Make "parent also has host effect param?" a span_delayed_bug.

Fixes rust-lang#121418.

Author: nnethercote

compiler/rustc_hir_analysis/src/collect/generics_of.rs

@@ -315,7 +315,7 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
 
             if is_host_effect {
                 if let Some(idx) = host_effect_index {
-                    tcx.dcx().span_bug(
+                    tcx.dcx().span_delayed_bug(
                         param.span,
                         format!("parent also has host effect param? index: {idx}, def: {def_id:?}"),
                     );

tests/ui/generics/issue-121418.rs

@@ -0,0 +1,13 @@
+#![feature(const_trait_impl)]
+#![feature(effects)]
+
+struct S;
+trait T {}
+
+impl const dyn T {
+    //~^ ERROR inherent impls cannot be `const`
+    //~| ERROR the const parameter `host` is not constrained by the impl trait, self type, or
+    pub const fn new() -> std::sync::Mutex<dyn T> {}
+}
+
+fn main() {}

tests/ui/generics/issue-121418.stderr

@@ -0,0 +1,22 @@
+error: inherent impls cannot be `const`
+  --> $DIR/issue-121418.rs:7:12
+   |
+LL | impl const dyn T {
+   |      ----- ^^^^^ inherent impl for this type
+   |      |
+   |      `const` because of this
+   |
+   = note: only trait implementations may be annotated with `const`
+
+error[E0207]: the const parameter `host` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/issue-121418.rs:7:6
+   |
+LL | impl const dyn T {
+   |      ^^^^^ unconstrained const parameter
+   |
+   = note: expressions using a const parameter must map each value to a distinct output value
+   = note: proving the result of expressions other than the parameter are unique is not supported
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0207`.