Oxc Coverage V8

V8 inspector coverage to Istanbul FileCoverage conversion, a Rust port of v8-to-istanbul.

Overview

V8's inspector protocol reports coverage as [startOffset, endOffset, count] ranges grouped by function. Istanbul reporters consume per-statement, per-function, and per-branch hit counts keyed by line and column. This crate takes a pre-built FileCoverage, typically produced by an AST-traversal pass in an instrumenter, and fills in its hit-count vectors from V8 ranges.

The input shape is the one Node's inspector and @vitest/coverage-v8 emit, so V8-collected data joins the reporting pipeline at the same point runtime-collected __coverage__ does.

Key Features

• Encoding normalization. Inspector offsets and Istanbul columns are UTF-16 code units, while Oxc branch spans are UTF-8 bytes, so source-side coordinates are converted before matching. Astral characters and mixed line endings are handled.
• Explicit wrapper base. Node wraps every CommonJS module in (function(exports,require,module,...){. Node inspector output is source-relative and reports a wrapper_length of zero; a nonzero value is an explicit UTF-16 base, for producers that already report wrapper-shifted ranges.
• Block coverage. With block coverage enabled, statement, function, and branch counts are populated by intersecting V8 ranges with locations recovered from a visit-only AST pass.
• Source-map recovery. Inline //# sourceMappingURL=data:... trailers are decoded automatically by extract_inline_source_map; external map references are reported by extract_external_source_mapping_url and resolve through the optional loader on v8_to_istanbul_with_loader.

use oxc_coverage_instrument::v8_to_istanbul_with_loader;

let fc = v8_to_istanbul_with_loader(source, "app.js", &functions, 0, |url| {
    std::fs::read_to_string(url).ok()
})?;

Architecture

apply_v8_coverage is the core: it walks the V8 ranges once, intersects each against the locations already recorded in the FileCoverage, and writes the resulting hit counts into the s, f, and b vectors. Because the entry maps are supplied rather than derived, the crate never parses source on the counting path, and the same FileCoverage shape is produced whether counts came from V8 or from injected counters.

The v8_to_istanbul entry points that build the FileCoverage from source live in oxc_coverage_instrument, which owns the AST pass; this crate stays free of a parser dependency on the conversion path.