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.
In combination with carnix we can now build crates that require a
specific edition of rust features. A few crates started requiring that
already and having this in nixpkgs is just logical.
This commit adds test based on real-world crates (brotli).
There were a few more edge cases that were missing beforehand. Also it
turned out that we can get rid of the `finalBins` list since that will
now be handled during runtime.
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.
Cargo has a few odd (old) ways of picking source files if the `bin.path`
attribute isn't given in the Cargo.toml. This commit adds support for
some of those. The previous behaviour always defaulted to `src/main.rs`
which was not always the right choice.
Since there is look-ahead into the unpacked sources before running the
actual builder the path selection logic has to be embedded within the
build script.
`buildRustCrate` currently supports two ways of running building
binaries when processing a crate:
- Explicit definition of all the binaries (& optionally the paths to
their respective `main.rs`) and,
- if not binary was explictly configured all files matching the patterns
`src/main.rs`, `src/bin/*.rs`.
When the explicit list is given without path information paths are now
being picked from a list of candidates. The first match wins. The order
is the same as within the cargo compatibility code.
If the crate does not provide any libraries the path `src/{bin_name}.rs`
is also considered.
All underscores within the binary names are translated into dashes (`-`)
before the lookups are made. This seems to be a common convention.
