Nested template literals in frontmatter throw "Unexpected token" (Astro 7 / new compiler)

Astro Info

Astro                    v7.1.3
Node                     v26.5.0
System                   macOS (arm64)
Package Manager          pnpm
Output                   static
Adapter                  none
Integrations             none

If this issue only occurs in one browser, which browser is a problem?

No response

Describe the Bug

A template literal containing another template literal nested inside a ${} expression fails to compile in the frontmatter, throwing [CompilerError] Unexpected token. This is valid JS/TS and worked in Astro 6 and earlier.

Reproduction

---
let hideTitle = false;
let title = "Hello";
let srcdoc = `<div>${hideTitle ? '' : `<span>${title}</span>`}</div>`;
---
<div>{srcdoc}</div>

Running astro build fails with:

[CompilerError] Unexpected token

pointing at the frontmatter block.

Expected behavior

Nested template literals should parse normally, as they do in standard JS/TS and in Astro 6.

Workaround

Pulling the inner template literal out into its own variable avoids the issue:

---
let inner = hideTitle ? '' : `<span>${title}</span>`;
let srcdoc = `<div>${inner}</div>`;
---

Additional context

This appears to be a limitation in the new compiler-rs frontmatter compiler introduced as part of the Rolldown/Vite 8 migration, it doesn't seem to track backtick nesting depth correctly inside ${} expressions.

What's the expected result?

Nested template literals should parse normally, as they do in standard JS/TS and in Astro 6.

Link to Minimal Reproducible Example

NA

Participation

• I am willing to submit a pull request for this issue.