From b5d7da878fa329b37f06d37c6a1a4d049953e7a0 Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <n.nethercote@gmail.com>
Date: Fri, 1 Mar 2024 13:00:06 +1100
Subject: [PATCH 1/2] Decouple `DummyAstNode` and `DummyResult`.

They are two different ways of creating dummy results, with two
different purposes. Their implementations are separate except for
crates, where `DummyResult` depends on `DummyAstNode`.

This commit removes that dependency, so they are now fully separate. It
also expands the comment on `DummyAstNode`.
---
 compiler/rustc_ast/src/mut_visit.rs |  5 ++++-
 compiler/rustc_expand/src/base.rs   | 14 ++++++++++++--
 2 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/compiler/rustc_ast/src/mut_visit.rs b/compiler/rustc_ast/src/mut_visit.rs
index bbbb34f7f2f48..da4328d8ce267 100644
--- a/compiler/rustc_ast/src/mut_visit.rs
+++ b/compiler/rustc_ast/src/mut_visit.rs
@@ -1604,7 +1604,10 @@ pub fn noop_visit_capture_by<T: MutVisitor>(capture_by: &mut CaptureBy, vis: &mu
     }
 }
 
-/// Some value for the AST node that is valid but possibly meaningless.
+/// Some value for the AST node that is valid but possibly meaningless. Similar
+/// to `Default` but not intended for wide use. The value will never be used
+/// meaningfully, it exists just to support unwinding in `visit_clobber` in the
+/// case where its closure panics.
 pub trait DummyAstNode {
     fn dummy() -> Self;
 }
diff --git a/compiler/rustc_expand/src/base.rs b/compiler/rustc_expand/src/base.rs
index 485f0e7e46deb..d8ac4a540378e 100644
--- a/compiler/rustc_expand/src/base.rs
+++ b/compiler/rustc_expand/src/base.rs
@@ -4,7 +4,6 @@ use crate::expand::{self, AstFragment, Invocation};
 use crate::module::DirOwnership;
 
 use rustc_ast::attr::MarkedAttrs;
-use rustc_ast::mut_visit::DummyAstNode;
 use rustc_ast::ptr::P;
 use rustc_ast::token::{self, Nonterminal};
 use rustc_ast::tokenstream::TokenStream;
@@ -582,6 +581,17 @@ impl DummyResult {
             tokens: None,
         })
     }
+
+    /// A plain dummy crate.
+    pub fn raw_crate() -> ast::Crate {
+        ast::Crate {
+            attrs: Default::default(),
+            items: Default::default(),
+            spans: Default::default(),
+            id: ast::DUMMY_NODE_ID,
+            is_placeholder: Default::default(),
+        }
+    }
 }
 
 impl MacResult for DummyResult {
@@ -650,7 +660,7 @@ impl MacResult for DummyResult {
     }
 
     fn make_crate(self: Box<DummyResult>) -> Option<ast::Crate> {
-        Some(DummyAstNode::dummy())
+        Some(DummyResult::raw_crate())
     }
 }
 

From a9dff2d931ad6844344c1b81f343ccdd1f4abdd9 Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <n.nethercote@gmail.com>
Date: Sat, 2 Mar 2024 08:24:45 +1100
Subject: [PATCH 2/2] Remove unused `impl DummyAstNode for Block`.

---
 compiler/rustc_ast/src/mut_visit.rs | 13 -------------
 1 file changed, 13 deletions(-)

diff --git a/compiler/rustc_ast/src/mut_visit.rs b/compiler/rustc_ast/src/mut_visit.rs
index da4328d8ce267..c517c6138ceb6 100644
--- a/compiler/rustc_ast/src/mut_visit.rs
+++ b/compiler/rustc_ast/src/mut_visit.rs
@@ -1682,19 +1682,6 @@ impl DummyAstNode for Stmt {
     }
 }
 
-impl DummyAstNode for Block {
-    fn dummy() -> Self {
-        Block {
-            stmts: Default::default(),
-            id: DUMMY_NODE_ID,
-            rules: BlockCheckMode::Default,
-            span: Default::default(),
-            tokens: Default::default(),
-            could_be_bare_literal: Default::default(),
-        }
-    }
-}
-
 impl DummyAstNode for Crate {
     fn dummy() -> Self {
         Crate {