Recover from missing expression in for loop

Author: obeis

compiler/rustc_error_messages/locales/en-US/parse.ftl

@@ -128,6 +128,9 @@ parse_missing_in_in_for_loop = missing `in` in `for` loop
     .use_in_not_of = try using `in` here instead
     .add_in = try adding `in` here
 
+parse_missing_expression_in_for_loop = missing expression to iterate on in `for` loop
+    .suggestion = try adding an expression to the `for` loop
+
 parse_missing_comma_after_match_arm = expected `,` following `match` arm
     .suggestion = missing a comma here to end this `match` arm
 

compiler/rustc_parse/src/errors.rs

@@ -433,6 +433,18 @@ pub(crate) enum MissingInInForLoopSub {
     AddIn(#[primary_span] Span),
 }
 
+#[derive(Diagnostic)]
+#[diag(parse_missing_expression_in_for_loop)]
+pub(crate) struct MissingExpressionInForLoop {
+    #[primary_span]
+    #[suggestion(
+        code = "/* expression */ ",
+        applicability = "has-placeholders",
+        style = "verbose"
+    )]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(parse_missing_comma_after_match_arm)]
 pub(crate) struct MissingCommaAfterMatchArm {

compiler/rustc_parse/src/parser/expr.rs

@@ -2471,6 +2471,21 @@ impl<'a> Parser<'a> {
 
         let pat = self.recover_parens_around_for_head(pat, begin_paren);
 
+        // Recover from missing expression in `for` loop
+        if matches!(expr.kind, ExprKind::Block(..))
+            && !matches!(self.token.kind, token::OpenDelim(token::Delimiter::Brace))
+            && self.may_recover()
+        {
+            self.sess
+                .emit_err(errors::MissingExpressionInForLoop { span: expr.span.shrink_to_lo() });
+            let err_expr = self.mk_expr(expr.span, ExprKind::Err);
+            let block = self.mk_block(vec![], BlockCheckMode::Default, self.prev_token.span);
+            return Ok(self.mk_expr(
+                lo.to(self.prev_token.span),
+                ExprKind::ForLoop(pat, err_expr, block, opt_label),
+            ));
+        }
+
         let (attrs, loop_block) = self.parse_inner_attrs_and_block()?;
 
         let kind = ExprKind::ForLoop(pat, expr, loop_block, opt_label);