From d2881c32e67ada6f4273b0ad232af0389ddc16b7 Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Sat, 24 Aug 2024 00:00:49 -0700
Subject: [PATCH] Pretty-print tail calls (`become`)

---
 Cargo.toml  |  4 ++--
 src/expr.rs | 17 +++++++++++++++++
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/Cargo.toml b/Cargo.toml
index d951596..56dd9bc 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -19,13 +19,13 @@ verbatim = ["syn/parsing"]
 
 [dependencies]
 proc-macro2 = { version = "1.0.80", default-features = false }
-syn = { version = "2.0.59", default-features = false, features = ["full"] }
+syn = { version = "2.0.76", default-features = false, features = ["full"] }
 
 [dev-dependencies]
 indoc = "2"
 proc-macro2 = { version = "1.0.80", default-features = false }
 quote = { version = "1.0.35", default-features = false }
-syn = { version = "2.0.59", default-features = false, features = ["parsing"] }
+syn = { version = "2.0.76", default-features = false, features = ["parsing"] }
 
 [lib]
 doc-scrape-examples = false
diff --git a/src/expr.rs b/src/expr.rs
index 4927c86..1521a29 100644
--- a/src/expr.rs
+++ b/src/expr.rs
@@ -681,10 +681,16 @@ impl Printer {
         enum ExprVerbatim {
             Empty,
             Ellipsis,
+            Become(Become),
             Builtin(Builtin),
             RawReference(RawReference),
         }
 
+        struct Become {
+            attrs: Vec<Attribute>,
+            tail_call: Expr,
+        }
+
         struct Builtin {
             attrs: Vec<Attribute>,
             name: Ident,
@@ -709,6 +715,11 @@ impl Printer {
                 let lookahead = ahead.lookahead1();
                 if input.is_empty() {
                     Ok(ExprVerbatim::Empty)
+                } else if lookahead.peek(Token![become]) {
+                    input.advance_to(&ahead);
+                    input.parse::<Token![become]>()?;
+                    let tail_call: Expr = input.parse()?;
+                    Ok(ExprVerbatim::Become(Become { attrs, tail_call }))
                 } else if lookahead.peek(kw::builtin) {
                     input.advance_to(&ahead);
                     input.parse::<kw::builtin>()?;
@@ -751,6 +762,12 @@ impl Printer {
             ExprVerbatim::Ellipsis => {
                 self.word("...");
             }
+            ExprVerbatim::Become(expr) => {
+                self.outer_attrs(&expr.attrs);
+                self.word("become");
+                self.nbsp();
+                self.expr(&expr.tail_call);
+            }
             ExprVerbatim::Builtin(expr) => {
                 self.outer_attrs(&expr.attrs);
                 self.word("builtin # ");