Detect single-line previews like #Preview("x") { foo() }

[AllDmeat]

Problem

Single-line previews whose closure opens and closes on the same line as the #Preview macro are silently skipped — no snapshot test is generated for them:

#Preview("MyView") { previewMyView() }

There is no warning or error; the preview simply never produces a test.

Root cause

Two spots, both needed for the case to work end to end:

1. PreviewLoader.previewBodies starts tracking the brace balance only once a line has a non-zero brace change (braceChange != 0). On a single-line preview the opening and closing braces cancel out (braceChange == 0), so the balance is never initialised and the braceBalance == 0 close condition never fires — the body is never collected.

2. RawPreviewModel.init derives the first line via dropLast(2) + removeFirst(). For a single-line body the collection is empty after dropLast(2), so removeFirst() traps.

Fix

1. previewBodies now starts tracking the brace balance as soon as a line contains the opening brace, regardless of whether it nets to zero on that line.
2. RawPreviewModel.init takes the first line directly. The lines slice after removeFirst() was never used — only the first line matters here (for displayName/traits); body and properties are already parsed from the full macro body via SwiftSyntax, so nothing else changes.

Multi-line previews are unaffected: their #Preview { line already has a non-zero brace change, so balance tracking starts exactly as before.

Tests

• PreviewLoaderTests: single-line only, single-line + multi-line mixed, and single-line under defaultEnabled: false.
• RawPreviewModelTests: single-line body parses displayName, body, and traits correctly.

Full PrefireExecutable suite: 45 tests, 0 failures.

[BarredEwe]

@AllDmeat Hi! Thank you for improvements.

This fixes the single-line case, but it also makes extraction depend on any { appearing on the #Preview line, not on the actual preview closure opening.

For example this now gets truncated to only the first line:

#Preview("{braced}")
{
    Text("TestView")
}

Previously this worked because braceBalance did not start until the real { line. I think we should avoid extending the line-based brace scanner here and instead extract #Preview macros with SwiftSyntax, which is already a dependency in this target and already used by RawPreviewModel. That would correctly handle braces in strings/comments/raw strings/nested closures without adding more ad-hoc parsing.

[AllDmeat]

Good catch, thanks. Switched the extraction to SwiftSyntax as you suggested — PreviewLoader now walks the file for #Preview macro expansions (MacroExpansionDeclSyntax/MacroExpansionExprSyntax) and takes the real source range from the # token to the end of the closure, instead of the line-based brace scanner. So braces in strings/comments/raw strings/nested closures no longer confuse it.

Added a regression test for your counterexample:

#Preview("{braced}")
{
    Text("TestView")
}

All existing single-line/multi-line/Allman/@available/prefireIgnored cases still pass.

[BarredEwe]

Super, thanks for the enhancement!