C++ compiler is selected via a constraint

[comius]

Description of the bug:

Bazel selects C++ compiler using a compiler constraint. Because of this, users need to create a new platform for each compiler. This doesn't really match the expectation of what a platform should describe.

The problem is most obvious on Windows, which has several C++ compilers.

Proposed solution

What should happen instead is that the compiler is selected by --compiler flag (that has arbitrary string value). This approach is well tested and works well at Google.

Caveat is that this is now an incompatible change. Compiler constraint was introduced very early, before Bazel team build up experience with platforms and toolchains.

[comius]

cc @fmeum @meteorcloudy @gregestren @katre @mai93

[fmeum]

Bazel selects C++ compiler using a compiler constraint. Because of this, users need to create a new platform for each compiler. This doesn't really match the expectation of what a platform should describe.

I would say it depends. If the compiler is not hermetic, but rather discovered on the host and accessed from PATH, then it does make sense to select it via an exec constraint (never a target constraint) on a platform, with the constraint meaning "the execution platform needs to have gcc/clang/clang-cl available in PATH". As far as I know, this is the situation on Windows.

But if there are multiple hermetic toolchains available, required_settings seems more appropriate as the toolchains can be used on any exec platform supporting their general OS/arch constraints.