Deny #[cfg_attr(pred, type_const)]

compiler/rustc_expand/messages.ftl

@@ -194,6 +194,9 @@ expand_trailing_semi_macro = trailing semicolon in macro used in expression posi
     .note1 = macro invocations at the end of a block are treated as expressions
     .note2 = to ignore the value produced by the macro, add a semicolon after the invocation of `{$name}`
 
+expand_type_const_in_cfg_attr =
+    `type_const` within an `#[cfg_attr]` attribute is forbidden
+
 expand_unknown_macro_variable = unknown macro variable `{$name}`
 
 expand_unsupported_key_value =

compiler/rustc_expand/src/config.rs

@@ -29,7 +29,7 @@ use tracing::instrument;
 use crate::errors::{
     CrateNameInCfgAttr, CrateTypeInCfgAttr, FeatureNotAllowed, FeatureRemoved,
     FeatureRemovedReason, InvalidCfg, MalformedFeatureAttribute, MalformedFeatureAttributeHelp,
-    RemoveExprNotSupported,
+    RemoveExprNotSupported, TypeConstInCfgAttr,
 };
 
 /// A folder that strips out items that do not belong in the current configuration.
@@ -319,11 +319,13 @@ impl<'a> StripUnconfigured<'a> {
             //  `#[cfg_attr(false, cfg_attr(true, some_attr))]`.
             let expanded_attrs = expanded_attrs
                 .into_iter()
-                .flat_map(|item| self.process_cfg_attr(&self.expand_cfg_attr_item(cfg_attr, item)));
+                .flat_map(|item| self.expand_cfg_attr_item(cfg_attr, item))
+                .flat_map(|attr| self.process_cfg_attr(&attr));
             iter::once(trace_attr).chain(expanded_attrs).collect()
         } else {
-            let expanded_attrs =
-                expanded_attrs.into_iter().map(|item| self.expand_cfg_attr_item(cfg_attr, item));
+            let expanded_attrs = expanded_attrs
+                .into_iter()
+                .flat_map(|item| self.expand_cfg_attr_item(cfg_attr, item));
             iter::once(trace_attr).chain(expanded_attrs).collect()
         }
     }
@@ -332,7 +334,7 @@ impl<'a> StripUnconfigured<'a> {
         &self,
         cfg_attr: &Attribute,
         (item, item_span): (ast::AttrItem, Span),
-    ) -> Attribute {
+    ) -> Option<Attribute> {
         // Convert `#[cfg_attr(pred, attr)]` to `#[attr]`.
 
         // Use the `#` from `#[cfg_attr(pred, attr)]` in the result `#[attr]`.
@@ -388,7 +390,11 @@ impl<'a> StripUnconfigured<'a> {
         if attr.has_name(sym::crate_name) {
             self.sess.dcx().emit_err(CrateNameInCfgAttr { span: attr.span });
         }
-        attr
+        if attr.has_name(sym::type_const) {
+            self.sess.dcx().emit_err(TypeConstInCfgAttr { span: attr.span });
+            return None;
+        }
+        Some(attr)
     }
 
     /// Determines if a node with the given attributes should be included in this configuration.

compiler/rustc_expand/src/errors.rs

@@ -439,6 +439,13 @@ pub(crate) struct CrateTypeInCfgAttr {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(expand_type_const_in_cfg_attr)]
+pub(crate) struct TypeConstInCfgAttr {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(expand_glob_delegation_traitless_qpath)]
 pub(crate) struct GlobDelegationTraitlessQpath {

tests/ui/const-generics/mgca/type-const-within-cfg-attr.rs

@@ -0,0 +1,16 @@
+//@ needs-rustc-debug-assertions
+
+#![feature(min_generic_const_args)]
+#![expect(incomplete_features)]
+
+pub trait Foo {
+    #[cfg_attr(true, type_const)] //~ ERROR `type_const` within an `#[cfg_attr]` attribute is forbidden
+    const N: usize;
+}
+
+impl Foo for u32 {
+    #[cfg_attr(true, type_const)] //~ ERROR `type_const` within an `#[cfg_attr]` attribute is forbidden
+    const N: usize = 99;
+}
+
+fn main() {}

tests/ui/const-generics/mgca/type-const-within-cfg-attr.stderr

@@ -0,0 +1,14 @@
+error: `type_const` within an `#[cfg_attr]` attribute is forbidden
+  --> $DIR/type-const-within-cfg-attr.rs:7:22
+   |
+LL |     #[cfg_attr(true, type_const)]
+   |                      ^^^^^^^^^^
+
+error: `type_const` within an `#[cfg_attr]` attribute is forbidden
+  --> $DIR/type-const-within-cfg-attr.rs:12:22
+   |
+LL |     #[cfg_attr(true, type_const)]
+   |                      ^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+