Rollup merge of #92037 - fee1-dead:fix_env_dmbic, r=oli-obk

Use a const ParamEnv when in default_method_body_is_const

r? `@oli-obk`

This PR fixes the param_env function to return `constness: Const` correctly for trait methods marked with `#[default_method_body_is_const]`. The snippet below is erroneously accepted by the compiler and has been fixed by this change. ([Playground](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=12dc6681b2eeee5f604203d96259eeb4))

```rust
#![feature(const_fn_trait_bound)]
#![feature(const_trait_impl)]

trait Tr {}
impl Tr for () {}

const fn foo<T>() where T: ~const Tr {}

pub trait Foo {
    #[default_method_body_is_const]
    fn foo() {
        foo::<()>();
    }
}
```

compiler/rustc_ty_utils/src/ty.rs

@@ -3,7 +3,7 @@ use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::ty::subst::Subst;
 use rustc_middle::ty::{self, Binder, Predicate, PredicateKind, ToPredicate, Ty, TyCtxt};
-use rustc_span::Span;
+use rustc_span::{sym, Span};
 use rustc_trait_selection::traits;
 
 fn sized_constraint_for_ty<'tcx>(
@@ -285,6 +285,12 @@ fn param_env(tcx: TyCtxt<'_>, def_id: DefId) -> ty::ParamEnv<'_> {
 
     let constness = match hir_id {
         Some(hir_id) => match tcx.hir().get(hir_id) {
+            hir::Node::TraitItem(hir::TraitItem { kind: hir::TraitItemKind::Fn(..), .. })
+                if tcx.has_attr(def_id, sym::default_method_body_is_const) =>
+            {
+                hir::Constness::Const
+            }
+
             hir::Node::Item(hir::Item { kind: hir::ItemKind::Const(..), .. })
             | hir::Node::Item(hir::Item { kind: hir::ItemKind::Static(..), .. })
             | hir::Node::TraitItem(hir::TraitItem {

src/test/ui/rfc-2632-const-trait-impl/default-method-body-is-const-body-checking.rs

@@ -0,0 +1,17 @@
+#![feature(const_fn_trait_bound)]
+#![feature(const_trait_impl)]
+
+trait Tr {}
+impl Tr for () {}
+
+const fn foo<T>() where T: ~const Tr {}
+
+pub trait Foo {
+    #[default_method_body_is_const]
+    fn foo() {
+        foo::<()>();
+        //~^ ERROR the trait bound `(): Tr` is not satisfied
+    }
+}
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/default-method-body-is-const-body-checking.stderr

@@ -0,0 +1,19 @@
+error[E0277]: the trait bound `(): Tr` is not satisfied
+  --> $DIR/default-method-body-is-const-body-checking.rs:12:15
+   |
+LL |         foo::<()>();
+   |               ^^ the trait `Tr` is not implemented for `()`
+   |
+note: required by a bound in `foo`
+  --> $DIR/default-method-body-is-const-body-checking.rs:7:28
+   |
+LL | const fn foo<T>() where T: ~const Tr {}
+   |                            ^^^^^^^^^ required by this bound in `foo`
+help: consider introducing a `where` bound, but there might be an alternative better way to express this requirement
+   |
+LL | pub trait Foo where (): Tr {
+   |               ++++++++++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.