RFC: Compiler Selection - Platform Constraints vs Starlark Flags

[srikantharun]

Background

Currently, rules_cc uses platform constraints inconsistently:

• Windows toolchain uses constraints on exec_platform
• Some users expect constraints on target_platform
• Neither may be semantically correct for all use cases

Design Dimensions

1. Platform Constraints vs Starlark Flags

Approach	Best For	Limitations
Platform constraints	Hermetic toolchains, cross-compilation, RBE	Less flexible for ad-hoc overrides
Starlark flags	Non-hermetic toolchains, user overrides, backward compat	Breaks hermeticity, poor cross-compilation support

Note: Starlark flags can integrate with toolchain resolution via required_settings attribute.

2. Exec Platform vs Target Platform

Constraint Location	Semantic Meaning
exec_platform	"Compiler available on this machine" (RBE workers, licensed compilers)
target_platform	"Code targets this environment" (embedded, cross-compile)
Neither	"User preference" (gcc vs clang on same platform)

3. Hermetic vs Non-Hermetic Toolchains

• Hermetic: Downloaded by Bazel, reproducible → Platform constraints work well
• Non-Hermetic: Preinstalled on host/RBE → May need exec_platform constraints or flags

Use Cases to Consider

1. Cross-compilation - Building for ARM on x86_64 host
2. RBE with licensed compilers - Commercial compiler preinstalled on RBE workers
3. CI matrix testing - Same code with gcc/clang/msvc
4. Code compatibility - Codebase requires specific compiler (MSVC extensions, GCC builtins)
5. Hermetic builds - Enforcing exact compiler version across team

Questions for Discussion

1. Should compiler be a platform property or user configuration?
2. If constraints, should they be on exec_platform, target_platform, or both?
3. Should we support hybrid (constraints + flag override)?
4. How do rules_go and rules_rust handle this? Should rules_cc align?

Prior Art

• rules_go: Platform constraints for SDK selection, hermetic downloads
• rules_rust: Platform constraints with explicit exec/target distinction
• rules_cc (Windows): Exec platform constraints (current)

References

• PR Expose compiler constraint values in public API #562: Expose Compiler Constraint Values
• Issue Platform compatibility issues for Bazel 7+8 #330: Platform compatibility across Bazel versions
• Bazel docs: Toolchain Resolution

cc @fmeum