Don't synthesize host effect args inside trait object types

compiler/rustc_ast_lowering/src/lib.rs

@@ -1454,19 +1454,21 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 let (bounds, lifetime_bound) = self.with_dyn_type_scope(true, |this| {
                     let bounds =
                         this.arena.alloc_from_iter(bounds.iter().filter_map(|bound| match bound {
-                            GenericBound::Trait(
-                                ty,
-                                TraitBoundModifiers {
-                                    polarity: BoundPolarity::Positive | BoundPolarity::Negative(_),
-                                    constness,
-                                },
-                            ) => Some(this.lower_poly_trait_ref(ty, itctx, *constness)),
-                            // We can safely ignore constness here, since AST validation
-                            // will take care of invalid modifier combinations.
-                            GenericBound::Trait(
-                                _,
-                                TraitBoundModifiers { polarity: BoundPolarity::Maybe(_), .. },
-                            ) => None,
+                            // We can safely ignore constness here since AST validation
+                            // takes care of rejecting invalid modifier combinations and
+                            // const trait bounds in trait object types.
+                            GenericBound::Trait(ty, modifiers) => match modifiers.polarity {
+                                BoundPolarity::Positive | BoundPolarity::Negative(_) => {
+                                    Some(this.lower_poly_trait_ref(
+                                        ty,
+                                        itctx,
+                                        // Still, don't pass along the constness here; we don't want to
+                                        // synthesize any host effect args, it'd only cause problems.
+                                        ast::BoundConstness::Never,
+                                    ))
+                                }
+                                BoundPolarity::Maybe(_) => None,
+                            },
                             GenericBound::Outlives(lifetime) => {
                                 if lifetime_bound.is_none() {
                                     lifetime_bound = Some(this.lower_lifetime(lifetime));

tests/ui/rfcs/rfc-2632-const-trait-impl/const-trait-bounds-trait-objects.rs

@@ -1,9 +1,17 @@
-#![feature(const_trait_impl)]
+#![feature(const_trait_impl, effects)]
 // edition: 2021
 
 #[const_trait]
 trait Trait {}
 
 fn main() {
     let _: &dyn const Trait; //~ ERROR const trait bounds are not allowed in trait object types
+    let _: &dyn ~const Trait; //~ ERROR `~const` is not allowed here
 }
+
+// Regression test for issue #119525.
+trait NonConst {}
+const fn handle(_: &dyn const NonConst) {}
+//~^ ERROR const trait bounds are not allowed in trait object types
+const fn take(_: &dyn ~const NonConst) {}
+//~^ ERROR `~const` is not allowed here

tests/ui/rfcs/rfc-2632-const-trait-impl/const-trait-bounds-trait-objects.stderr

@@ -4,5 +4,27 @@ error: const trait bounds are not allowed in trait object types
 LL |     let _: &dyn const Trait;
    |                 ^^^^^^^^^^^
 
-error: aborting due to 1 previous error
+error: `~const` is not allowed here
+  --> $DIR/const-trait-bounds-trait-objects.rs:9:17
+   |
+LL |     let _: &dyn ~const Trait;
+   |                 ^^^^^^
+   |
+   = note: trait objects cannot have `~const` trait bounds
+
+error: const trait bounds are not allowed in trait object types
+  --> $DIR/const-trait-bounds-trait-objects.rs:14:25
+   |
+LL | const fn handle(_: &dyn const NonConst) {}
+   |                         ^^^^^^^^^^^^^^
+
+error: `~const` is not allowed here
+  --> $DIR/const-trait-bounds-trait-objects.rs:16:23
+   |
+LL | const fn take(_: &dyn ~const NonConst) {}
+   |                       ^^^^^^
+   |
+   = note: trait objects cannot have `~const` trait bounds
+
+error: aborting due to 4 previous errors