Rollup merge of #121258 - fmease:assoc-const-eq-reject-overly-generic-tys, r=compiler-errors

Reject overly generic assoc const binding types

Split off from #119385 to make #119385 easier to review.

---

In the *instantiated* type of assoc const bindings

1. reject **early-bound generic params**
   * Provide a rich error message instead of ICE'ing ([#108271](https://github.com/rust-lang/rust/issues/108271)).
   * This is a temporary and semi-artificial restriction until the arrival of *generic const generics*.
   * It's quite possible that rustc could already perfectly support this subset of generic const generics if we just removed some checks (some `.no_bound_vars().expect(…)`) but even if that was the case, I'd rather gate it behind a new feature flag. Reporting an error instead of ICE'ing is a good first step towards an eventual feature gate error.
2. reject **escaping late-bound generic params**
   * They lead to ICEs before & I'm pretty sure that they remain incorrect even in a world with *generic const generics*

---

Together with #118668 & #119385, this supersedes #118360.
Fixes #108271.

compiler/rustc_hir_analysis/messages.ftl

@@ -118,6 +118,11 @@ hir_analysis_enum_discriminant_overflowed = enum discriminant overflowed
     .label = overflowed on value after {$discr}
     .note = explicitly set `{$item_name} = {$wrapped_discr}` if that is desired outcome
 
+hir_analysis_escaping_bound_var_in_ty_of_assoc_const_binding =
+    the type of the associated constant `{$assoc_const}` cannot capture late-bound generic parameters
+    .label = its type cannot capture the late-bound {$var_def_kind} `{$var_name}`
+    .var_defined_here_label = the late-bound {$var_def_kind} `{$var_name}` is defined here
+
 hir_analysis_field_already_declared =
     field `{$field_name}` is already declared
     .label = field already declared
@@ -316,6 +321,22 @@ hir_analysis_opaque_captures_higher_ranked_lifetime = `impl Trait` cannot captur
     .label = `impl Trait` implicitly captures all lifetimes in scope
     .note = lifetime declared here
 
+hir_analysis_param_in_ty_of_assoc_const_binding =
+    the type of the associated constant `{$assoc_const}` must not depend on {$param_category ->
+        [self] `Self`
+        [synthetic] `impl Trait`
+        *[normal] generic parameters
+    }
+    .label = its type must not depend on {$param_category ->
+        [self] `Self`
+        [synthetic] `impl Trait`
+        *[normal] the {$param_def_kind} `{$param_name}`
+    }
+    .param_defined_here_label = {$param_category ->
+        [synthetic] the `impl Trait` is specified here
+        *[normal] the {$param_def_kind} `{$param_name}` is defined here
+    }
+
 hir_analysis_paren_sugar_attribute = the `#[rustc_paren_sugar]` attribute is a temporary means of controlling which traits can use parenthetical notation
     .help = add `#![feature(unboxed_closures)]` to the crate attributes to use it
 
@@ -432,6 +453,8 @@ hir_analysis_transparent_non_zero_sized_enum = the variant of a transparent {$de
     .label = needs at most one field with non-trivial size or alignment, but has {$field_count}
     .labels = this field has non-zero size or requires alignment
 
+hir_analysis_ty_of_assoc_const_binding_note = `{$assoc_const}` has type `{$ty}`
+
 hir_analysis_ty_param_first_local = type parameter `{$param_ty}` must be covered by another type when it appears before the first local type (`{$local_type}`)
     .label = type parameter `{$param_ty}` must be covered by another type when it appears before the first local type (`{$local_type}`)
     .note = implementing a foreign trait is only possible if at least one of the types for which it is implemented is local, and no uncovered type parameters appear before that first local type

compiler/rustc_hir_analysis/src/astconv/bounds.rs

@@ -1,12 +1,15 @@
-use rustc_data_structures::fx::FxIndexMap;
+use std::ops::ControlFlow;
+
+use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
 use rustc_errors::{codes::*, struct_span_code_err};
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{DefId, LocalDefId};
-use rustc_middle::ty::{self as ty, Ty};
+use rustc_middle::ty::{self as ty, IsSuggestable, Ty, TyCtxt};
 use rustc_span::symbol::Ident;
-use rustc_span::{ErrorGuaranteed, Span};
+use rustc_span::{ErrorGuaranteed, Span, Symbol};
 use rustc_trait_selection::traits;
+use rustc_type_ir::visit::{TypeSuperVisitable, TypeVisitable, TypeVisitableExt, TypeVisitor};
 use smallvec::SmallVec;
 
 use crate::astconv::{AstConv, OnlySelfBounds, PredicateFilter};
@@ -433,14 +436,8 @@ impl<'tcx> dyn AstConv<'tcx> + '_ {
                 binding.kind
             {
                 let ty = alias_ty.map_bound(|ty| tcx.type_of(ty.def_id).instantiate(tcx, ty.args));
-                // Since the arguments passed to the alias type above may contain early-bound
-                // generic parameters, the instantiated type may contain some as well.
-                // Therefore wrap it in `EarlyBinder`.
-                // FIXME(fmease): Reject escaping late-bound vars.
-                tcx.feed_anon_const_type(
-                    anon_const.def_id,
-                    ty::EarlyBinder::bind(ty.skip_binder()),
-                );
+                let ty = check_assoc_const_binding_type(tcx, assoc_ident, ty, binding.hir_id);
+                tcx.feed_anon_const_type(anon_const.def_id, ty::EarlyBinder::bind(ty));
             }
 
             alias_ty
@@ -530,3 +527,167 @@ impl<'tcx> dyn AstConv<'tcx> + '_ {
         Ok(())
     }
 }
+
+/// Detect and reject early-bound & escaping late-bound generic params in the type of assoc const bindings.
+///
+/// FIXME(const_generics): This is a temporary and semi-artifical restriction until the
+/// arrival of *generic const generics*[^1].
+///
+/// It might actually be possible that we can already support early-bound generic params
+/// in such types if we just lifted some more checks in other places, too, for example
+/// inside [`ty::Const::from_anon_const`]. However, even if that were the case, we should
+/// probably gate this behind another feature flag.
+///
+/// [^1]: <https://github.com/rust-lang/project-const-generics/issues/28>.
+fn check_assoc_const_binding_type<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    assoc_const: Ident,
+    ty: ty::Binder<'tcx, Ty<'tcx>>,
+    hir_id: hir::HirId,
+) -> Ty<'tcx> {
+    // We can't perform the checks for early-bound params during name resolution unlike E0770
+    // because this information depends on *type* resolution.
+    // We can't perform these checks in `resolve_bound_vars` either for the same reason.
+    // Consider the trait ref `for<'a> Trait<'a, C = { &0 }>`. We need to know the fully
+    // resolved type of `Trait::C` in order to know if it references `'a` or not.
+
+    let ty = ty.skip_binder();
+    if !ty.has_param() && !ty.has_escaping_bound_vars() {
+        return ty;
+    }
+
+    let mut collector = GenericParamAndBoundVarCollector {
+        tcx,
+        params: Default::default(),
+        vars: Default::default(),
+        depth: ty::INNERMOST,
+    };
+    let mut guar = ty.visit_with(&mut collector).break_value();
+
+    let ty_note = ty
+        .make_suggestable(tcx, false)
+        .map(|ty| crate::errors::TyOfAssocConstBindingNote { assoc_const, ty });
+
+    let enclosing_item_owner_id = tcx
+        .hir()
+        .parent_owner_iter(hir_id)
+        .find_map(|(owner_id, parent)| parent.generics().map(|_| owner_id))
+        .unwrap();
+    let generics = tcx.generics_of(enclosing_item_owner_id);
+    for index in collector.params {
+        let param = generics.param_at(index as _, tcx);
+        let is_self_param = param.name == rustc_span::symbol::kw::SelfUpper;
+        guar.get_or_insert(tcx.dcx().emit_err(crate::errors::ParamInTyOfAssocConstBinding {
+            span: assoc_const.span,
+            assoc_const,
+            param_name: param.name,
+            param_def_kind: tcx.def_descr(param.def_id),
+            param_category: if is_self_param {
+                "self"
+            } else if param.kind.is_synthetic() {
+                "synthetic"
+            } else {
+                "normal"
+            },
+            param_defined_here_label:
+                (!is_self_param).then(|| tcx.def_ident_span(param.def_id).unwrap()),
+            ty_note,
+        }));
+    }
+    for (var_def_id, var_name) in collector.vars {
+        guar.get_or_insert(tcx.dcx().emit_err(
+            crate::errors::EscapingBoundVarInTyOfAssocConstBinding {
+                span: assoc_const.span,
+                assoc_const,
+                var_name,
+                var_def_kind: tcx.def_descr(var_def_id),
+                var_defined_here_label: tcx.def_ident_span(var_def_id).unwrap(),
+                ty_note,
+            },
+        ));
+    }
+
+    let guar = guar.unwrap_or_else(|| bug!("failed to find gen params or bound vars in ty"));
+    Ty::new_error(tcx, guar)
+}
+
+struct GenericParamAndBoundVarCollector<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    params: FxIndexSet<u32>,
+    vars: FxIndexSet<(DefId, Symbol)>,
+    depth: ty::DebruijnIndex,
+}
+
+impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for GenericParamAndBoundVarCollector<'tcx> {
+    type Result = ControlFlow<ErrorGuaranteed>;
+
+    fn visit_binder<T: TypeVisitable<TyCtxt<'tcx>>>(
+        &mut self,
+        binder: &ty::Binder<'tcx, T>,
+    ) -> Self::Result {
+        self.depth.shift_in(1);
+        let result = binder.super_visit_with(self);
+        self.depth.shift_out(1);
+        result
+    }
+
+    fn visit_ty(&mut self, ty: Ty<'tcx>) -> Self::Result {
+        match ty.kind() {
+            ty::Param(param) => {
+                self.params.insert(param.index);
+            }
+            ty::Bound(db, bt) if *db >= self.depth => {
+                self.vars.insert(match bt.kind {
+                    ty::BoundTyKind::Param(def_id, name) => (def_id, name),
+                    ty::BoundTyKind::Anon => {
+                        let reported = self
+                            .tcx
+                            .dcx()
+                            .delayed_bug(format!("unexpected anon bound ty: {:?}", bt.var));
+                        return ControlFlow::Break(reported);
+                    }
+                });
+            }
+            _ if ty.has_param() || ty.has_bound_vars() => return ty.super_visit_with(self),
+            _ => {}
+        }
+        ControlFlow::Continue(())
+    }
+
+    fn visit_region(&mut self, re: ty::Region<'tcx>) -> Self::Result {
+        match re.kind() {
+            ty::ReEarlyParam(param) => {
+                self.params.insert(param.index);
+            }
+            ty::ReBound(db, br) if db >= self.depth => {
+                self.vars.insert(match br.kind {
+                    ty::BrNamed(def_id, name) => (def_id, name),
+                    ty::BrAnon | ty::BrEnv => {
+                        let guar = self
+                            .tcx
+                            .dcx()
+                            .delayed_bug(format!("unexpected bound region kind: {:?}", br.kind));
+                        return ControlFlow::Break(guar);
+                    }
+                });
+            }
+            _ => {}
+        }
+        ControlFlow::Continue(())
+    }
+
+    fn visit_const(&mut self, ct: ty::Const<'tcx>) -> Self::Result {
+        match ct.kind() {
+            ty::ConstKind::Param(param) => {
+                self.params.insert(param.index);
+            }
+            ty::ConstKind::Bound(db, ty::BoundVar { .. }) if db >= self.depth => {
+                let guar = self.tcx.dcx().delayed_bug("unexpected escaping late-bound const var");
+                return ControlFlow::Break(guar);
+            }
+            _ if ct.has_param() || ct.has_bound_vars() => return ct.super_visit_with(self),
+            _ => {}
+        }
+        ControlFlow::Continue(())
+    }
+}

compiler/rustc_hir_analysis/src/errors.rs

@@ -295,6 +295,44 @@ pub struct AssocTypeBindingNotAllowed {
     pub fn_trait_expansion: Option<ParenthesizedFnTraitExpansion>,
 }
 
+#[derive(Diagnostic)]
+#[diag(hir_analysis_param_in_ty_of_assoc_const_binding)]
+pub(crate) struct ParamInTyOfAssocConstBinding<'tcx> {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    pub assoc_const: Ident,
+    pub param_name: Symbol,
+    pub param_def_kind: &'static str,
+    pub param_category: &'static str,
+    #[label(hir_analysis_param_defined_here_label)]
+    pub param_defined_here_label: Option<Span>,
+    #[subdiagnostic]
+    pub ty_note: Option<TyOfAssocConstBindingNote<'tcx>>,
+}
+
+#[derive(Subdiagnostic, Clone, Copy)]
+#[note(hir_analysis_ty_of_assoc_const_binding_note)]
+pub(crate) struct TyOfAssocConstBindingNote<'tcx> {
+    pub assoc_const: Ident,
+    pub ty: Ty<'tcx>,
+}
+
+#[derive(Diagnostic)]
+#[diag(hir_analysis_escaping_bound_var_in_ty_of_assoc_const_binding)]
+pub(crate) struct EscapingBoundVarInTyOfAssocConstBinding<'tcx> {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    pub assoc_const: Ident,
+    pub var_name: Symbol,
+    pub var_def_kind: &'static str,
+    #[label(hir_analysis_var_defined_here_label)]
+    pub var_defined_here_label: Span,
+    #[subdiagnostic]
+    pub ty_note: Option<TyOfAssocConstBindingNote<'tcx>>,
+}
+
 #[derive(Subdiagnostic)]
 #[help(hir_analysis_parenthesized_fn_trait_expansion)]
 pub struct ParenthesizedFnTraitExpansion {