fix(discover): make/cargo dep-info `.d` files are parsed with the D grammar

[Fieldnote-Echo]

Version

built from source, main @ 2278498 (v0.11.0-186)

Platform

Linux (x64)

Install channel

Built from source

Binary variant

standard

What happened, and what did you expect?

.d always maps to D (src/discover/language.c:406). Make-style dependency files written by cargo/rustc, gcc/clang -MD and CMake therefore go through the D grammar and then the unbudgeted unified walk (internal/cbm/cbm.c:301). They yield nothing useful, and the cost grows superlinearly with size: a 128 KB cargo dep-info file takes about 2 s to parse and 13 s to walk. Discovery always skips only target/, so a cargo --target-dir with another name, or in-tree -MD output that is not gitignored, gets indexed.

My fix adds cbm_disambiguate_d, a 4 KB first-line sniff in the shape of the .frm/.cls/.m disambiguators (#2074) and the new .res one (#2352). It drops a .d file only when its first non-blank line is a make rule whose targets all look like paths, and keeps D on any doubt. Dep-info files lose their File/Module/Folder nodes (they were never D source), so please confirm that is the behavior you want.

Reproduction

Repro: copy the .d files from a cargo target/debug/deps into a directory with a few .rs files and index it.

Linux x86_64, 32 logical CPUs, GCC 16	main	fixed
Cold index, 200 real cargo dep-info files (16-205 KB) + 3 .rs	109 s, 858 CPU-s, 327 MB RSS, 17 parse timeouts	1.3 s, 1.0 CPU-s, 81 MB, 0 timeouts
Real cargo dep-info files detected as D	26,157 of 26,157	0
Real D files detected as D (phobos, dmd, ldc, dub, vibe.d)	12,062 of 12,062	12,062 of 12,062

Logs

Diagnostics trajectory (memory / performance / leak issues)

Not a leak: the CPU time and peak RSS above are for one cold index run of the repro directory.

Project scale (if relevant)

No response

Confirmations

• I searched existing issues and this is not a duplicate.
• My reproduction uses shareable code (a dummy snippet or a public OSS repository), not proprietary code.