The platforms flag should be single-valued

[katre]

Description of the feature request:

Right now, the --platforms flag accepts multiple values but only the first is actually used.

Instead, the flag should be single-valued and give an error if multiple values are passed.

Further, the flag should actually be --platform and uses should be migrated, but that may be harder to manage.

Which category does this issue belong to?

Configurability

What underlying problem are you trying to solve with this feature?

Poor decision decisions at the start of the platforms project

Which operating system are you running Bazel on?

No response

What is the output of bazel info release?

No response

If bazel info release returns development version or (@non-git), tell us how you built Bazel.

No response

What's the output of git remote get-url origin; git rev-parse master; git rev-parse HEAD ?

No response

Have you found anything relevant by searching the web?

No response

Any other information, logs, or outputs that you want to share?

No response

[katre]

@sdtwigg We discussed this recently

[chaselatta]

I put together a quick repro case which shows the issue we discussed. https://github.com/chaselatta/platform_bug_repro

[fmeum]

I remember a design doc by @brentleyjones that (if I recall correctly) gave meaning to a multi-valued --platforms flag: A target is built for the first platform in the list that it is target_compatible_with, which allows for pretty clean flagless multi-platform builds. I am noting this here to make sure that if we migrate, we are very sure that we aren't going to migrate back eventually.

[brentleyjones]

Yes, I would like to confirm if this means we will never adopt something like my proposal. And if so, if there are any future plans at all to support multi-platform builds in a nice way?

[katre]

@brentleyjones: Your proposal was very interesting, and we may decide to take it up in the future. At this point, however, I think the best approach is the one from https://github.com/bazelbuild/proposals/blob/main/designs/2023-06-08-standard-platform-transitions.md, where the controls are in BUILD files and Starlark rules.

The reason to handle this now are to reduce confusion, both for Bazel users (who frequently ask about what happens when they pass multiple values to --platforms) and for Bazel developers (who keep having to add defensive code around PlatformOptions.platforms).

If we decide to implement either @brentleyjones's proposal or something similar, we can change the flag semantics again to match whatever we decide to implement.

[katre]

Copying parts of my comment from #6519:

Steps for cleaning up --platforms:

1. Make the existing multi-valued flag single-valued, but leave the name.
2. Actually rename the flag to --platform (flag migrations are hard, sorry), leaving --platforms as a deprecated legacy name.
3. Actually remove --platforms.

[fmeum]

@katre Would step 1 include making it single-valued within transition implementation functions as well? I could see that being very breaking.

Could we keep //command_line_option:platforms list-valued until it is removed and introduce //command_line_option:platform as the separate single-valued alternative?

[katre]

Yes, that's part of why we're not moving on this faster.

Having two different flags also seems confusing. When we pick this up we'll consider which is less bad.

I consider multi-valued --platforms as one of the two biggest mistakes I made when starting the platforms project (the other was a separate toolchains parameter to rule instead of a new attr.toolchain attribute type).