feat(compiler): Support braced Unicode escapes in template string literals - #67269

Braced Unicode escapes in Angular template string literals

Scope (feature-only)

This PR adds support for ES6 braced Unicode escapes in Angular template expression string literals:

• {{ '\u{4f60}' }} -> 你 (CJK character)
• {{ '\u{1F600}' }} -> 😀 (emoji, above U+FFFF)
• {{ '\u4f60' }} -> still works (traditional 4-digit form)

Additional validated sequence example:

• \u{48}\u{65}\u{6C}\u{6C}\u{6F} -> Hello

Working example:

What changed

• Extended expression lexer string escape handling to support both:
  • \uXXXX
  • \u{...}
• Added code point handling above BMP (> 0xFFFF) via String.fromCodePoint(...).
• Added parser tests for:
  • CJK braced escape (你)
  • Emoji braced escape (😀)
  • Legacy 4-digit escape compatibility
  • Multi-escape sequence producing Hello

Character mapping

Key takeaways

• The Why: Standard \uXXXX is limited to 16 bits (65,536 combinations). Modern Unicode extends to 21-bit code points (astral planes) used by many emoji and historic scripts.
• Braced Escapes: \u{...} is the modern ES6 form. It directly expresses code points without manual surrogate math or zero-padding.
• Surrogate Pairs: Legacy UTF-16 workaround where one non-BMP code point is encoded as two 16-bit units (high + low surrogate).

Tests run

• pnpm bazel test //packages/compiler/test:test --test_filter=".*unicode braced escapes.*" --test_output=errors (passed)
• pnpm bazel test //packages/compiler/test:test --test_output=errors (passed)

Notes

• This PR intentionally ports only the braced-unicode part of broader template-expression work.
• Legacy \uXXXX behavior is preserved.
• Fixes Support braced Unicode escapes #67279

AI disclosure

AI assisted with implementation and drafting under user direction; all changes were reviewed and validated with local tests before opening this PR.