Doesn't support JSON targets

[thejpster]

If you supply a target as a JSON file, the build.rs can't detect the target. This is because target is a full path - in my case starts this starts C:\, which != thumbv.

I think the fix is to take the file_stem() of the path and use that. This should work for both JSON targets and built-in targets (that don't contain .). If the built-in target does use . (e.g. thumbv8-m.base) we'll need to be a bit cleverer.

[japaric]

I think the fix is to take the file_stem() of the path and use that.

This doesn't always work. There's no guarantee the file name matches the LLVM name or it's even a proper triple, e.g. foo.json is a valid name for a custom specification file. The proper way is to run rustc --print cfg --target $TARGET and then parse the output. However, on beta / stable the output doesn't contain enough information to discern ARM Linux from ARM Cortex-M. For example, target_feature="mclass" only appears in the nightly output of rustc --print cfg --target thumbv7m-none-eabi and it's not stable, i.e. it may disappear on any nightly update.

If we want to "support" custom targets the documentation should state that this is best effort support and than nightly updates may break things.

[thejpster]

I agree it requires the user to name their JSON file correctly, but at least they have that option. Currently, cortex-m and cortex-m-rt don't work at all with a JSON target file (i.e. thumbv8m)