Don't synthesize host effect params for trait assoc fns marked const

compiler/rustc_ast_lowering/src/item.rs

@@ -1254,11 +1254,13 @@ impl<'hir> LoweringContext<'_, 'hir> {
         coroutine_kind: Option<CoroutineKind>,
     ) -> (&'hir hir::Generics<'hir>, hir::FnSig<'hir>) {
         let header = self.lower_fn_header(sig.header);
+        // Don't pass along the user-provided constness of trait associated functions; we don't want to
+        // synthesize a host effect param for them. We reject `const` on them during AST validation.
+        let constness = if kind == FnDeclKind::Inherent { sig.header.constness } else { Const::No };
         let itctx = ImplTraitContext::Universal;
-        let (generics, decl) =
-            self.lower_generics(generics, sig.header.constness, id, &itctx, |this| {
-                this.lower_fn_decl(&sig.decl, id, sig.span, kind, coroutine_kind)
-            });
+        let (generics, decl) = self.lower_generics(generics, constness, id, &itctx, |this| {
+            this.lower_fn_decl(&sig.decl, id, sig.span, kind, coroutine_kind)
+        });
         (generics, hir::FnSig { header, decl, span: self.lower_span(sig.span) })
     }
 

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

@@ -0,0 +1,13 @@
+// Regression test for issue #113378.
+#![feature(const_trait_impl, effects)]
+
+#[const_trait]
+trait Trait {
+    const fn fun(); //~ ERROR functions in traits cannot be declared const
+}
+
+impl const Trait for () {
+    const fn fun() {}  //~ ERROR functions in traits cannot be declared const
+}
+
+fn main() {}

tests/ui/rfcs/rfc-2632-const-trait-impl/effects/trait-fn-const.stderr

@@ -0,0 +1,15 @@
+error[E0379]: functions in traits cannot be declared const
+  --> $DIR/trait-fn-const.rs:6:5
+   |
+LL |     const fn fun();
+   |     ^^^^^ functions in traits cannot be const
+
+error[E0379]: functions in traits cannot be declared const
+  --> $DIR/trait-fn-const.rs:10:5
+   |
+LL |     const fn fun() {}
+   |     ^^^^^ functions in traits cannot be const
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0379`.