--incompatible_exclude_starlark_flags_from_exec_config

[gregestren]

Motivation

Stop Starlark flags from propagating to the exec config by default.

i.e. on:

$ bazel build //foo --//my:flag=some_custom_value

stop compilers, code generators, and other tools from also building with --//my:flag=some_custom_value.

The vast majority of the time these settings are intended for production code, not compiler tools. Passing them to tools compromises caching, making builds slower and more resource-heavy for no good reason.

Description

With --incompatible_exclude_starlark_flags_from_exec_config=true, --//my:flag=some_custom_value resets to its default value when building compiler tools, and any other exec-configured targets.

Incompatible Flag

--incompatible_exclude_starlark_flags_from_exec_config

Migration Guide

This only affects you if you need a Starlark flag to pass its value to exec-configured targets. While this isn't common, there are valid use cases. For example a flag that modifies rules_foo behavior may want to apply to all rules_foo targets, no matter their configuration.

To define custom exec propagation logic for a flag, do one of the following:

1. (preferred) Add specific exec config logic to the flag definition. Part of a pending proposal. Not live yet, but we don't intend to flip this flag until that API is live
2. Set --experimental_propagate_custom_flag=//my:flag (at the command line or in a .bazerlc). With this set, the above example keeps the flag set to some_custom_value in the exec config. That's equivalent to --incompatible_exclude_starlark_flags_from_exec_config being disabled for just this flag.

In which Bazel LTS version will this incompatible change be enabled?

Bazel 9

Additional Context

No response

TODO List

• Implement Starlark flag propagation API so migration step 1 is viable
• Flip the flag

[gregestren]

Flag definition in #26899 (it's a rename from --experimental_exclude_starlark_flags_from_exec_config).