debug;
 
 #[derive(Clone, PartialEq, Eq, Hash, Debug)]
@@ -51,7 +51,7 @@ pub(crate) fn parameters_for<'tcx>(
     include_nonconstraining: bool,
 ) -> Vec<Parameter> {
     let mut collector = ParameterCollector { parameters: vec![], include_nonconstraining };
-    let value = if !include_nonconstraining { tcx.expand_weak_alias_tys(value) } else { value };
+    let value = if !include_nonconstraining { expand_weak_alias_tys(tcx, value) } else { value };
     value.visit_with(&mut collector);
     collector.parameters
 }

compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs

@@ -11,7 +11,10 @@ use rustc_middle::bug;
 use rustc_middle::ty::{self as ty, IsSuggestable, Ty, TyCtxt};
 use rustc_span::{ErrorGuaranteed, Ident, Span, Symbol, kw, sym};
 use rustc_trait_selection::traits;
-use rustc_type_ir::visit::{TypeSuperVisitable, TypeVisitable, TypeVisitableExt, TypeVisitor};
+use rustc_type_ir::visit::{
+    TypeSuperVisitable, TypeVisitable, TypeVisitableExt, TypeVisitor,
+    collect_constrained_late_bound_regions, collect_referenced_late_bound_regions,
+};
 use smallvec::SmallVec;
 use tracing::{debug, instrument};
 
@@ -358,9 +361,9 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 //     for<'a> <T as Iterator>::Item = &'a str // <-- 'a is bad
                 //     for<'a> <T as FnMut<(&'a u32,)>>::Output = &'a str // <-- 'a is ok
                 let late_bound_in_projection_ty =
-                    tcx.collect_constrained_late_bound_regions(projection_term);
+                    collect_constrained_late_bound_regions(tcx, projection_term);
                 let late_bound_in_term =
-                    tcx.collect_referenced_late_bound_regions(trait_ref.rebind(term));
+                    collect_referenced_late_bound_regions(tcx, trait_ref.rebind(term));
                 debug!(?late_bound_in_projection_ty);
                 debug!(?late_bound_in_term);
 

compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_compatibility.rs

@@ -13,6 +13,9 @@ use rustc_span::{ErrorGuaranteed, Span};
 use rustc_trait_selection::error_reporting::traits::report_dyn_incompatibility;
 use rustc_trait_selection::traits::{self, hir_ty_lowering_dyn_compatibility_violations};
 use rustc_type_ir::elaborate::ClauseWithSupertraitSpan;
+use rustc_type_ir::visit::{
+    collect_constrained_late_bound_regions, collect_referenced_late_bound_regions,
+};
 use smallvec::{SmallVec, smallvec};
 use tracing::{debug, instrument};
 
@@ -362,10 +365,12 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         //
         //     for<'a> <T as Iterator>::Item = &'a str // <-- 'a is bad
         //     for<'a> <T as FnMut<(&'a u32,)>>::Output = &'a str // <-- 'a is ok
-        let late_bound_in_projection_term =
-            tcx.collect_constrained_late_bound_regions(pred.map_bound(|pred| pred.projection_term));
+        let late_bound_in_projection_term = collect_constrained_late_bound_regions(
+            tcx,
+            pred.map_bound(|pred| pred.projection_term),
+        );
         let late_bound_in_term =
-            tcx.collect_referenced_late_bound_regions(pred.map_bound(|pred| pred.term));
+            collect_referenced_late_bound_regions(tcx, pred.map_bound(|pred| pred.term));
         debug!(?late_bound_in_projection_term);
         debug!(?late_bound_in_term);
 

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -49,6 +49,9 @@ use rustc_span::{DUMMY_SP, Ident, Span, Symbol, kw};
 use rustc_trait_selection::infer::InferCtxtExt;
 use rustc_trait_selection::traits::wf::object_region_bounds;
 use rustc_trait_selection::traits::{self, ObligationCtxt};
+use rustc_type_ir::visit::{
+    collect_constrained_late_bound_regions, collect_referenced_late_bound_regions,
+};
 use tracing::{debug, instrument};
 
 use crate::bounds::Bounds;
@@ -2606,9 +2609,9 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         //     for<'a> fn(&'a String) -> &'a str <-- 'a is ok
         let inputs = bare_fn_ty.inputs();
         let late_bound_in_args =
-            tcx.collect_constrained_late_bound_regions(inputs.map_bound(|i| i.to_owned()));
+            collect_constrained_late_bound_regions(tcx, inputs.map_bound(|i| i.to_owned()));
         let output = bare_fn_ty.output();
-        let late_bound_in_ret = tcx.collect_referenced_late_bound_regions(output);
+        let late_bound_in_ret = collect_referenced_late_bound_regions(tcx, output);
 
         self.validate_late_bound_regions(late_bound_in_args, late_bound_in_ret, |br_name| {
             struct_span_code_err!(
@@ -2665,12 +2668,12 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     #[instrument(level = "trace", skip(self, generate_err))]
     fn validate_late_bound_regions<'cx>(
         &'cx self,
-        constrained_regions: FxIndexSet<ty::BoundRegionKind>,
-        referenced_regions: FxIndexSet<ty::BoundRegionKind>,
+        constrained_regions: FxIndexSet<ty::BoundRegion>,
+        referenced_regions: FxIndexSet<ty::BoundRegion>,
         generate_err: impl Fn(&str) -> Diag<'cx>,
     ) {
         for br in referenced_regions.difference(&constrained_regions) {
-            let br_name = match *br {
+            let br_name = match br.kind {
                 ty::BoundRegionKind::Named(_, kw::UnderscoreLifetime)
                 | ty::BoundRegionKind::Anon
                 | ty::BoundRegionKind::ClosureEnv => "an anonymous lifetime".to_string(),
@@ -2680,7 +2683,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             let mut err = generate_err(&br_name);
 
             if let ty::BoundRegionKind::Named(_, kw::UnderscoreLifetime)
-            | ty::BoundRegionKind::Anon = *br
+            | ty::BoundRegionKind::Anon = br.kind
             {
                 // The only way for an anonymous lifetime to wind up
                 // in the return type but **also** be unconstrained is

compiler/rustc_middle/src/ty/context.rs

@@ -305,8 +305,8 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
         self.parent(def_id)
     }
 
-    fn recursion_limit(self) -> usize {
-        self.recursion_limit().0
+    fn recursion_limit(self) -> Limit {
+        self.recursion_limit()
     }
 
     type Features = &'tcx rustc_feature::Features;

compiler/rustc_middle/src/ty/util.rs

@@ -6,7 +6,6 @@ use rustc_abi::{ExternAbi, Float, Integer, IntegerType, Size};
 use rustc_apfloat::Float as _;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::stable_hasher::{Hash128, HashStable, StableHasher};
-use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_errors::ErrorGuaranteed;
 use rustc_hir as hir;
 use rustc_hir::def::{CtorOf, DefKind, Res};
@@ -895,30 +894,6 @@ impl<'tcx> TyCtxt<'tcx> {
             || self.extern_crate(key).is_some_and(|e| e.is_direct())
     }
 
-    /// Expand any [weak alias types][weak] contained within the given `value`.
-    ///
-    /// This should be used over other normalization routines in situations where
-    /// it's important not to normalize other alias types and where the predicates
-    /// on the corresponding type alias shouldn't be taken into consideration.
-    ///
-    /// Whenever possible **prefer not to use this function**! Instead, use standard
-    /// normalization routines or if feasible don't normalize at all.
-    ///
-    /// This function comes in handy if you want to mimic the behavior of eager
-    /// type alias expansion in a localized manner.
-    ///
-    /// <div class="warning">
-    /// This delays a bug on overflow! Therefore you need to be certain that the
-    /// contained types get fully normalized at a later stage. Note that even on
-    /// overflow all well-behaved weak alias types get expanded correctly, so the
-    /// result is still useful.
-    /// </div>
-    ///
-    /// [weak]: ty::Weak
-    pub fn expand_weak_alias_tys<T: TypeFoldable<TyCtxt<'tcx>>>(self, value: T) -> T {
-        value.fold_with(&mut WeakAliasTypeExpander { tcx: self, depth: 0 })
-    }
-
     /// Peel off all [weak alias types] in this type until there are none left.
     ///
     /// This only expands weak alias types in “head” / outermost positions. It can
@@ -1088,42 +1063,6 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for OpaqueTypeExpander<'tcx> {
     }
 }
 
-struct WeakAliasTypeExpander<'tcx> {
-    tcx: TyCtxt<'tcx>,
-    depth: usize,
-}
-
-impl<'tcx> TypeFolder<TyCtxt<'tcx>> for WeakAliasTypeExpander<'tcx> {
-    fn cx(&self) -> TyCtxt<'tcx> {
-        self.tcx
-    }
-
-    fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
-        if !ty.has_type_flags(ty::TypeFlags::HAS_TY_WEAK) {
-            return ty;
-        }
-        let ty::Alias(ty::Weak, alias) = ty.kind() else {
-            return ty.super_fold_with(self);
-        };
-        if !self.tcx.recursion_limit().value_within_limit(self.depth) {
-            let guar = self.tcx.dcx().delayed_bug("overflow expanding weak alias type");
-            return Ty::new_error(self.tcx, guar);
-        }
-
-        self.depth += 1;
-        ensure_sufficient_stack(|| {
-            self.tcx.type_of(alias.def_id).instantiate(self.tcx, alias.args).fold_with(self)
-        })
-    }
-
-    fn fold_const(&mut self, ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
-        if !ct.has_type_flags(ty::TypeFlags::HAS_TY_WEAK) {
-            return ct;
-        }
-        ct.super_fold_with(self)
-    }
-}
-
 /// Indicates the form of `AsyncDestruct::Destructor`. Used to simplify async
 /// drop glue for types not using async drop.
 #[derive(Clone, Copy, PartialEq, Eq, Debug)]