[Option] Add a feature flag for -Isystem

[ian-twilightcoder]

swift-build needs a flag to know if -Isystem is available.

rdar://152540225

[cachemeifyoucan]

Can you just check on swift driver side for if a frontend flag is available? Or is this more involved?

[ian-twilightcoder]

Can you just check on swift driver side for if a frontend flag is available? Or is this more involved?

I don't think so, swift-build uses the current version of swift-driver that knows about Isystem, but it uses an old version of swift-frontend that doesn't know about it.

[ian-twilightcoder]

Related to swiftlang/swift-build#553

[ian-twilightcoder]

@swift-ci smoke test

[cachemeifyoucan]

Merged?

What are you going to do on build system side if flag is not available? Pass as "-F" instead? You can eat the unsupported flag in swift driver and pass it as something else. I don't like a small thing like a flag available is listed as feature and we need to teach build system about it but I don't exactly know what you want to do. This implementation is also fine so no need to revert.

[ian-twilightcoder]

Merged?

I enabled auto-merge and it merged with no approvals. 🤦‍♂️

What are you going to do on build system side if flag is not available? Pass as "-F" instead? You can eat the unsupported flag in swift driver and pass it as something else. I don't like a small thing like a flag available is listed as feature and we need to teach build system about it but I don't exactly know what you want to do. This implementation is also fine so no need to revert.

No, pass as -I. swiftlang/swift-build#553 This approach was the one suggested by the swift-build team. I didn't see anything in the driver that knows about what flags are available in the front end, my first instinct was for swift-build to use the driver to query the front end but there doesn't seem to be such a thing.

[artemcm]

I didn't see anything in the driver that knows about what flags are available in the front end, my first instinct was for swift-build to use the driver to query the front end but there doesn't seem to be such a thing.

The driver queries the toolchain's compiler at the beginning of planning to get supported flags. And then throughout the driver code we use:

if isFrontendArgSupported(.<someFlag>) {
   // Specify this flag to the actual job command
}

And swift-build, similarly, can query which flags its driver supports.

[ian-twilightcoder]

I didn't see anything in the driver that knows about what flags are available in the front end, my first instinct was for swift-build to use the driver to query the front end but there doesn't seem to be such a thing.

The driver queries the toolchain's compiler at the beginning of planning to get supported flags. And then throughout the driver code we use:

if isFrontendArgSupported(.<someFlag>) {
   // Specify this flag to the actual job command
}

And swift-build, similarly, can query which flags its driver supports.

Iiinteresting. That's awkward to hook up in SwiftCompilerSpec since that doesn't have a Driver, but I like the idea of swapping the flag in the driver and then we don't need this feature flag. Let me see if I can get that going today and then revert this and the swift-build adoption too.

[ian-twilightcoder]

I didn't see anything in the driver that knows about what flags are available in the front end, my first instinct was for swift-build to use the driver to query the front end but there doesn't seem to be such a thing.

The driver queries the toolchain's compiler at the beginning of planning to get supported flags. And then throughout the driver code we use:

if isFrontendArgSupported(.<someFlag>) {
   // Specify this flag to the actual job command
}

And swift-build, similarly, can query which flags its driver supports.

Iiinteresting. That's awkward to hook up in SwiftCompilerSpec since that doesn't have a Driver, but I like the idea of swapping the flag in the driver and then we don't need this feature flag. Let me see if I can get that going today and then revert this and the swift-build adoption too.

Although... If SWIFT_USE_INTEGRATED_DRIVER is set to NO then I guess swift-build needs to look at the features directly. So maybe we do need this feature flag after all.