Reverts the prior session's -Z json-target-spec addition
that was breaking the build on nightly-2025-10-03 (the
kernel's rust-toolchain.toml specified toolchain). The
flag did not exist in that nightly; only nightly-2026-04-11
has it. Since the prior toolchain can build custom .json
target specs without any cargo-level gating (just pass
-Zunstable-options through -- separator to rustc),
the cleanest fix is to use rustc's -- directly:
cd SOURCE && cargo rustc -Z build-std=core,alloc ...
--bin kernel --target FILE --release
-- -C link-arg=...
RUSTUP_TOOLCHAIN=nightly-2025-10-03 is explicit so the
Makefile build works regardless of which toolchain the
outer shell has.
Also: restore rust-toolchain.toml to nightly-2025-10-03
(the version pinned in this fork). The 2026-04-01 bump
was a workaround attempt that did not work.
And: add .cargo/config.toml with [unstable]
json-target-spec = true as the new standard way (cargo
PR #16557) of enabling custom .json target specs. This
is harmless on older toolchains that don't have the feature
(cargo ignores unknown config keys).
Discovered via research into the nightly-2026-04-11 vs
nightly-2025-10-03 divergence after the redbear-mini build
failed with 'unknown -Z flag specified: json-target-spec'.
vasilito
5098d1651f