regression: worker-thread stack overflow while building stage1 rustc on sparcv9-sun-solaris

I am seeing a regression when building Rust on sparcv9-sun-solaris.

Environment

• host: sparcv9-sun-solaris
• OS: Solaris 11.4 on sun4v
• build type: normal dist/nightly build

Bisection
git bisect points to this as the first bad commit: https://github.com/rust-lang/rust/commit/3f34c31af945def1ace8c74216de2f60ae866d8c

Author: Edvin Bryntesson epost@edvinbryntesson.se
Date: Sun Feb 22 15:31:39 2026 +0100

  add variant `ExpectedNameValueAsLastArgument` to `AttributeParseErrorReason`

  compiler/rustc_attr_parsing/src/context.rs
  compiler/rustc_attr_parsing/src/session_diagnostics.rs

Failure
The build fails while compiling rustc_attr_parsing:

thread 'lto cgu.04' (26) has overflowed its stack
fatal runtime error: stack overflow, aborting
rustc exited with signal: 6 (SIGABRT) (core dumped)
error: could not compile `rustc_attr_parsing` (lib)

Notes

• Changing ulimit -s did not help.
• The overflow is in a Rust-created worker thread, not the shell/main thread.
• From source inspection, rustc worker threads use the internal default stack size unless overridden via RUST_MIN_STACK.

Workaround
Setting:

RUST_MIN_STACK=16777216

makes the same build pass on this machine.

So the current picture is:

• default worker-thread stack (8 MiB) fails on this target/build
• 16 MiB is sufficient in my testing

Start with the bisected commit and inspect compiler/rustc_attr_parsing/src/context.rs and compiler/rustc_attr_parsing/src/session_diagnostics.rs. Reproduce the stage1 dist/nightly build on sparcv9-sun-solaris, comparing the default worker-thread stack with RUST_MIN_STACK=16777216. Done means the build no longer overflows the worker-thread stack without relying on that workaround.