The default compiler

On every native platform, the top-level v executable contains only the experimental V3 C compiler (whose source lives in vlib/v3). Every direct C build, including compiler self-builds, is compiled by V3 in-process. The CLI and tool commands remain in cmd/v; commands such as test and fmt are external tools, and non-C backends remain separate builder tools.

The standard bootstrap builds a sibling v1_fallback executable (v1_fallback.exe on Windows). -old-compiler launches it explicitly, and ordinary user builds retry through it after a V3 compiler or C compilation failure. Explicit -new-compiler builds and native compiler self-builds remain strict V3 operations, except for -new-compiler -cc msvc on Windows. V3 does not yet generate MSVC command lines, so that combination intentionally launches v1_fallback.exe. A separately built V3-only executable without the sibling cannot use the fallback. -new-compiler remains accepted for command-line compatibility and otherwise selects the same embedded driver.

Portable cross-VC snapshots do not embed V3 and retain the established compiler from vlib/v.