Rollup merge of #141430 - fee1-dead-contrib:push-nmzoprvtsvww, r=petrochenkov

remove `visit_clobber` and move `DummyAstNode` to `rustc_expand`

`visit_clobber` is not really useful except for one niche purpose
involving generic code. We should just use the replace logic where we
can.

3 files changed, 20 insertions, 104 deletions

diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
index 2ececee8751..7b103126e45 100644
--- a/compiler/rustc_ast/src/ast.rs
+++ b/compiler/rustc_ast/src/ast.rs
@@ -32,7 +32,7 @@ use rustc_data_structures::tagged_ptr::Tag;
 use rustc_macros::{Decodable, Encodable, HashStable_Generic};
 pub use rustc_span::AttrId;
 use rustc_span::source_map::{Spanned, respan};
-use rustc_span::{ErrorGuaranteed, Ident, Span, Symbol, kw, sym};
+use rustc_span::{DUMMY_SP, ErrorGuaranteed, Ident, Span, Symbol, kw, sym};
 use thin_vec::{ThinVec, thin_vec};
 
 pub use crate::format::*;
@@ -1526,6 +1526,19 @@ impl Expr {
                 | ExprKind::Struct(_)
         )
     }
+
+    /// Creates a dummy `P<Expr>`.
+    ///
+    /// Should only be used when it will be replaced afterwards or as a return value when an error was encountered.
+    pub fn dummy() -> P<Expr> {
+        P(Expr {
+            id: DUMMY_NODE_ID,
+            kind: ExprKind::Dummy,
+            span: DUMMY_SP,
+            attrs: ThinVec::new(),
+            tokens: None,
+        })
+    }
 }
 
 #[derive(Clone, Encodable, Decodable, Debug)]
diff --git a/compiler/rustc_ast/src/ast_traits.rs b/compiler/rustc_ast/src/ast_traits.rs
index 21de7ff7719..797ab297319 100644
--- a/compiler/rustc_ast/src/ast_traits.rs
+++ b/compiler/rustc_ast/src/ast_traits.rs
@@ -304,6 +304,7 @@ impl HasAttrs for Stmt {
 }
 
 /// A newtype around an AST node that implements the traits above if the node implements them.
+#[repr(transparent)]
 pub struct AstNodeWrapper<Wrapped, Tag> {
     pub wrapped: Wrapped,
     pub tag: PhantomData<Tag>,
@@ -313,6 +314,11 @@ impl<Wrapped, Tag> AstNodeWrapper<Wrapped, Tag> {
     pub fn new(wrapped: Wrapped, _tag: Tag) -> AstNodeWrapper<Wrapped, Tag> {
         AstNodeWrapper { wrapped, tag: Default::default() }
     }
+
+    pub fn from_mut(wrapped: &mut Wrapped, _tag: Tag) -> &mut AstNodeWrapper<Wrapped, Tag> {
+        // SAFETY: `AstNodeWrapper` is `repr(transparent)` w.r.t `Wrapped`
+        unsafe { &mut *<*mut Wrapped>::cast(wrapped) }
+    }
 }
 
 impl<Wrapped: HasNodeId, Tag> HasNodeId for AstNodeWrapper<Wrapped, Tag> {
diff --git a/compiler/rustc_ast/src/mut_visit.rs b/compiler/rustc_ast/src/mut_visit.rs
index 1700c701e8e..f1b183e25b5 100644
--- a/compiler/rustc_ast/src/mut_visit.rs
+++ b/compiler/rustc_ast/src/mut_visit.rs
@@ -368,14 +368,6 @@ pub trait MutVisitor: Sized {
 
 super::common_visitor_and_walkers!((mut) MutVisitor);
 
-/// Use a map-style function (`FnOnce(T) -> T`) to overwrite a `&mut T`. Useful
-/// when using a `flat_map_*` or `filter_map_*` method within a `visit_`
-/// method.
-pub fn visit_clobber<T: DummyAstNode>(t: &mut T, f: impl FnOnce(T) -> T) {
-    let old_t = std::mem::replace(t, T::dummy());
-    *t = f(old_t);
-}
-
 #[inline]
 fn visit_vec<T, F>(elems: &mut Vec<T>, mut visit_elem: F)
 where
@@ -1417,101 +1409,6 @@ fn walk_capture_by<T: MutVisitor>(vis: &mut T, capture_by: &mut CaptureBy) {
     }
 }
 
-/// 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;
-}
-
-impl<T> DummyAstNode for Option<T> {
-    fn dummy() -> Self {
-        Default::default()
-    }
-}
-
-impl<T: DummyAstNode + 'static> DummyAstNode for P<T> {
-    fn dummy() -> Self {
-        P(DummyAstNode::dummy())
-    }
-}
-
-impl DummyAstNode for Item {
-    fn dummy() -> Self {
-        Item {
-            attrs: Default::default(),
-            id: DUMMY_NODE_ID,
-            span: Default::default(),
-            vis: Visibility {
-                kind: VisibilityKind::Public,
-                span: Default::default(),
-                tokens: Default::default(),
-            },
-            kind: ItemKind::ExternCrate(None, Ident::dummy()),
-            tokens: Default::default(),
-        }
-    }
-}
-
-impl DummyAstNode for Expr {
-    fn dummy() -> Self {
-        Expr {
-            id: DUMMY_NODE_ID,
-            kind: ExprKind::Dummy,
-            span: Default::default(),
-            attrs: Default::default(),
-            tokens: Default::default(),
-        }
-    }
-}
-
-impl DummyAstNode for Ty {
-    fn dummy() -> Self {
-        Ty {
-            id: DUMMY_NODE_ID,
-            kind: TyKind::Dummy,
-            span: Default::default(),
-            tokens: Default::default(),
-        }
-    }
-}
-
-impl DummyAstNode for Pat {
-    fn dummy() -> Self {
-        Pat {
-            id: DUMMY_NODE_ID,
-            kind: PatKind::Wild,
-            span: Default::default(),
-            tokens: Default::default(),
-        }
-    }
-}
-
-impl DummyAstNode for Stmt {
-    fn dummy() -> Self {
-        Stmt { id: DUMMY_NODE_ID, kind: StmtKind::Empty, span: Default::default() }
-    }
-}
-
-impl DummyAstNode for Crate {
-    fn dummy() -> Self {
-        Crate {
-            attrs: Default::default(),
-            items: Default::default(),
-            spans: Default::default(),
-            id: DUMMY_NODE_ID,
-            is_placeholder: Default::default(),
-        }
-    }
-}
-
-impl<N: DummyAstNode, T: DummyAstNode> DummyAstNode for crate::ast_traits::AstNodeWrapper<N, T> {
-    fn dummy() -> Self {
-        crate::ast_traits::AstNodeWrapper::new(N::dummy(), T::dummy())
-    }
-}
-
 #[derive(Debug)]
 pub enum FnKind<'a> {
     /// E.g., `fn foo()`, `fn foo(&self)`, or `extern "Abi" fn foo()`.