Add ast::ExprKind::Dummy

compiler/rustc_ast/src/ast.rs

@@ -1296,23 +1296,10 @@ pub fn precedence(&self) -> ExprPrecedence {
             ExprKind::Yeet(..) => ExprPrecedence::Yeet,
             ExprKind::FormatArgs(..) => ExprPrecedence::FormatArgs,
             ExprKind::Become(..) => ExprPrecedence::Become,
-            ExprKind::Err => ExprPrecedence::Err,
+            ExprKind::Err | ExprKind::Dummy => ExprPrecedence::Err,
         }
     }
 
-    pub fn take(&mut self) -> Self {
-        mem::replace(
-            self,
-            Expr {
-                id: DUMMY_NODE_ID,
-                kind: ExprKind::Err,
-                span: DUMMY_SP,
-                attrs: AttrVec::new(),
-                tokens: None,
-            },
-        )
-    }
-
     /// To a first-order approximation, is this a pattern?
     pub fn is_approximately_pattern(&self) -> bool {
         matches!(
@@ -1532,6 +1519,9 @@ pub enum ExprKind {
 
     /// Placeholder for an expression that wasn't syntactically well formed in some way.
     Err,
+
+    /// Acts as a null expression. Lowering it will always emit a bug.
+    Dummy,
 }
 
 /// Used to differentiate between `for` loops and `for await` loops.

compiler/rustc_ast/src/mut_visit.rs

@@ -1526,7 +1526,7 @@ pub fn noop_visit_expr<T: MutVisitor>(
         }
         ExprKind::Try(expr) => vis.visit_expr(expr),
         ExprKind::TryBlock(body) => vis.visit_block(body),
-        ExprKind::Lit(_) | ExprKind::IncludedBytes(..) | ExprKind::Err => {}
+        ExprKind::Lit(_) | ExprKind::IncludedBytes(..) | ExprKind::Err | ExprKind::Dummy => {}
     }
     vis.visit_id(id);
     vis.visit_span(span);
@@ -1642,7 +1642,7 @@ impl DummyAstNode for Expr {
     fn dummy() -> Self {
         Expr {
             id: DUMMY_NODE_ID,
-            kind: ExprKind::Err,
+            kind: ExprKind::Dummy,
             span: Default::default(),
             attrs: Default::default(),
             tokens: Default::default(),

compiler/rustc_ast/src/util/classify.rs

@@ -89,7 +89,8 @@ pub fn expr_trailing_brace(mut expr: &ast::Expr) -> Option<&ast::Expr> {
             | Paren(_)
             | Try(_)
             | Yeet(None)
-            | Err => break None,
+            | Err
+            | Dummy => break None,
         }
     }
 }

compiler/rustc_ast/src/visit.rs

@@ -1063,7 +1063,7 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) -> V
         }
         ExprKind::Try(subexpression) => try_visit!(visitor.visit_expr(subexpression)),
         ExprKind::TryBlock(body) => try_visit!(visitor.visit_block(body)),
-        ExprKind::Lit(_) | ExprKind::IncludedBytes(..) | ExprKind::Err => {}
+        ExprKind::Lit(_) | ExprKind::IncludedBytes(..) | ExprKind::Err | ExprKind::Dummy => {}
     }
 
     visitor.visit_expr_post(expression)

compiler/rustc_ast_lowering/src/expr.rs

@@ -14,6 +14,7 @@
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
+use rustc_middle::span_bug;
 use rustc_session::errors::report_lit_error;
 use rustc_span::source_map::{respan, Spanned};
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
@@ -331,6 +332,11 @@ pub(super) fn lower_expr_mut(&mut self, e: &Expr) -> hir::Expr<'hir> {
                 ExprKind::Err => {
                     hir::ExprKind::Err(self.dcx().span_delayed_bug(e.span, "lowered ExprKind::Err"))
                 }
+
+                ExprKind::Dummy => {
+                    span_bug!(e.span, "lowered ExprKind::Dummy")
+                }
+
                 ExprKind::Try(sub_expr) => self.lower_expr_try(e.span, sub_expr),
 
                 ExprKind::Paren(_) | ExprKind::ForLoop { .. } => {

compiler/rustc_ast_lowering/src/pat.rs

@@ -331,7 +331,8 @@ fn lower_expr_within_pat(&mut self, expr: &Expr, allow_paths: bool) -> &'hir hir
             ExprKind::Lit(..)
             | ExprKind::ConstBlock(..)
             | ExprKind::IncludedBytes(..)
-            | ExprKind::Err => {}
+            | ExprKind::Err
+            | ExprKind::Dummy => {}
             ExprKind::Path(..) if allow_paths => {}
             ExprKind::Unary(UnOp::Neg, inner) if matches!(inner.kind, ExprKind::Lit(_)) => {}
             _ => {

compiler/rustc_ast_pretty/src/pprust/state/expr.rs

@@ -898,6 +898,11 @@ pub(super) fn print_expr_outer_attr_style(
                 self.word("/*ERROR*/");
                 self.pclose()
             }
+            ast::ExprKind::Dummy => {
+                self.popen();
+                self.word("/*DUMMY*/");
+                self.pclose();
+            }
         }
 
         self.ann.post(self, AnnNode::Expr(expr));

compiler/rustc_builtin_macros/src/assert/context.rs

@@ -303,6 +303,7 @@ fn manage_cond_expr(&mut self, expr: &mut P<Expr>) {
             | ExprKind::Closure(_)
             | ExprKind::ConstBlock(_)
             | ExprKind::Continue(_)
+            | ExprKind::Dummy
             | ExprKind::Err
             | ExprKind::Field(_, _)
             | ExprKind::ForLoop { .. }

compiler/rustc_builtin_macros/src/concat.rs

@@ -68,6 +68,7 @@ pub fn expand_concat(
             ast::ExprKind::Err => {
                 has_errors = true;
             }
+            ast::ExprKind::Dummy => cx.dcx().span_bug(e.span, "concatenating `ExprKind::Dummy`"),
             _ => {
                 missing_literal.push(e.span);
             }

compiler/rustc_builtin_macros/src/concat_bytes.rs

@@ -176,6 +176,7 @@ pub fn expand_concat_bytes(
             ast::ExprKind::Err => {
                 has_errors = true;
             }
+            ast::ExprKind::Dummy => cx.dcx().span_bug(e.span, "concatenating `ExprKind::Dummy`"),
             _ => {
                 missing_literals.push(e.span);
             }

compiler/rustc_expand/src/base.rs

@@ -1279,6 +1279,9 @@ pub fn expr_to_spanned_string<'a>(
             _ => Some((cx.dcx().struct_span_err(expr.span, err_msg), false)),
         },
         ast::ExprKind::Err => None,
+        ast::ExprKind::Dummy => {
+            cx.dcx().span_bug(expr.span, "tried to get a string literal from `ExprKind::Dummy`")
+        }
         _ => Some((cx.dcx().struct_span_err(expr.span, err_msg), false)),
     })
 }

compiler/rustc_parse/src/parser/expr.rs

@@ -3949,7 +3949,8 @@ fn visit_expr(&mut self, e: &mut P<Expr>) {
             | ExprKind::Become(_)
             | ExprKind::IncludedBytes(_)
             | ExprKind::FormatArgs(_)
-            | ExprKind::Err => {
+            | ExprKind::Err
+            | ExprKind::Dummy => {
                 // These would forbid any let expressions they contain already.
             }
         }

compiler/rustc_parse/src/parser/pat.rs

@@ -388,7 +388,7 @@ fn maybe_recover_trailing_expr(
         // Parse `?`, `.f`, `(arg0, arg1, ...)` or `[expr]` until they've all been eaten.
         if let Ok(expr) = snapshot
             .parse_expr_dot_or_call_with(
-                self.mk_expr_err(pat_span), // equivalent to transforming the parsed pattern into an `Expr`
+                self.mk_expr(pat_span, ExprKind::Dummy), // equivalent to transforming the parsed pattern into an `Expr`
                 pat_span,
                 AttrVec::new(),
             )

compiler/rustc_passes/src/hir_stats.rs

@@ -589,7 +589,7 @@ fn visit_expr(&mut self, e: &'v ast::Expr) {
                 If, While, ForLoop, Loop, Match, Closure, Block, Await, TryBlock, Assign,
                 AssignOp, Field, Index, Range, Underscore, Path, AddrOf, Break, Continue, Ret,
                 InlineAsm, FormatArgs, OffsetOf, MacCall, Struct, Repeat, Paren, Try, Yield, Yeet,
-                Become, IncludedBytes, Gen, Err
+                Become, IncludedBytes, Gen, Err, Dummy
             ]
         );
         ast_visit::walk_expr(self, e)

src/tools/clippy/clippy_utils/src/ast_utils.rs

@@ -144,6 +144,7 @@ pub fn eq_expr(l: &Expr, r: &Expr) -> bool {
         (Paren(l), _) => eq_expr(l, r),
         (_, Paren(r)) => eq_expr(l, r),
         (Err, Err) => true,
+        (Dummy, _) | (_, Dummy) => unreachable!("comparing `ExprKind::Dummy`"),
         (Try(l), Try(r)) | (Await(l, _), Await(r, _)) => eq_expr(l, r),
         (Array(l), Array(r)) => over(l, r, |l, r| eq_expr(l, r)),
         (Tup(l), Tup(r)) => over(l, r, |l, r| eq_expr(l, r)),

src/tools/clippy/clippy_utils/src/sugg.rs

@@ -222,7 +222,8 @@ pub fn ast(
             | ast::ExprKind::Array(..)
             | ast::ExprKind::While(..)
             | ast::ExprKind::Await(..)
-            | ast::ExprKind::Err => Sugg::NonParen(snippet_with_context(cx, expr.span, ctxt, default, app).0),
+            | ast::ExprKind::Err
+            | ast::ExprKind::Dummy => Sugg::NonParen(snippet_with_context(cx, expr.span, ctxt, default, app).0),
             ast::ExprKind::Range(ref lhs, ref rhs, RangeLimits::HalfOpen) => Sugg::BinOp(
                 AssocOp::DotDot,
                 lhs.as_ref().map_or("".into(), |lhs| {

src/tools/rustfmt/src/expr.rs

@@ -404,7 +404,7 @@ fn needs_space_after_range(rhs: &ast::Expr) -> bool {
             // These do not occur in the AST because macros aren't expanded.
             unreachable!()
         }
-        ast::ExprKind::Err => None,
+        ast::ExprKind::Err | ast::ExprKind::Dummy => None,
     };
 
     expr_rw

src/tools/rustfmt/src/utils.rs

@@ -497,6 +497,7 @@ pub(crate) fn is_block_expr(context: &RewriteContext<'_>, expr: &ast::Expr, repr
         | ast::ExprKind::Break(..)
         | ast::ExprKind::Cast(..)
         | ast::ExprKind::Continue(..)
+        | ast::ExprKind::Dummy
         | ast::ExprKind::Err
         | ast::ExprKind::Field(..)
         | ast::ExprKind::IncludedBytes(..)