AST/HIR: Introduce `ExprKind::Err` for better error recovery in the front-end

5 files changed, 17 insertions, 2 deletions

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index 7e228d8d7cf..f0e567f9cd6 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -1001,6 +1001,7 @@ impl Expr {
             ExprKind::Paren(..) => ExprPrecedence::Paren,
             ExprKind::Try(..) => ExprPrecedence::Try,
             ExprKind::Yield(..) => ExprPrecedence::Yield,
+            ExprKind::Err => ExprPrecedence::Err,
         }
     }
 }
@@ -1160,6 +1161,9 @@ pub enum ExprKind {
 
     /// A `yield`, with an optional value to be yielded.
     Yield(Option<P<Expr>>),
+
+    /// Placeholder for an expression that wasn't syntactically well formed in some way.
+    Err,
 }
 
 /// The explicit `Self` type in a "qualified path". The actual
diff --git a/src/libsyntax/fold.rs b/src/libsyntax/fold.rs
index 0abc74c2314..b8990264c5d 100644
--- a/src/libsyntax/fold.rs
+++ b/src/libsyntax/fold.rs
@@ -1367,6 +1367,7 @@ pub fn noop_fold_expr<T: Folder>(Expr {id, node, span, attrs}: Expr, folder: &mu
             ExprKind::Yield(ex) => ExprKind::Yield(ex.map(|x| folder.fold_expr(x))),
             ExprKind::Try(ex) => ExprKind::Try(folder.fold_expr(ex)),
             ExprKind::TryBlock(body) => ExprKind::TryBlock(folder.fold_block(body)),
+            ExprKind::Err => ExprKind::Err,
         },
         id: folder.new_id(id),
         span: folder.new_span(span),
diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index 931e91e9c79..2ad3d3a6d64 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -1093,7 +1093,9 @@ impl<'a> State<'a> {
                 self.s.word("_")?;
             }
             ast::TyKind::Err => {
-                self.s.word("?")?;
+                self.popen()?;
+                self.s.word("/*ERROR*/")?;
+                self.pclose()?;
             }
             ast::TyKind::ImplicitSelf => {
                 self.s.word("Self")?;
@@ -2391,6 +2393,11 @@ impl<'a> State<'a> {
                 self.s.space()?;
                 self.print_block_with_attrs(blk, attrs)?
             }
+            ast::ExprKind::Err => {
+                self.popen()?;
+                self.s.word("/*ERROR*/")?;
+                self.pclose()?
+            }
         }
         self.ann.post(self, AnnNode::Expr(expr))?;
         self.end()
diff --git a/src/libsyntax/util/parser.rs b/src/libsyntax/util/parser.rs
index 52390fa5b1d..89d4e53b8d1 100644
--- a/src/libsyntax/util/parser.rs
+++ b/src/libsyntax/util/parser.rs
@@ -267,6 +267,7 @@ pub enum ExprPrecedence {
     TryBlock,
     Struct,
     Async,
+    Err,
 }
 
 impl ExprPrecedence {
@@ -325,7 +326,8 @@ impl ExprPrecedence {
             ExprPrecedence::Block |
             ExprPrecedence::TryBlock |
             ExprPrecedence::Async |
-            ExprPrecedence::Struct => PREC_PAREN,
+            ExprPrecedence::Struct |
+            ExprPrecedence::Err => PREC_PAREN,
         }
     }
 }
diff --git a/src/libsyntax/visit.rs b/src/libsyntax/visit.rs
index 43f8a13609e..156546bbba9 100644
--- a/src/libsyntax/visit.rs
+++ b/src/libsyntax/visit.rs
@@ -802,6 +802,7 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
         ExprKind::TryBlock(ref body) => {
             visitor.visit_block(body)
         }
+        ExprKind::Err => {}
     }
 
     visitor.visit_expr_post(expression)