From 5d3edf9d7e57f8a3080b12f96ab7f1f6e491061e Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Thu, 31 Oct 2024 14:25:18 -0700
Subject: [PATCH] Improve error on malformed format attribute

---
 impl/src/attr.rs               | 12 ++++++++----
 tests/ui/concat-display.stderr |  2 +-
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/impl/src/attr.rs b/impl/src/attr.rs
index e0ac02b1..e59b5191 100644
--- a/impl/src/attr.rs
+++ b/impl/src/attr.rs
@@ -91,7 +91,11 @@ fn parse_error_attribute<'a>(attrs: &mut Attrs<'a>, attr: &'a Attribute) -> Resu
     syn::custom_keyword!(transparent);
 
     attr.parse_args_with(|input: ParseStream| {
-        if let Some(kw) = input.parse::<Option<transparent>>()? {
+        let lookahead = input.lookahead1();
+        let fmt = if lookahead.peek(LitStr) {
+            input.parse::<LitStr>()?
+        } else if lookahead.peek(transparent) {
+            let kw: transparent = input.parse()?;
             if attrs.transparent.is_some() {
                 return Err(Error::new_spanned(
                     attr,
@@ -103,9 +107,9 @@ fn parse_error_attribute<'a>(attrs: &mut Attrs<'a>, attr: &'a Attribute) -> Resu
                 span: kw.span,
             });
             return Ok(());
-        }
-
-        let fmt: LitStr = input.parse()?;
+        } else {
+            return Err(lookahead.error());
+        };
 
         let ahead = input.fork();
         ahead.parse::<Option<Token![,]>>()?;
diff --git a/tests/ui/concat-display.stderr b/tests/ui/concat-display.stderr
index dbecd69f..d92e635a 100644
--- a/tests/ui/concat-display.stderr
+++ b/tests/ui/concat-display.stderr
@@ -1,4 +1,4 @@
-error: expected string literal
+error: expected string literal or `transparent`
   --> tests/ui/concat-display.rs:8:17
    |
 8  |         #[error(concat!("invalid ", $what))]