effects: support ~const in assoc fns in trait impls

Author: fmease

compiler/rustc_ast_lowering/src/item.rs

@@ -12,6 +12,7 @@ use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{LocalDefId, CRATE_DEF_ID};
 use rustc_hir::PredicateOrigin;
 use rustc_index::{Idx, IndexSlice, IndexVec};
+use rustc_middle::span_bug;
 use rustc_middle::ty::{ResolverAstLowering, TyCtxt};
 use rustc_span::edit_distance::find_best_match_for_name;
 use rustc_span::symbol::{kw, sym, Ident};
@@ -572,23 +573,25 @@ impl<'hir> LoweringContext<'_, 'hir> {
         // This is used to track which lifetimes have already been defined,
         // and which need to be replicated when lowering an async fn.
 
-        match parent_hir.node().expect_item().kind {
+        let generics = match parent_hir.node().expect_item().kind {
             hir::ItemKind::Impl(impl_) => {
                 self.is_in_trait_impl = impl_.of_trait.is_some();
+                &impl_.generics
             }
-            hir::ItemKind::Trait(_, _, generics, _, _) if self.tcx.features().effects => {
-                self.host_param_id = generics
-                    .params
-                    .iter()
-                    .find(|param| {
-                        matches!(
-                            param.kind,
-                            hir::GenericParamKind::Const { is_host_effect: true, .. }
-                        )
-                    })
-                    .map(|param| param.def_id);
+            hir::ItemKind::Trait(_, _, generics, _, _) => generics,
+            kind => {
+                span_bug!(item.span, "assoc item has unexpected kind of parent: {}", kind.descr())
             }
-            _ => {}
+        };
+
+        if self.tcx.features().effects {
+            self.host_param_id = generics
+                .params
+                .iter()
+                .find(|param| {
+                    matches!(param.kind, hir::GenericParamKind::Const { is_host_effect: true, .. })
+                })
+                .map(|param| param.def_id);
         }
 
         match ctxt {

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

@@ -1,27 +1,30 @@
-error[E0277]: can't compare `impl PartialEq + Destruct + Copy` with `impl PartialEq + Destruct + Copy`
-  --> $DIR/const-impl-trait.rs:28:17
+error[E0277]: can't compare `()` with `()`
+  --> $DIR/const-impl-trait.rs:35:17
    |
-LL |     fn huh() -> impl ~const PartialEq + ~const Destruct + Copy {
-   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no implementation for `impl PartialEq + Destruct + Copy == impl PartialEq + Destruct + Copy`
+LL |     assert!(cmp(&()));
+   |             --- ^^^ no implementation for `() == ()`
+   |             |
+   |             required by a bound introduced by this call
    |
-   = help: the trait `~const PartialEq` is not implemented for `impl PartialEq + Destruct + Copy`
-note: required by a bound in `Foo::{opaque#0}`
-  --> $DIR/const-impl-trait.rs:24:22
+   = help: the trait `const PartialEq` is not implemented for `()`
+   = help: the trait `PartialEq` is implemented for `()`
+note: required by a bound in `cmp`
+  --> $DIR/const-impl-trait.rs:12:23
    |
-LL |     fn huh() -> impl ~const PartialEq + ~const Destruct + Copy;
-   |                      ^^^^^^^^^^^^^^^^ required by this bound in `Foo::{opaque#0}`
+LL | const fn cmp(a: &impl ~const PartialEq) -> bool {
+   |                       ^^^^^^^^^^^^^^^^ required by this bound in `cmp`
 
-error[E0277]: can't drop `impl PartialEq + Destruct + Copy`
-  --> $DIR/const-impl-trait.rs:28:17
+error[E0277]: can't compare `&impl ~const PartialEq` with `&impl ~const PartialEq`
+  --> $DIR/const-impl-trait.rs:13:7
    |
-LL |     fn huh() -> impl ~const PartialEq + ~const Destruct + Copy {
-   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `~const Destruct` is not implemented for `impl PartialEq + Destruct + Copy`
+LL |     a == a
+   |       ^^ no implementation for `&impl ~const PartialEq == &impl ~const PartialEq`
    |
-note: required by a bound in `Foo::{opaque#0}`
-  --> $DIR/const-impl-trait.rs:24:41
+   = help: the trait `~const PartialEq<&impl ~const PartialEq>` is not implemented for `&impl ~const PartialEq`
+help: consider dereferencing both sides of the expression
    |
-LL |     fn huh() -> impl ~const PartialEq + ~const Destruct + Copy;
-   |                                         ^^^^^^^^^^^^^^^ required by this bound in `Foo::{opaque#0}`
+LL |     *a == *a
+   |     +     +
 
 error: aborting due to 2 previous errors
 

tests/ui/rfcs/rfc-2632-const-trait-impl/tilde-const-assoc-fn-in-trait-impl.rs

@@ -0,0 +1,29 @@
+// Regression test for issue #119700.
+// check-pass
+
+#![feature(const_trait_impl, effects)]
+
+#[const_trait]
+trait Main {
+    fn compute<T: ~const Aux>() -> u32;
+}
+
+impl const Main for () {
+    fn compute<T: ~const Aux>() -> u32 {
+        T::generate()
+    }
+}
+
+#[const_trait]
+trait Aux {
+    fn generate() -> u32;
+}
+
+impl const Aux for () {
+    fn generate() -> u32 { 1024 }
+}
+
+fn main() {
+    const _: u32 = <()>::compute::<()>();
+    let _ = <()>::compute::<()>();
+}