Emit fatal on defaults for generic params in binders if with nested defs

compiler/rustc_ast_lowering/src/expr.rs

@@ -29,7 +29,7 @@ use super::{
 use crate::errors::{InvalidLegacyConstGenericArg, UseConstGenericArg, YieldInClosure};
 use crate::{AllowReturnTypeNotation, FnDeclKind, ImplTraitPosition, TryBlockScope};
 
-struct WillCreateDefIdsVisitor {}
+pub(super) struct WillCreateDefIdsVisitor;
 
 impl<'v> rustc_ast::visit::Visitor<'v> for WillCreateDefIdsVisitor {
     type Result = ControlFlow<Span>;
@@ -479,18 +479,18 @@ impl<'hir, R: ResolverAstLoweringExt<'hir>> LoweringContext<'_, 'hir, R> {
                     DefPathData::LateAnonConst,
                     f.span,
                 );
-                let mut visitor = WillCreateDefIdsVisitor {};
-                let const_value = if let ControlFlow::Break(span) = visitor.visit_expr(&arg) {
-                    Box::new(Expr {
-                        id: self.next_node_id(),
-                        kind: ExprKind::Err(invalid_expr_error(self.tcx, span)),
-                        span: f.span,
-                        attrs: [].into(),
-                        tokens: None,
-                    })
-                } else {
-                    arg
-                };
+                let const_value =
+                    if let ControlFlow::Break(span) = WillCreateDefIdsVisitor.visit_expr(&arg) {
+                        Box::new(Expr {
+                            id: self.next_node_id(),
+                            kind: ExprKind::Err(invalid_expr_error(self.tcx, span)),
+                            span: f.span,
+                            attrs: [].into(),
+                            tokens: None,
+                        })
+                    } else {
+                        arg
+                    };
 
                 let anon_const = AnonConst {
                     id: node_id,

compiler/rustc_ast_lowering/src/lib.rs

@@ -39,7 +39,7 @@ use std::mem;
 use std::sync::Arc;
 
 use rustc_ast::node_id::NodeMap;
-use rustc_ast::visit::AssocCtxt;
+use rustc_ast::visit::{AssocCtxt, Visitor};
 use rustc_ast::{self as ast, *};
 use rustc_attr_parsing::{AttributeParser, Late, OmitDoc};
 use rustc_data_structures::fingerprint::Fingerprint;
@@ -2236,14 +2236,19 @@ impl<'hir, R: ResolverAstLoweringExt<'hir>> LoweringContext<'_, 'hir, R> {
                 // since later compiler stages cannot handle them (and shouldn't need to be able to).
                 let default = default
                     .as_ref()
-                    .filter(|_| match source {
+                    .filter(|anon_const| match source {
                         hir::GenericParamSource::Generics => true,
                         hir::GenericParamSource::Binder => {
-                            self.dcx().emit_err(errors::GenericParamDefaultInBinder {
-                                span: param.span(),
-                            });
-
-                            false
+                            let err = errors::GenericParamDefaultInBinder { span: param.span() };
+                            if expr::WillCreateDefIdsVisitor
+                                .visit_expr(&anon_const.value)
+                                .is_break()
+                            {
+                                self.dcx().emit_fatal(err)
+                            } else {
+                                self.dcx().emit_err(err);
+                                false
+                            }
                         }
                     })
                     .map(|def| self.lower_anon_const_to_const_arg_and_alloc(def));

tests/ui/const-generics/generic_const_exprs/bad-defaults-for-generic-param-in-binder-123629.rs

@@ -0,0 +1,12 @@
+#![feature(generic_assert)]
+
+fn foo()
+where
+    for<const N: usize = { assert!(u) }> ():,
+    //~^ ERROR cannot find value `u` in this scope
+    //~^^ ERROR only lifetime parameters can be used in this context
+    //~^^^ ERROR defaults for generic parameters are not allowed in `for<...>` binders
+{
+}
+
+fn main() {}

tests/ui/const-generics/generic_const_exprs/bad-defaults-for-generic-param-in-binder-123629.stderr

@@ -0,0 +1,26 @@
+error[E0425]: cannot find value `u` in this scope
+  --> $DIR/bad-defaults-for-generic-param-in-binder-123629.rs:5:36
+   |
+LL |     for<const N: usize = { assert!(u) }> ():,
+   |                                    ^ not found in this scope
+
+error[E0658]: only lifetime parameters can be used in this context
+  --> $DIR/bad-defaults-for-generic-param-in-binder-123629.rs:5:15
+   |
+LL |     for<const N: usize = { assert!(u) }> ():,
+   |               ^
+   |
+   = note: see issue #108185 <https://github.com/rust-lang/rust/issues/108185> for more information
+   = help: add `#![feature(non_lifetime_binders)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: defaults for generic parameters are not allowed in `for<...>` binders
+  --> $DIR/bad-defaults-for-generic-param-in-binder-123629.rs:5:9
+   |
+LL |     for<const N: usize = { assert!(u) }> ():,
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0425, E0658.
+For more information about an error, try `rustc --explain E0425`.