Rollup merge of #154957 - lapla-cogito:issue_153891, r=oli-obk

Fix ICE when const closure appears inside a non-const trait method

Fixes rust-lang/rust#153891

`hir_body_const_context()` unconditionally delegated to the parent's const context for const closures, returning `None` when the parent had no const context. This caused `mir_const_qualif()` to hit a `span_bug!`, since `mir_promoted()` had already decided to call it based on the closure's own syntactic constness. Fall back to `ConstContext::ConstFn` when the parent's const context is `None`, so that the const closure body is still properly const-checked rather than triggering an ICE.

Examining [another attempt](https://github.com/rust-lang/rust/pull/153900/changes) at this issue (which has already been closed), I thought that its approach represents a workaround fix to avoid inconsistencies in the caller of `mir_promoted()`, whereas I think the correct behavior is for `hir_body_const_context()` itself to return the proper value.

compiler/rustc_middle/src/hir/map.rs

@@ -319,7 +319,17 @@ pub fn hir_body_const_context(self, local_def_id: LocalDefId) -> Option<ConstCon
             BodyOwnerKind::Fn if self.is_constructor(def_id) => return None,
             // Const closures use their parent's const context
             BodyOwnerKind::Closure if self.is_const_fn(def_id) => {
-                return self.hir_body_const_context(self.local_parent(local_def_id));
+                return Some(
+                    self.hir_body_const_context(self.local_parent(local_def_id)).unwrap_or_else(
+                        || {
+                            assert!(
+                                self.dcx().has_errors().is_some(),
+                                "`const` closure with no enclosing const context",
+                            );
+                            ConstContext::ConstFn
+                        },
+                    ),
+                );
             }
             BodyOwnerKind::Fn if self.is_const_fn(def_id) => ConstContext::ConstFn,
             BodyOwnerKind::Fn | BodyOwnerKind::Closure | BodyOwnerKind::GlobalAsm => return None,

tests/ui/traits/const-traits/const-closure-in-non-const-trait-impl-method.rs

@@ -0,0 +1,17 @@
+#![feature(const_trait_impl)]
+#![feature(const_closures)]
+
+// Regression test for https://github.com/rust-lang/rust/issues/153891
+
+const trait Foo {
+    fn test() -> impl [const] Fn();
+}
+
+impl<T: Foo> Foo for &mut T {
+    const fn test() -> impl [const] Fn() {
+        //~^ ERROR functions in trait impls cannot be declared const
+        const move || {}
+    }
+}
+
+fn main() {}

tests/ui/traits/const-traits/const-closure-in-non-const-trait-impl-method.stderr

@@ -0,0 +1,19 @@
+error[E0379]: functions in trait impls cannot be declared const
+  --> $DIR/const-closure-in-non-const-trait-impl-method.rs:11:5
+   |
+LL |     const fn test() -> impl [const] Fn() {
+   |     ^^^^^ functions in trait impls cannot be const
+   |
+help: remove the `const` ...
+   |
+LL -     const fn test() -> impl [const] Fn() {
+LL +     fn test() -> impl [const] Fn() {
+   |
+help: ... and declare the impl to be const instead
+   |
+LL | impl<T: Foo> const Foo for &mut T {
+   |              +++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0379`.

tests/ui/traits/const-traits/const-closure-in-non-const-trait-method.rs

@@ -0,0 +1,12 @@
+#![feature(const_closures)]
+
+// Regression test for https://github.com/rust-lang/rust/issues/153891
+
+trait Tr {
+    const fn test() {
+        //~^ ERROR functions in traits cannot be declared const
+        (const || {})()
+    }
+}
+
+fn main() {}

tests/ui/traits/const-traits/const-closure-in-non-const-trait-method.stderr

@@ -0,0 +1,12 @@
+error[E0379]: functions in traits cannot be declared const
+  --> $DIR/const-closure-in-non-const-trait-method.rs:6:5
+   |
+LL |     const fn test() {
+   |     ^^^^^-
+   |     |
+   |     functions in traits cannot be const
+   |     help: remove the `const`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0379`.