Rollup merge of #144000 - jacob-greenfield:stable-defid-parent, r=celinval

Add `DefId::parent()` accessor for `rustc_public`

Adds a `parent()` method to `DefId` (the `rustc_pub` version) which exposes the parent path, ie. `foo::bar::baz` -> `foo::bar`.

This is useful for organizing/grouping definitions into a tree, and is probably simpler and less brittle than attempting to parse the fully-qualified name into path components (e.g. especially when handling path components with qualified generic parameters).

compiler/rustc_public/src/compiler_interface.rs

@@ -249,6 +249,14 @@ impl<'tcx> CompilerInterface<'tcx> {
         cx.def_name(did, trimmed)
     }
 
+    /// Returns the parent of the given `DefId`.
+    pub(crate) fn def_parent(&self, def_id: DefId) -> Option<DefId> {
+        let mut tables = self.tables.borrow_mut();
+        let cx = &*self.cx.borrow();
+        let did = tables[def_id];
+        cx.def_parent(did).map(|did| tables.create_def_id(did))
+    }
+
     /// Return registered tool attributes with the given attribute name.
     ///
     /// FIXME(jdonszelmann): may panic on non-tool attributes. After more attribute work, non-tool

compiler/rustc_public/src/crate_def.rs

@@ -28,6 +28,12 @@ impl DefId {
     pub fn trimmed_name(&self) -> Symbol {
         with(|cx| cx.def_name(*self, true))
     }
+
+    /// Return the parent of this definition, or `None` if this is the root of a
+    /// crate.
+    pub fn parent(&self) -> Option<DefId> {
+        with(|cx| cx.def_parent(*self))
+    }
 }
 
 /// A trait for retrieving information about a particular definition.