Compute proper module parent during resolution

Fixes #75982

The direct parent of a module may not be a module
(e.g. `const _: () =  { #[path = "foo.rs"] mod foo; };`).

To find the parent of a module for purposes of resolution, we need to
walk up the tree until we hit a module or a crate root.

compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

@@ -8,6 +8,7 @@ use rustc_ast as ast;
 use rustc_ast::expand::allocator::AllocatorKind;
 use rustc_data_structures::svh::Svh;
 use rustc_hir as hir;
+use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{CrateNum, DefId, DefIdMap, CRATE_DEF_INDEX, LOCAL_CRATE};
 use rustc_hir::definitions::{DefKey, DefPath, DefPathHash};
 use rustc_middle::hir::exports::Export;
@@ -487,6 +488,10 @@ impl CrateStore for CStore {
         self.get_crate_data(def.krate).def_key(def.index)
     }
 
+    fn def_kind(&self, def: DefId) -> DefKind {
+        self.get_crate_data(def.krate).def_kind(def.index)
+    }
+
     fn def_path(&self, def: DefId) -> DefPath {
         self.get_crate_data(def.krate).def_path(def.index)
     }

compiler/rustc_middle/src/middle/cstore.rs

@@ -8,6 +8,7 @@ use rustc_ast as ast;
 use rustc_ast::expand::allocator::AllocatorKind;
 use rustc_data_structures::svh::Svh;
 use rustc_data_structures::sync::{self, MetadataRef};
+use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{CrateNum, DefId, LOCAL_CRATE};
 use rustc_hir::definitions::{DefKey, DefPath, DefPathHash};
 use rustc_macros::HashStable;
@@ -185,6 +186,7 @@ pub trait CrateStore {
 
     // resolve
     fn def_key(&self, def: DefId) -> DefKey;
+    fn def_kind(&self, def: DefId) -> DefKind;
     fn def_path(&self, def: DefId) -> DefPath;
     fn def_path_hash(&self, def: DefId) -> DefPathHash;
     fn all_def_path_hashes_and_def_ids(&self, cnum: CrateNum) -> Vec<(DefPathHash, DefId)>;

compiler/rustc_resolve/src/build_reduced_graph.rs

@@ -95,6 +95,27 @@ impl<'a> Resolver<'a> {
         }
     }
 
+    /// Walks up the tree of definitions starting at `def_id`,
+    /// stopping at the first `DefKind::Mod` encountered
+    fn nearest_mod_parent(&mut self, def_id: DefId) -> Module<'a> {
+        let def_key = self.cstore().def_key(def_id);
+
+        let mut parent_id = DefId {
+            krate: def_id.krate,
+            index: def_key.parent.expect("failed to get parent for module"),
+        };
+        // The immediate parent may not be a module
+        // (e.g. `const _: () =  { #[path = "foo.rs"] mod foo; };`)
+        // Walk up the tree until we hit a module or the crate root.
+        while parent_id.index != CRATE_DEF_INDEX
+            && self.cstore().def_kind(parent_id) != DefKind::Mod
+        {
+            let parent_def_key = self.cstore().def_key(parent_id);
+            parent_id.index = parent_def_key.parent.expect("failed to get parent for module");
+        }
+        self.get_module(parent_id)
+    }
+
     crate fn get_module(&mut self, def_id: DefId) -> Module<'a> {
         // If this is a local module, it will be in `module_map`, no need to recalculate it.
         if let Some(def_id) = def_id.as_local() {
@@ -116,11 +137,8 @@ impl<'a> Resolver<'a> {
                 .data
                 .get_opt_name()
                 .expect("given a DefId that wasn't a module");
-            // This unwrap is safe since we know this isn't the root
-            let parent = Some(self.get_module(DefId {
-                index: def_key.parent.expect("failed to get parent for module"),
-                ..def_id
-            }));
+
+            let parent = Some(self.nearest_mod_parent(def_id));
             (name, parent)
         };
 
@@ -145,8 +163,24 @@ impl<'a> Resolver<'a> {
         if let Some(id) = def_id.as_local() {
             self.local_macro_def_scopes[&id]
         } else {
-            let module_def_id = ty::DefIdTree::parent(&*self, def_id).unwrap();
-            self.get_module(module_def_id)
+            // This is not entirely correct - a `macro_rules!` macro may occur
+            // inside a 'block' module:
+            //
+            // ```rust
+            // const _: () = {
+            // #[macro_export]
+            // macro_rules! my_macro {
+            //     () => {};
+            // }
+            // `
+            // We don't record this information for external crates, so
+            // the module we compute here will be the closest 'mod' item
+            // (not necesssarily the actual parent of the `macro_rules!`
+            // macro). `macro_rules!` macros can't use def-site hygiene,
+            // so this hopefully won't be a problem.
+            //
+            // See https://github.com/rust-lang/rust/pull/77984#issuecomment-712445508
+            self.nearest_mod_parent(def_id)
         }
     }
 

src/test/ui/macros/auxiliary/issue-75982.rs

@@ -0,0 +1,12 @@
+const _: () = {
+    #[macro_export]
+    macro_rules! first_macro {
+        () => {}
+    }
+    mod foo {
+        #[macro_export]
+        macro_rules! second_macro {
+            () => {}
+        }
+    }
+};

src/test/ui/macros/issue-75982-foreign-macro-weird-mod.rs

@@ -0,0 +1,13 @@
+// aux-build:issue-75982.rs
+// check-pass
+
+// Regression test for issue #75982
+// Tests that don't ICE when invoking a foreign macro
+// that occurs inside a module with a weird parent.
+
+extern crate issue_75982;
+
+fn main() {
+    issue_75982::first_macro!();
+    issue_75982::second_macro!();
+}