Setting a lower RUST_MIN_STACK than DEFAULT_STACK_SIZE increases stack???

[biabbas]

While working on this issue #122357, I found that setting the RUST_MIN_STACK variable size to 7mb or more, the build is successful. But the default stack size in rust is 8mb. Why do we have to set RUST_MIN_STACK again for this to work?

Steps to create the example directory are in this link #122357 (comment)
To verify this please follow these steps.

1. setup the example from above link.
2. Run cargo build in the example, this time the build would fail.
3. Export RUST_MIN_STACK=$((7*1024*1024) and run cargo build again. This time the build is successful.

[ChrisDenton]

The default stack size of new threads is 2 MiB. However, the default stack size of the main thread varies depending on the platform and possibly platform-dependent build configuration.

[workingjubilee]

Rust does not override every last platform-specific behavior. To do so would likely be quite harmful.

[workingjubilee]

Actually, wait. I'm confused. For rustc, this shouldn't matter because of the DEFAULT_STACK_SIZE (not to be confused with the default stack size).

[workingjubilee]

@biabbas What version of rustc did you test with?

[workingjubilee]

No repro on x86_64-unknown-linux-gnu.

[workingjubilee]

Wait, I misapplied some directions.

It... does repro? What.

[biabbas]

@biabbas What version of rustc did you test with?

I was able to reproduce this behavior on rust 1.75, 1.78, 1.79 stable.

[biabbas]

The default stack size of new threads is 2 MiB. However, the default stack size of the main thread varies depending on the platform and possibly platform-dependent build configuration.

You are right, default stack size for new threads is 2mb. I went through the std thread code and I see that min_stack uses this RUST_MIN_STACK variable to influence the size of thread builder stack size. In the test case what stack is overflowed and why RUST_MIN_STACK workaround fixes it?

[biabbas]

Actually, wait. I'm confused. For rustc, this shouldn't matter because of the DEFAULT_STACK_SIZE (not to be confused with the default stack size).

Could you please elaborate and differentiate between the two.

[workingjubilee]

@biabbas DEFAULT_STACK_SIZE is a constant used by rustc to set the thread builder's stack amount, the "default stack size" we've mentioned in a few cases in this thread is the platform's implicit stack size for the main thread.

[biabbas]

Can rustc be using parallelism( sub threads with limit around 2mb) for program compilation? In that case the error message should emit the actual stack that is overflowed. I know cargo uses parallelism for build but not sure if rustc also uses this.

I had access to reproduce and work on #113612. This example would compile with RUST_MIN_STACT set to 3mb.
Maybe the thread builder stack size is overwriting the main threads stack size, in which case it needs a fix.

[workingjubilee]

Yes, we deliberately set the stack size by spawning a thread.

However, I would expect we are setting it to 8MB.

We emit a suggestion that suggests setting RUST_MIN_STACK to double the previous value.