From 93931a4f243665a1e50748072b9fac1adbcebb7c Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Mon, 1 Jul 2024 19:42:22 -0700
Subject: [PATCH 1/2] Add fixup test for break with leading label

---
 tests/test_expr.rs | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tests/test_expr.rs b/tests/test_expr.rs
index 961e3c4ea3..364386c5f8 100644
--- a/tests/test_expr.rs
+++ b/tests/test_expr.rs
@@ -672,6 +672,7 @@ fn test_fixup() {
         quote! { match m { _ => ({}) - 1 } },
         quote! { if let _ = (a && b) && c {} },
         quote! { if let _ = (S {}) {} },
+        quote! { break ('a: loop { break 'a 1 } + 1) },
     ] {
         let original: Expr = syn::parse2(tokens).unwrap();
 

From 4e71c1caa413122a644f9e3fcb8222dfc20d9f8d Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Sun, 19 May 2024 20:17:44 -0700
Subject: [PATCH 2/2] Parenthesize labeled loops inside break value

---
 src/classify.rs | 53 +++++++++++++++++++++++++++++++++++++++++++++++++
 src/expr.rs     | 11 ++++++++--
 2 files changed, 62 insertions(+), 2 deletions(-)

diff --git a/src/classify.rs b/src/classify.rs
index 1b0ff30040..cc176d06dc 100644
--- a/src/classify.rs
+++ b/src/classify.rs
@@ -248,6 +248,59 @@ pub(crate) fn confusable_with_adjacent_lt(mut expr: &Expr) -> bool {
     }
 }
 
+/// Whether the expression's first token is the label of a loop/block.
+#[cfg(all(feature = "printing", feature = "full"))]
+pub(crate) fn expr_leading_label(mut expr: &Expr) -> bool {
+    loop {
+        match expr {
+            Expr::Block(e) => return e.label.is_some(),
+            Expr::ForLoop(e) => return e.label.is_some(),
+            Expr::Loop(e) => return e.label.is_some(),
+            Expr::While(e) => return e.label.is_some(),
+
+            Expr::Assign(e) => expr = &e.left,
+            Expr::Await(e) => expr = &e.base,
+            Expr::Binary(e) => expr = &e.left,
+            Expr::Call(e) => expr = &e.func,
+            Expr::Cast(e) => expr = &e.expr,
+            Expr::Field(e) => expr = &e.base,
+            Expr::Index(e) => expr = &e.expr,
+            Expr::MethodCall(e) => expr = &e.receiver,
+            Expr::Range(e) => match &e.start {
+                Some(start) => expr = start,
+                None => return false,
+            },
+            Expr::Try(e) => expr = &e.expr,
+
+            Expr::Array(_)
+            | Expr::Async(_)
+            | Expr::Break(_)
+            | Expr::Closure(_)
+            | Expr::Const(_)
+            | Expr::Continue(_)
+            | Expr::Group(_)
+            | Expr::If(_)
+            | Expr::Infer(_)
+            | Expr::Let(_)
+            | Expr::Lit(_)
+            | Expr::Macro(_)
+            | Expr::Match(_)
+            | Expr::Paren(_)
+            | Expr::Path(_)
+            | Expr::Reference(_)
+            | Expr::Repeat(_)
+            | Expr::Return(_)
+            | Expr::Struct(_)
+            | Expr::TryBlock(_)
+            | Expr::Tuple(_)
+            | Expr::Unary(_)
+            | Expr::Unsafe(_)
+            | Expr::Verbatim(_)
+            | Expr::Yield(_) => return false,
+        }
+    }
+}
+
 /// Whether the expression's last token is `}`.
 #[cfg(feature = "full")]
 pub(crate) fn expr_trailing_brace(mut expr: &Expr) -> bool {
diff --git a/src/expr.rs b/src/expr.rs
index 30bf9c76c6..e684068caa 100644
--- a/src/expr.rs
+++ b/src/expr.rs
@@ -3286,8 +3286,15 @@ pub(crate) mod printing {
         outer_attrs_to_tokens(&e.attrs, tokens);
         e.break_token.to_tokens(tokens);
         e.label.to_tokens(tokens);
-        if let Some(expr) = &e.expr {
-            print_expr(expr, tokens, fixup.subsequent_subexpression());
+        if let Some(value) = &e.expr {
+            print_subexpression(
+                value,
+                // Parenthesize `break 'inner: loop { break 'inner 1 } + 1`
+                //                     ^---------------------------------^
+                e.label.is_none() && classify::expr_leading_label(value),
+                tokens,
+                fixup.subsequent_subexpression(),
+            );
         }
     }