Design For Transitive Features

[everythingfunctional]

In the recent Fortran-Lang Monthly Call, @perazz and I discussed a bit about how to design, using the [features] feature, how to conveniently depend on a package that has optional features. The current design already enables this (sort of), but without it being particularly convenient.

For example, say there are two packages we'd like to depend on, each with a feature defined, and we'd like to also build on these features.

name = "pkg_a"
# ...

[features]
feat_a.preprocess.cpp.macros = ["WITH_FEAT_A"]

name = "pkg_b"
# ...

[features]
feat_b.preprocess.cpp.macros = ["WITH_FEAT_B"]

name = "my_pkg"
# ...

[features]
feat_a.dependencies.pkg_a = { path = "../depends/pkg_a", features = ["feat_a"] }
feat_a.preprocess.cpp.macros = ["WITH_FEAT_A"]

feat_b.dependencies.pgk_b = { path = "../depends/pkg_b", features = ["feat_b"] }
feat_b.preprocess.cpp.macros = ["WITH_FEAT_B"]

So I think all that is needed is just a shorthand to make it more convenient. Something like the following.

name = "my_pkg"
# ...

[dependencies]
pkg_a = { path = "../depends/pkg_a", import_features = ["feat_a"] }
pkg_b = { path = "../depends/pkg_b", import_features = ["feat_b"] }

The usage of the import_features entry says that, for each feature listed, add to the features table of this package an entry with the same name that includes a .dependencies.pkg_name entry with that feature listed, and adds entries identical to the entries for that feature from the named package.