Can't run doc tests with `-Znext-solver=globally`

[nazar-pc]

Problem

Looks like RUSTFLAGS are not propagated when running doc tests properly

Steps

1. Have code that uses generic_const_args
2. Run RUSTFLAGS="-Znext-solver=globally" cargo t
3. Observe an error (normal tests are compiled file, doc tests are not despite environment variable set correctly):

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

     Running tests/balanced.rs (/home/nazar-pc/.cache/cargo/target/debug/deps/balanced-7d61a7da24839a02)

running 6 tests
test mt_balanced_16_leaves ... ok
test mt_balanced_2_leaves ... ok
test mt_balanced_4_leaves ... ok
test mt_balanced_32_leaves ... ok
test mt_balanced_8_leaves ... ok
test mt_balanced_64_leaves ... ok

test result: ok. 6 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

   Doc-tests ab_merkle_tree
error: `generic_const_args` requires -Znext-solver=globally to be enabled
  --> crates/shared/ab-merkle-tree/src/lib.rs:36:5
   |
36 |     generic_const_args,
   |     ^^^^^^^^^^^^^^^^^^
   |
   = help: enable all of these features

error: aborting due to 1 previous error

error: doctest failed, to rerun pass `--doc`

Possible Solution(s)

The only explanation I can see if RUSTFLAGS not being propagated when building doc tests, but I'm not 100% sure where the issue is exactly. Seems like Cargo, but feel free to move the issue elsewhere if not.

Notes

No response

Version

cargo 1.98.0-nightly (fbb61be30 2026-05-26)
release: 1.98.0-nightly
commit-hash: fbb61be30e5f9ac3a6ad58e56a5c0f5db2d2b3ef
commit-date: 2026-05-26
host: x86_64-unknown-linux-gnu
libgit2: 1.9.2 (sys:0.20.4 vendored)
libcurl: 8.20.0-DEV (sys:0.4.88+curl-8.20.0 vendored ssl:OpenSSL/3.6.2)
ssl: OpenSSL 3.6.2 7 Apr 2026
os: Ubuntu 24.4.0 (noble) [64-bit]

[ehuss]

You can use RUSTDOCFLAGS to set flags for doctests.

[nazar-pc]

Thanks, that would be a workaround, but should I have to for things like -Znext-solver=globally specifically?

It is specified only because things wouldn't compile otherwise, so I totally expect Cargo to forward it to doc test compilation as well since it is most likely that doc tests would fail to compile without it as well.

[ehuss]

Yes. RUSTFLAGS is specifically for executing rustc and RUSTDOCFLAGS is specifically for executing rustdoc. Although they have a large overlap in the flags they accept, they aren't identical (and the overlap in the past was much smaller).

I understand it is awkward, and not enentirely intuitive. However, these are intended as low-level flags for dipping down directly to the tooling.

Perhaps in the future, doctests may be redesigned so that they are not driven by rustdoc, but I believe that might be a long ways off.

[weihanglo]

Perhaps in the future, doctests may be redesigned so that they are not driven by rustdoc, but I believe that might be a long ways off.

Speaking of this, we made some progress during all-hands this year: rust-lang/all-hands-2026#8

(not progress in terms of implementation, but cross-team communications and idea brainstorming)