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Last of the boundary trilogy, after #36729 and #36730 (both merged); rebased onto main as a standalone 3-commit change. JS strings are WTF-16: a lone surrogate in source (`"\uD83E"`) is a legal string value, but Rust `String` is UTF-8 and cannot hold it. The compiler previously leaned on the napi bridge's `__SURROGATE_XXXX__` marker encoding end to end, so the core compiler compared, concatenated, and constant-folded marker text as if it were the actual string value. `JsString` (in `react_compiler_diagnostics`, our lowest layer) holds the common well-formed case as a plain UTF-8 `String` with zero overhead and falls back to UTF-16 code units only for ill-formed values. `StringLiteral.value` and `PrimitiveValue::String` carry it through the pipeline; constant folding concatenates via code units so split surrogate halves re-pair exactly as they do in JS; markers are emitted only at the napi edge, where the babel bridge requires them (serde_json can neither parse nor emit a lone `\uXXXX` escape). The representation is encapsulated behind an opaque struct (private `Repr` enum, borrowed `JsStringRef` view via `as_ref`) so the "well-formed values are always UTF-8" invariant that makes the derived `PartialEq`/`Hash` sound holds by construction, per review feedback. The marker decoder scans byte-wise (an earlier draft range-sliced at fixed offsets and panicked on multibyte UTF-8 following `__SURROGATE_`), validates hex digits, and accepts uppercase only, exactly mirroring what the bridge emits; lowercase marker-shaped user text survives verbatim. The HIR debug printer renders unpaired surrogates as `\uXXXX` escapes byte-identical to the TS printer. This closes the lone-surrogate divergence on the e2e harness. Verified on the rebased branch: cargo workspace tests, Rust snap channel 1804/1804, and HIR + Code parity on the lone-surrogate fixture through the comparison harness.
React Compiler
React Compiler is a compiler that optimizes React applications, ensuring that only the minimal parts of components and hooks will re-render when state changes. The compiler also validates that components and hooks follow the Rules of React.
More information about the design and architecture of the compiler are covered in the Design Goals.
More information about developing the compiler itself is covered in the Development Guide.