fix(embedded): Disallow ambiguous multiple frontmatters

We could say that we delegate this to rustc but if they add support for
multiple frontmatters, we need to update to be able to know which we
should read, so its better to error on our side.

Author: epage

src/cargo/util/toml/embedded.rs

@@ -143,6 +143,11 @@ impl<'s> ScriptSource<'s> {
         let frontmatter_len = input.len() - rest.len();
         source.content = &input[frontmatter_len..];
 
+        let repeat = Self::parse(source.content)?;
+        if repeat.frontmatter.is_some() {
+            anyhow::bail!("only one frontmatter is supported");
+        }
+
         Ok(source)
     }
 
@@ -362,6 +367,7 @@ content: "\n//@ check-pass\n\n// check that frontmatter blocks can have tokens t
 
     #[test]
     fn rustc_frontmatter_whitespace_1() {
+        // Deferred to rustc since this requires knowledge of Rust grammar
         assert_source(
             r#"  ---
 //~^ ERROR: invalid preceding whitespace for frontmatter opening
@@ -539,8 +545,9 @@ content: "\nfn main() {}\n"
 
     #[test]
     fn rustc_multifrontmatter() {
-        assert_source(
-            r#"---
+        assert_err(
+            ScriptSource::parse(
+                r#"---
 ---
 
 ---
@@ -552,13 +559,8 @@ content: "\nfn main() {}\n"
 
 fn main() {}
 "#,
-            str![[r#"
-shebang: None
-info: None
-frontmatter: ""
-content: "\n---\n//~^ ERROR: expected item, found `-`\n// FIXME(frontmatter): make this diagnostic better\n---\n\n// test that we do not parse another frontmatter block after the first one.\n\nfn main() {}\n"
-
-"#]],
+            ),
+            str!["only one frontmatter is supported"],
         );
     }