[naga] constants aren't named expressions

[sagudev]

Connections
Fix #6207

Description
Do not threat local constants as named expressions, and do not emit constable expr as constant in wgsl-out.

Testing
Existing tests

Squash or Rebase? Squash

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.

[andyleiserson]

I think this was already clear to everyone else from the discussion in the issue, but I'll note that removing constants from named_expressions has the side effect that they're less likely to appear in the output (see here), and if they do appear, they will appear without their names and not as const.

Otherwise, this looks good to me, normally I would approve, but I think it would be good to have a stamp from somebody more familiar with the code than I am.

It would be nice to merge this soon, because it is going to conflict with the tests I am writing for my fix for #7356.

[jimblandy]

I think this was already clear to everyone else from the discussion in the issue, but I'll note that removing constants from named_expressions has the side effect that they're less likely to appear in the output (see here), and if they do appear, they will appear without their names and not as const.

What Naga promises is to preserve your shader's behavior. What we aspire to is legible output that has a reasonable relation to what you wrote. So it's not really a blocker if local const expressions get removed.

Note that everything with abstract type will always be removed.

The rationale for omitting local consts is explained in #6207 (comment). This isn't the ideal solution, but it will do for now.

[sagudev]

Rebased!

[sagudev]

Gentle ping @jimblandy