Specific error for positional args after named args in `format!()`

When writing positional arguments after named arguments in the
`format!()` and `println!()` macros, provide a targeted diagnostic.

1 files changed, 8 insertions, 2 deletions

diff --git a/src/test/ui/macros/format-parse-errors.rs b/src/test/ui/macros/format-parse-errors.rs
index 96aee5e6aee..ffa7a2817ff 100644
--- a/src/test/ui/macros/format-parse-errors.rs
+++ b/src/test/ui/macros/format-parse-errors.rs
@@ -1,9 +1,15 @@
 fn main() {
+    let foo = "";
+    let bar = "";
     format!(); //~ ERROR requires at least a format string argument
     format!(struct); //~ ERROR expected expression
     format!("s", name =); //~ ERROR expected expression
-    format!("s", foo = foo, bar); //~ ERROR expected `=`
-    format!("s", foo = struct); //~ ERROR expected expression
+    format!(
+        "s {foo} {} {}",
+        foo = foo,
+        bar, //~ ERROR positional arguments cannot follow named arguments
+    );
+    format!("s {foo}", foo = struct); //~ ERROR expected expression
     format!("s", struct); //~ ERROR expected expression
 
     // This error should come after parsing errors to ensure they are non-fatal.