dev-guide instructions for running tests are incomplete

[tlyu]

It's not completely clear whether this is a Reference bug, or a bug in mdBook.

The provided build setup instructions in the dev-guide are incomplete. Context: I currently have my rustup default toolchain set to stable. Installing the nightly toolchain and setting a rustup override in the reference directory (as instructed) seems to be sufficient for building, but running tests doesn't always work. Setting rustup default nightly seems to be necessary to run the tests directly from mdbook test.

Running cargo xtask test-all succeeds for some reason, even without rustup default nightly. Directly running mdbook test from the reference directory succeeds if rustup default nightly has been set, or if RUSTUP_TOOLCHAIN=nightly is set in the environment.

As far as I can tell from its source code, mdbook test uses a temporary directory (based on the OS default), which in general won't be subject to the override set in the reference directory. I don't know what additional setup is being propagated either from cargo run or xtask that is causing cargo xtask test-all to succeed.

macOS 15.7.5 (24G624) in case that matters.

Sample failure output

test expressions/operator-expr.md - Operator_expressions::Type_cast_expressions::Semantics::Pointer_to_pointer_cast (line 2074) - compile fail ... ok

failures:

---- expressions/operator-expr.md - Operator_expressions::The_try_propagation_expression (line 785) stdout ----
error[E0554]: `#![feature]` may not be used on the stable release channel
   --> expressions/operator-expr.md:785:1
    |
785 | #![ feature(try_trait_v2) ]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0554`.
Couldn't compile the test.

[tlyu]

Additional information: rustup show reveals that rust-toolchain.toml sets an override to nightly, so the rustup override command isn't strictly necessary anymore for getting cargo xtask test-all to work.

On the other hand, running a cargo that's proxied through rustup will set RUSTUP_TOOLCHAIN in the environment according to the overrides, so when mdbook later runs rustdoc, it will use the nightly version.

Running mdbook directly from the command line doesn't use the rustup proxy, so it will invoke the same default rustdoc that would run when invoked from the temporary directory that mdbook creates for it.

Suggested solution: add an xtask command to run mdbook test, which will pick up the toolchain override, via the cargo proxy.