Fix fallback fmt expression parser hang

Author: dtolnay

impl/src/fmt.rs

@@ -193,6 +193,8 @@ fn fallback_explicit_named_args(input: ParseStream) -> Result<FmtArguments> {
             let ident = input.call(Ident::parse_any)?;
             input.parse::<Token![=]>()?;
             args.named.insert(ident);
+        } else {
+            input.parse::<TokenTree>()?;
         }
     }
 

tests/ui/expression-fallback.stderr

@@ -0,0 +1,19 @@
+error: expected `,`, found `.`
+ --> tests/ui/expression-fallback.rs:4:11
+  |
+4 | #[error("".yellow)]
+  |           ^ expected `,`
+
+error: argument never used
+ --> tests/ui/expression-fallback.rs:4:12
+  |
+4 | #[error("".yellow)]
+  |         -- ^^^^^^ argument never used
+  |         |
+  |         formatting specifier missing
+
+error[E0425]: cannot find value `yellow` in this scope
+ --> tests/ui/expression-fallback.rs:4:12
+  |
+4 | #[error("".yellow)]
+  |            ^^^^^^ not found in this scope