Constant-related build errors on 32-bit platforms

[thombles]

tracing: 6d8b995c99f1d7b8758da687e5b7df25456f9559
This commit has tracing-subscriber depending on sharded-slab 0.0.6

When building tracing-subscriber for a 32 bit target, e.g. cargo build --target i686-linux-android, this results in build errors in sharded-slab code.

   Compiling sharded-slab v0.0.6
...
   Compiling tracing-subscriber v0.2.0-alpha.1 (/Users/tk/repos/tracing/tracing-subscriber)
error[E0080]: could not evaluate constant
  --> /Users/tk/.cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/sharded-slab-0.0.6/src/page/slot.rs:54:24
   |
54 |     const LEN: usize = (cfg::WIDTH - C::RESERVED_BITS) - Self::SHIFT;
   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempt to subtract with overflow

error[E0080]: could not evaluate constant
   --> /Users/tk/.cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/sharded-slab-0.0.6/src/lib.rs:691:26
    |
691 |         let shift = 1 << (Self::LEN - 1);
    |                          ^^^^^^^^^^^^^^^ referenced constant has errors

error[E0080]: could not evaluate constant
  --> /Users/tk/.cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/sharded-slab-0.0.6/src/cfg.rs:40:30
   |
40 |     const USED_BITS: usize = Generation::<Self>::LEN + Generation::<Self>::SHIFT;
   |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ referenced constant has errors

error[E0080]: could not evaluate constant
   --> /Users/tk/.cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/sharded-slab-0.0.6/src/cfg.rs:142:34
    |
142 |             .field("used_bits", &C::USED_BITS)
    |                                  ^^^^^^^^^^^^ referenced constant has errors

error[E0080]: could not evaluate constant
   --> /Users/tk/.cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/sharded-slab-0.0.6/src/lib.rs:704:25
    |
704 |     const MASK: usize = Self::BITS << Self::SHIFT;
    |                         ^^^^^^^^^^^^^^^^^^^^^^^^^ referenced constant has errors

error[E0080]: could not evaluate constant
   --> /Users/tk/.cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/sharded-slab-0.0.6/src/page/slot.rs:560:24
    |
560 |     const LEN: usize = Generation::<C>::LEN;
    |                        ^^^^^^^^^^^^^^^^^^^^ referenced constant has errors

error[E0080]: could not evaluate constant
   --> /Users/tk/.cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/sharded-slab-0.0.6/src/page/slot.rs:495:37
    |
495 |     const LEN: usize = cfg::WIDTH - (Lifecycle::<C>::LEN + Generation::<C>::LEN);
    |                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ referenced constant has errors

error[E0080]: could not evaluate constant
   --> /Users/tk/.cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/sharded-slab-0.0.6/src/lib.rs:699:26
    |
699 |     const SHIFT: usize = Self::Prev::SHIFT + Self::Prev::LEN;
    |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ referenced constant has errors

error: aborting due to 8 previous errors

For more information about this error, try `rustc --explain E0080`.
error: could not compile `tracing-subscriber`.

[hawkw]

I think the default MAX_THREADS value is way too high on 32-bit platforms — it is currently half of the 64-bit MAX_THREADS. On 64-bit targets, MAX_THREADS is 4096 (12 bits), and on 32-bit, it's 2048 (11 bits). It should be half as many bits, not half as high a number

Also both of those values should be one less, since 0 is a valid thread ID.

[hawkw]

@thombles I think the reason that this crate builds for you but tracing-subscriber (which depends on it) does not is that building the lib never actually evaluated the default config. I think that trying to build this crate's tests for a 32-bit target should fail?

Can I get you to do the following:

• try to cargo build --test for a 32-bit target, and confirm that it fails
• check out this branch
• try building the tests for a 32-bit target again and let me know if that branch fixes it?

Thanks! I don't currently have a 32-bit toolchain installed, so I figured I'd ask you to check it out.

[thombles]

Using the eliza/fix-max-threads-bits branch of sharded-slab I can successfully build both this crate, and tracing-subscriber, using the i686-linux-android target. 🎉

In terms of your specific instructions it's a bit more complicated.

Can I get you to do the following:

• try to cargo build --test for a 32-bit target, and confirm that it fails

It does fail, but not for the same reason - the loom dev dependency pulls in generator which currently only supports a handful of 64-bit architectures with its assembly.

• check out this branch
• try building the tests for a 32-bit target again and let me know if that branch fixes it?

It doesn't help with the generator issue of course. But it has solved my actual problem!