Before this change, buildRustCrate always called rustc with
--extern libName=[...]libName[...]
However, Cargo permits using a different name under which a dependency
is known to a crate. For example, rand 0.7.0 uses:
[dependencies]
getrandom_package = { version = "0.1.1", package = "getrandom", optional = true }
Which introduces the getrandom dependency such that it is known as
getrandom_package to the rand crate. In this case, the correct extern
flag is of the form
--extern getrandom_package=[...]getrandom[...]
which is currently not supported. In order to support such cases, this
change introduces a crateRenames argument to buildRustCrate. This
argument is an attribute set of dependencies that should be renamed. In
this case, crateRenames would be:
{
"getrandom" = "getrandom_package";
}
The extern options are then built such that if the libName occurs as
an attribute in this set, it value will be used as the local
name. Otherwise libName will be used as before.
While it is not obvious from the source, cargo sets CARGO_MANIFEST_DIR to an absolute directory. This let to a build problem with the popular "tera" crate using the "pest" crate.
## Cargo details
The variable is set here:
f7c91ba622/src/cargo/core/compiler/compilation.rs (L229)
and computed from the `manifest_path`:
f7c91ba622/src/cargo/core/package.rs (L163)
The manifest path is also exported via `cargo metadata` where you can see that it is absolute.
The build expression got quiet large over time and to make it a bit
easier to grasp the different scripts involved in the build are now
separated from the nix file.
