Merge pull request #1531 from dtolnay/breaklabel

Improve parsing of labeled loop as value expression for break

src/expr.rs

@@ -2463,18 +2463,36 @@ pub(crate) mod parsing {
 
  #[cfg(feature = "full")]
  fn expr_break(input: ParseStream, allow_struct: AllowStruct) -> Result<ExprBreak> {
+ let break_token: Token![break] = input.parse()?;
+
+ let ahead = input.fork();
+ let label: Option<Lifetime> = ahead.parse()?;
+ if label.is_some() && ahead.peek(Token![:]) {
+ // Not allowed: `break 'label: loop {...}`
+ // Parentheses are required. `break ('label: loop {...})`
+ let _ = ambiguous_expr(input, allow_struct)?;
+ let start_span = label.unwrap().apostrophe;
+ let end_span = input.cursor().prev_span();
+ return Err(crate::error::new2(
+ start_span,
+ end_span,
+ "parentheses required",
+ ));
+ }
+
+ input.advance_to(&ahead);
+ let expr = if can_begin_expr(input) && (allow_struct.0 || !input.peek(token::Brace)) {
+ let expr = ambiguous_expr(input, allow_struct)?;
+ Some(Box::new(expr))
+ } else {
+ None
+ };
+
  Ok(ExprBreak {
  attrs: Vec::new(),
- break_token: input.parse()?,
- label: input.parse()?,
- expr: {
- if can_begin_expr(input) && (allow_struct.0 || !input.peek(token::Brace)) {
- let expr = ambiguous_expr(input, allow_struct)?;
- Some(Box::new(expr))
- } else {
- None
- }
- },
+ break_token,
+ label,
+ expr,
  })
  }
 

tests/test_expr.rs

@@ -1,11 +1,11 @@
-#![allow(clippy::uninlined_format_args)]
+#![allow(clippy::single_element_loop, clippy::uninlined_format_args)]
 
 #[macro_use]
 mod macros;
 
 use proc_macro2::{Delimiter, Group, Ident, Punct, Spacing, Span, TokenStream, TokenTree};
 use quote::quote;
-use syn::{Expr, ExprRange};
+use syn::{Expr, ExprRange, Stmt};
 
 #[test]
 fn test_expr_parse() {
@@ -310,3 +310,32 @@ fn test_ranges() {
  syn::parse_str::<Expr>("lo...").unwrap_err();
  syn::parse_str::<Expr>("lo...hi").unwrap_err();
 }
+
+#[test]
+fn test_ambiguous_label() {
+ for stmt in [
+ quote! {
+ return 'label: loop { break 'label 42; };
+ },
+ quote! {
+ break ('label: loop { break 'label 42; });
+ },
+ quote! {
+ break 1 + 'label: loop { break 'label 42; };
+ },
+ quote! {
+ break 'outer 'inner: loop { break 'inner 42; };
+ },
+ ] {
+ syn::parse2::<Stmt>(stmt).unwrap();
+ }
+
+ for stmt in [
+ // Parentheses required. See https:/rust-lang/rust/pull/87026.
+ quote! {
+ break 'label: loop { break 'label 42; };
+ },
+ ] {
+ syn::parse2::<Stmt>(stmt).unwrap_err();
+ }
+}