bootstrap should download rust-analyzer in stage0 toolchain

Summary

By default, loading the proc macro server in RA on rust-lang/rust will almost always fail because rust-analyzer's ABI doesn't match. Bootstrap could download stage0 rust-analyzer (to match stage0 proc-macro-srv), which would allow people to rustup link stage0 and then rustup override set stage0, making the ABIs match up.

Command used

(opened neovim on a file in compiler/)

Expected behaviour

Rust-analyzer loads all proc macros without error.

Actual behaviour

[ERROR][2026-01-16 04:40:26] ...p/_transport.lua:36	"rpc"	"rust-analyzer"	"stderr"	"2026-01-16T04:40:26.119484-05:00  WARN proc-macro loading for /Users/jyn/.local/lib/rustup/toolchains/nightly-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/lib/libdarling_macro-4cfb05fdfed43de3.dylib failed: proc macro server error: Cannot create expander for /Users/jyn/.local/lib/rustup/toolchains/nightly-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/lib/libdarling_macro-4cfb05fdfed43de3.dylib: mismatched ABI expected: `rustc 1.92.0-beta.1 (3b4dd9bf1 2025-10-28)`, got `rustc 1.94.0-nightly (0aced202c 2026-01-06)`\n"

Bootstrap configuration (bootstrap.toml)

complicated. anyway the thing you care about is my RA config, which is the same as src/etc/rust_analyzer_zed.json on HEAD except that i have checkOnSave set to false.

Operating system

macOS Sequoia 15.7.3

HEAD

branched from cec70080fd4

Additional context

<log>

Start by reading the bootstrap configuration and the rust-analyzer setup represented by src/etc/rust_analyzer_zed.json, then trace how the stage0 toolchain and proc-macro-srv are selected. Done means bootstrap obtains a stage0-compatible rust-analyzer so a linked stage0 toolchain can load proc macros without the reported ABI mismatch.