Add no_std support to bevy_log

[Shatur]

Objective

• Resurrects Allow partial support for bevy_log in no_std #18782, which was reverted in Revert "Allow partial support for bevy_log in no_std (#18782)" #18816. This time with proper no_std support.

@bushrat011899's work on adding portable-atomic support was merged. I saw that this was backported in tokio-rs/tracing#3283 and started working on the PR. Then I realized the release hadn’t been drafted yet: tokio-rs/tracing#3282 😢
Keeping this PR as a draft for now - it only requires uncommenting 2 lines once the release drops (see the TODO in bevy_log's Cargo.toml).

Solution

• Add std, web, and critical-section to bevy_log.
• Enable portable-atomic in bevy_log, similar to other crates.
• Make bevy_log non-optional. It became optional after the no_std migration (Add no_std support to bevy #17955) because it required std. In the original attempt (Allow partial support for bevy_log in no_std #18782), it was also made non-optional.

Testing

• CI

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Migration Guide

• If you were previously using bevy_log with default features disabled, enable the new std feature.
• If you were using the bevy_log feature, remove it.

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Please review the instructions for writing migration guides, then expand or revise the content in the migration guides directory to reflect your changes.

[Shatur]

@alice-i-cecile But I provided a migration guide. Probably the label was added by accident.

[james7132]

@\alice-i-cecile But I provided a migration guide. Probably the label was added by accident.

Might have been a bit out of date: migration guides now go in tree under the release-content directory.

[Shatur]

Ah, I didn't know! Will add once tracing releases then.

[james7132]

Just to jot this down somewhere: we're currently using log for bevy_ecs instead of tracing due for similar reasons why bevy_log was not supporting no_std. This PR wouldn't be a bad place to change that.

[Shatur]

There are many places where log needs to be replaced with bevy_log. I think it's better to open a follow-up for replacing log everywhere 🙂

But looks like tracing won't be released in time for 0.17 😢

[Shatur]

Updated to 0.18, but I'm still waiting for a new release: tokio-rs/tracing#3413 (comment)