Rollup merge of rust-lang#72380 - lcnr:const_context, r=estebank

Fix `is_const_context`, update `check_for_cast`

A better version of rust-lang#71477

Adds `fn enclosing_body_owner` and uses it in `is_const_context`.
`is_const_context` now uses the same mechanism as `mir_const_qualif` as it was previously incorrect.
Renames `is_const_context` to `is_inside_const_context`.

I also updated `check_for_cast` in the second commit, so r? @estebank

(I removed one lvl of indentation, so it might be easier to review by hiding whitespace changes)

Author: RalfJung

src/librustc_middle/hir/map/mod.rs

@@ -334,6 +334,16 @@ impl<'hir> Map<'hir> {
         }
     }
 
+    pub fn enclosing_body_owner(&self, hir_id: HirId) -> HirId {
+        for (parent, _) in self.parent_iter(hir_id) {
+            if let Some(body) = self.maybe_body_owned_by(parent) {
+                return self.body_owner(body);
+            }
+        }
+
+        bug!("no `enclosing_body_owner` for hir_id `{}`", hir_id);
+    }
+
     /// Returns the `HirId` that corresponds to the definition of
     /// which this is the body of, i.e., a `fn`, `const` or `static`
     /// item (possibly associated), a closure, or a `hir::AnonConst`.
@@ -536,18 +546,8 @@ impl<'hir> Map<'hir> {
 
     /// Whether the expression pointed at by `hir_id` belongs to a `const` evaluation context.
     /// Used exclusively for diagnostics, to avoid suggestion function calls.
-    pub fn is_const_context(&self, hir_id: HirId) -> bool {
-        let parent_id = self.get_parent_item(hir_id);
-        match self.get(parent_id) {
-            Node::Item(&Item { kind: ItemKind::Const(..) | ItemKind::Static(..), .. })
-            | Node::TraitItem(&TraitItem { kind: TraitItemKind::Const(..), .. })
-            | Node::ImplItem(&ImplItem { kind: ImplItemKind::Const(..), .. })
-            | Node::AnonConst(_) => true,
-            Node::Item(&Item { kind: ItemKind::Fn(ref sig, ..), .. }) => {
-                sig.header.constness == Constness::Const
-            }
-            _ => false,
-        }
+    pub fn is_inside_const_context(&self, hir_id: HirId) -> bool {
+        self.body_const_context(self.local_def_id(self.enclosing_body_owner(hir_id))).is_some()
     }
 
     /// Whether `hir_id` corresponds to a `mod` or a crate.

src/librustc_mir/borrow_check/diagnostics/mutability_errors.rs

@@ -495,7 +495,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         let closure_id = hir.as_local_hir_id(self.mir_def_id);
         let fn_call_id = hir.get_parent_node(closure_id);
         let node = hir.get(fn_call_id);
-        let item_id = hir.get_parent_item(fn_call_id);
+        let item_id = hir.enclosing_body_owner(fn_call_id);
         let mut look_at_return = true;
         // If we can detect the expression to be an `fn` call where the closure was an argument,
         // we point at the `fn` definition argument...