./x.py fmt is not supported on this channel when not using stage0 toolchain rustc

[tesuji]

I'm using nightly toolchain as toolchain to build bootstrap compiler with this config:

[build]
cargo = "/home/lzutao/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/cargo"
rustc = "/home/lzutao/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc"
#rustfmt = "/path/to/bin/rustfmt"

But I cannot use ./x.py fmt to use always-downloaded stage0 rustfmt to format code.
Instead I have this error from x.py:

$ ./x.py fmt
    Finished dev [unoptimized + debuginfo] target(s) in 0.25s
./x.py fmt is not supported on this channel

I'm working on main commits 20b1e05 .

Meta

rustc +nightly --version --verbose:

rustc 1.49.0-nightly (4760b8fb8 2020-10-25)
binary: rustc
commit-hash: 4760b8fb886a3702ae11bfa7868d495b2675b5ed
commit-date: 2020-10-25
host: x86_64-unknown-linux-gnu
release: 1.49.0-nightly
LLVM version: 11.0

@rustbot modify labels: T-infra

[Mark-Simulacrum]

You can "fix" this by manually setting the rustfmt path to the binary in build/.../stage0/bin/rustfmt. It can be fixed in rustbuild by explicitly threading the rustfmt path from python into Rust (or adding a fallback to look in the build/.../stage0/bin/ directory, which is probably harder but better in some sense).

[shepmaster]

I made some changes that might address this in #78842

[Enselic]

Triage: No one said "that didn't help", so I will assume it did. Closing as fixed.