add test

compiler/rustc_hir_analysis/src/collect/generics_of.rs

@@ -9,7 +9,7 @@ use rustc_hir::def_id::LocalDefId;
 use rustc_middle::ty::{self, TyCtxt};
 use rustc_session::lint;
 use rustc_span::symbol::{kw, Symbol};
-use rustc_span::Span;
+use rustc_span::{sym, Span};
 
 pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
     use rustc_hir::*;
@@ -295,7 +295,11 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
             })
         }
         GenericParamKind::Const { default, .. } => {
-            if !matches!(allow_defaults, Defaults::Allowed) && default.is_some() {
+            // `rustc_host` effect params are allowed to have defaults.
+            if !matches!(allow_defaults, Defaults::Allowed)
+                && default.is_some()
+                && !tcx.has_attr(param.def_id, sym::rustc_host)
+            {
                 tcx.sess.span_err(
                     param.span,
                     "defaults for const parameters are only allowed in \

tests/ui/rfcs/rfc-2632-const-trait-impl/effects/helloworld.rs

@@ -0,0 +1,29 @@
+// check-pass
+
+// gate-test-effects
+// ^ effects doesn't have a gate so we will trick tidy into thinking this is a gate test
+
+#![feature(const_trait_impl, effects, rustc_attrs)]
+
+// ensure we are passing in the correct host effect in always const contexts.
+
+pub const fn hmm</* T, */ #[rustc_host] const host: bool = true>() -> usize {
+    if host {
+        1
+    } else {
+        0
+    }
+}
+
+const _: () = {
+    let x = hmm();
+    assert!(0 == x);
+};
+
+/* FIXME(effects)
+pub const fn uwu(x: [u8; hmm::<()>()]) {
+    let [] = x;
+}
+*/
+
+fn main() {}