What

Adds SourceMap::lookup_token_approx (+ lookup_source_view_token_approx): like lookup_token, but when the queried column is before the first token on a line, it clamps to that first token instead of returning None. lookup_token is unchanged.

Why — with vs. without

lookup_token returns the greatest token <= (line, col), so a query before a line's first token returns None. Correct for a plain position lookup — but it silently drops mappings when composing two sourcemaps, and it only surfaces on indented lines.

Indented generated line (→ = leading tab):

    →globalThis.side = 1;
    ↑↑
    ↑└ col 1 — codegen's first token ("globalThis")
    └ col 0 — the tab, no token here

A composer remaps the line by querying column 0:

    lookup_token(line, 0)         →  None           ❌  the whole line drops out of the map
    lookup_token_approx(line, 0)  →  token @ col 1  ✅  the line stays mapped

Downstream, this is exactly what bit rolldown (rolldown/rolldown#10070): every indented statement that went through map composition lost its mapping, so a debugger could no longer jump from it back to source.

Prior art — Rollup's collapseSourcemaps does exactly this

Rollup's map composer approximates in this same situation instead of dropping the segment — src/utils/collapseSourcemaps.ts L128-L136:

// If a sourcemap does not have sufficient resolution to contain a
// necessary mapping, e.g. because it only contains line information or
// the column is not precise (e.g. the sourcemap is generated by esbuild, segment[0] may be shorter than the location of the first letter),
// we approximate by finding the closest segment whose segment[0] is less than the given column
if (segment[0] !== column && searchStart === searchEnd) {
    const approximatedSegmentIndex =
        segments[searchStart][0] > column ? Math.max(0, searchStart - 1) : searchStart;
    segment = segments[approximatedSegmentIndex];
}

When the column precedes the line's first segment (searchStart === 0 and segments[0][0] > column), Math.max(0, searchStart - 1) clamps to index 0 — the line's first segment. lookup_token_approx returns that same first token; for in-range columns it matches lookup_token (Rollup's exact-hit path). An empty/out-of-range line returns None in both.

Notes

• Additive only; lookup_token's "None before the first token" contract (and its test) is untouched.
• Both lookup_token and this method assume tokens are sorted by (dst_line, dst_col) — an existing invariant from generate_lookup_table; the approx variant adds no new assumption.
• Empty line / line past the table still returns None (nothing to clamp to).

Tests

A single end-to-end test, tests/lookup_token_approx.rs::compose_sourcemaps_with_approx_lookup_keeps_indented_lines, models the whole #10070 pipeline: codegen indents a line (first anchor at column 1), the bundler samples the moved line at column 0, and collapsing the chain resolves the bundle token through the codegen map — asserting the strict lookup drops the mapping while the approximating lookup keeps it. The composed map then round-trips through JSON and resolves the bundle position back to the original source via OwnedSourceMap (covering the delegation too).

(Maintainer edit: rebased on main, replaced the original unit tests with the e2e test above, and aligned the new method's binary search with #385's dst_col-only key.)