[detect escaping gen params]

compiler/rustc_hir_analysis/messages.ftl

@@ -285,6 +285,10 @@ hir_analysis_only_current_traits_primitive = only traits defined in the current
 
 hir_analysis_only_current_traits_ty = `{$ty}` is not defined in the current crate
 
+hir_analysis_param_in_ty_of_assoc_const = the type of the associated constant `{$assoc_const}` must not depend on generic parameters
+    .label = its type must not depend on the {$param_kind} parameter `{$param_name}`
+    .param_defined_here_label = the {$param_kind} parameter `{$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
 

compiler/rustc_hir_analysis/src/collect/type_of.rs

@@ -1,3 +1,5 @@
+use std::ops::ControlFlow;
+
 use rustc_errors::{Applicability, StashKey};
 use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
@@ -7,7 +9,8 @@ use rustc_middle::ty::print::with_forced_trimmed_paths;
 use rustc_middle::ty::util::IntTypeExt;
 use rustc_middle::ty::{self, ImplTraitInTraitData, IsSuggestable, Ty, TyCtxt, TypeVisitableExt};
 use rustc_span::symbol::Ident;
-use rustc_span::{Span, DUMMY_SP};
+use rustc_span::{ErrorGuaranteed, Span, Symbol, DUMMY_SP};
+use rustc_type_ir::visit::{TypeSuperVisitable, TypeVisitable, TypeVisitor};
 
 use super::ItemCtxt;
 use super::{bad_placeholder, is_suggestable_infer_ty};
@@ -66,10 +69,24 @@ fn anon_const_type_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Ty<'tcx> {
                 .expect("const parameter types cannot be generic");
         }
 
-        Node::TypeBinding(&TypeBinding { hir_id, .. }) => {
-            // FIXME(fmease): Reject “escaping” early-bound generic parameters.
+        Node::TypeBinding(&TypeBinding { hir_id, ident, .. }) => {
+            let ty = tcx.type_of_assoc_const_binding(hir_id);
+
+            // FIXME(const_generics): Support generic const generics.
+            let Some(ty) = ty.no_bound_vars() else {
+                // We can't possibly catch this in the resolver, therefore we need to handle it here.
+                let guar = report_assoc_const_depends_on_generic_params(
+                    tcx,
+                    ident,
+                    // FIXME(fmease): Improve this situation:
+                    ty.skip_binder().skip_binder(),
+                    hir_id,
+                );
+                return Ty::new_error(tcx, guar);
+            };
+
             // FIXME(fmease): Reject escaping late-bound vars.
-            return tcx.type_of_assoc_const_binding(hir_id).skip_binder().skip_binder();
+            return ty.skip_binder();
         }
 
         // This match arm is for when the def_id appears in a GAT whose
@@ -293,6 +310,64 @@ pub(super) fn type_of_assoc_const_binding<'tcx>(
     ty::EarlyBinder::bind(ty::Binder::dummy(Ty::new_error(tcx, guar)))
 }
 
+fn report_assoc_const_depends_on_generic_params<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    assoc_const: Ident,
+    ty: Ty<'tcx>,
+    hir_id: HirId,
+) -> ErrorGuaranteed {
+    // FIXME(fmease): Find all generic params instead of just the first, it's cleaner.
+    let ControlFlow::Break((param_index, param_name)) = ty.visit_with(&mut GenericParamFinder)
+    else {
+        // FIXME(fmease): Proper ICE message
+        bug!()
+    };
+
+    let body_owner = tcx.hir().enclosing_body_owner(hir_id);
+    let param_def = tcx.generics_of(body_owner).param_at(param_index as _, tcx);
+    struct GenericParamFinder;
+
+    impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for GenericParamFinder {
+        type BreakTy = (u32, Symbol);
+
+        fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
+            if let ty::Param(param) = ty.kind() {
+                return ControlFlow::Break((param.index, param.name));
+            }
+
+            ty.super_visit_with(self)
+        }
+
+        fn visit_region(&mut self, re: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
+            if let ty::ReEarlyParam(param) = re.kind() {
+                return ControlFlow::Break((param.index, param.name));
+            }
+
+            ControlFlow::Continue(())
+        }
+
+        fn visit_const(&mut self, ct: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
+            if let ty::ConstKind::Param(param) = ct.kind() {
+                return ControlFlow::Break((param.index, param.name));
+            }
+
+            ct.super_visit_with(self)
+        }
+    }
+
+    tcx.dcx().emit_err(crate::errors::ParamInTyOfAssocConst {
+        span: assoc_const.span,
+        assoc_const,
+        param_name,
+        param_kind: match &param_def.kind {
+            ty::GenericParamDefKind::Lifetime => "lifetime",
+            ty::GenericParamDefKind::Type { .. } => "type",
+            ty::GenericParamDefKind::Const { .. } => "const",
+        },
+        param_defined_here_label: tcx.def_ident_span(param_def.def_id).unwrap(),
+    })
+}
+
 fn get_path_containing_arg_in_pat<'hir>(
     pat: &'hir hir::Pat<'hir>,
     arg_id: HirId,

compiler/rustc_hir_analysis/src/errors.rs

@@ -254,6 +254,19 @@ pub struct AssocTypeBindingNotAllowed {
     pub fn_trait_expansion: Option<ParenthesizedFnTraitExpansion>,
 }
 
+#[derive(Diagnostic)]
+#[diag(hir_analysis_param_in_ty_of_assoc_const)]
+pub(crate) struct ParamInTyOfAssocConst {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    pub assoc_const: Ident,
+    pub param_name: Symbol,
+    pub param_kind: &'static str,
+    #[label(hir_analysis_param_defined_here_label)]
+    pub param_defined_here_label: Span,
+}
+
 #[derive(Subdiagnostic)]
 #[help(hir_analysis_parenthesized_fn_trait_expansion)]
 pub struct ParenthesizedFnTraitExpansion {

tests/ui/associated-consts/assoc-const-eq-param-in-ty.rs

@@ -0,0 +1,38 @@
+// Regression test for issue #108271.
+// Detect and reject generic params in the type of assoc consts used in an equality bound.
+#![feature(associated_const_equality)]
+
+trait Trait<'a, T, const N: usize> {
+    const K: &'a [T; N];
+}
+
+fn take0<'r>(_: impl Trait<'r, (), 0, K = { &[] }>) {}
+//~^ ERROR the type of the associated constant `K` must not depend on generic parameters
+//~| NOTE its type must not depend on the lifetime parameter `'r`
+//~| NOTE the lifetime parameter `'r` is defined here
+fn take1<A>(_: impl Trait<'static, A, 0, K = { &[] }>) {}
+//~^ ERROR the type of the associated constant `K` must not depend on generic parameters
+//~| NOTE its type must not depend on the type parameter `A`
+//~| NOTE the type parameter `A` is defined here
+fn take2<const Q: usize>(_: impl Trait<'static, (), Q, K = { [] }>) {}
+//~^ ERROR the type of the associated constant `K` must not depend on generic parameters
+//~| NOTE its type must not depend on the const parameter `Q`
+//~| NOTE the const parameter `Q` is defined here
+
+trait Project {
+    const S: Self;
+}
+
+// FIXME(fmease): Clean this up, treat synthetic type parameters differently in diags.
+
+fn take3(_: impl Project<S = {}>) {}
+//~^ ERROR the type of the associated constant `S` must not depend on generic parameters
+//~| NOTE its type must not depend on the type parameter `impl Project<S = const {}>`
+//~| NOTE the type parameter `impl Project<S = const {}>` is defined here
+
+fn take4<P: Project<S = {}>>(_: P) {}
+//~^ ERROR the type of the associated constant `S` must not depend on generic parameters
+//~| NOTE its type must not depend on the type parameter `P`
+//~| NOTE the type parameter `P` is defined here
+
+fn main() {}

tests/ui/associated-consts/assoc-const-eq-param-in-ty.stderr

@@ -0,0 +1,43 @@
+error: the type of the associated constant `K` must not depend on generic parameters
+  --> $DIR/assoc-const-eq-param-in-ty.rs:9:39
+   |
+LL | fn take0<'r>(_: impl Trait<'r, (), 0, K = { &[] }>) {}
+   |          --                           ^ its type must not depend on the lifetime parameter `'r`
+   |          |
+   |          the lifetime parameter `'r` is defined here
+
+error: the type of the associated constant `K` must not depend on generic parameters
+  --> $DIR/assoc-const-eq-param-in-ty.rs:13:42
+   |
+LL | fn take1<A>(_: impl Trait<'static, A, 0, K = { &[] }>) {}
+   |          -                               ^ its type must not depend on the type parameter `A`
+   |          |
+   |          the type parameter `A` is defined here
+
+error: the type of the associated constant `K` must not depend on generic parameters
+  --> $DIR/assoc-const-eq-param-in-ty.rs:17:56
+   |
+LL | fn take2<const Q: usize>(_: impl Trait<'static, (), Q, K = { [] }>) {}
+   |                -                                       ^ its type must not depend on the const parameter `Q`
+   |                |
+   |                the const parameter `Q` is defined here
+
+error: the type of the associated constant `S` must not depend on generic parameters
+  --> $DIR/assoc-const-eq-param-in-ty.rs:28:26
+   |
+LL | fn take3(_: impl Project<S = {}>) {}
+   |             -------------^------
+   |             |            |
+   |             |            its type must not depend on the type parameter `impl Project<S = const {}>`
+   |             the type parameter `impl Project<S = const {}>` is defined here
+
+error: the type of the associated constant `S` must not depend on generic parameters
+  --> $DIR/assoc-const-eq-param-in-ty.rs:33:21
+   |
+LL | fn take4<P: Project<S = {}>>(_: P) {}
+   |          -          ^ its type must not depend on the type parameter `P`
+   |          |
+   |          the type parameter `P` is defined here
+
+error: aborting due to 5 previous errors
+