Hi Chris,
Do you mean that if a crate X has an optional feature that requires
crate Y, then Cargo requires Y to be present when building X even if X
is being built with that feature disabled?
Correct, the source to crate Y must be present when crate X is being built, transitively, or otherwise. * If we have a “circular” dependency as part of dev-dependencies
(e.g. one crate pulls in an upstream crate during testing to ensure it
doesn’t break anything) we’ll need to detangle the dependency graph by
rewriting duplicate packages to include the #:skip-build? flag. I can
elaborate more on this in a separate email.
I think here, you're talking about the situation in which crate A depends on crate B, which in turn depends on crate A's source, and to break the cycle we will replace B's dependency on A with a dependency on A', where A' is effectively just a source build of A (via #:skip-build? #t). Is that basically right? I agree it would be good to discuss the resolution of circular dependencies in another email thread, but I just wanted to double check with you that my basic understanding of your intent is correct.
Yep, your understanding of the situation and potential solution is correct. I’ll send out a different email thread around this since we might need some small tweaks to the overall build system to support this.
What about Cargo packages that are
both libraries and applications? Do those even exist?
Yes these are a bit rare but they do exist. I don’t have any examples on
hand, but you can have something akin to curl which can be used
as a binary, as well as imported as a library to other projects.
In those rare cases, your changes are still good to go, right? We don't actually interact with the Cargo.lock file, and if there are any executables, your code will install them.
Yep, there’s no harm in building the “src” output of an application crate (outside of using up some store space) in case something else wants to depend on it. Something else occurred to me. In packages of C libraries, such as the
glibc package, we install libraries (e.g., ".so" files go into the "out"
output, and ".a" files go into "static" output). However, here we are
not installing any libraries like that. Do all Rust developers just use
Cargo.toml files to declare their dependencies, and then let Cargo
manage all the actual compiling and linking? Do they ever manually
install a Rust library (without using Cargo) when hacking on a project?
In general, cargo is used to perform all building an linking of the final rust outputs (binaries, shared libraries, and other rust artifacts). Cargo also supports defining arbitrary build scripts such as building some other non-rust dependency which is to be linked in the final outputs.
However, cargo does not perform the actual distribution of these “external” dependencies. Crates may vendor their own external sources, or they may expect them to be present in the usual places, or they may support an environment variable which points them in the right direction.
One example is the `jemalloc-sys` crate. If built with an environment variable pointing to a compiled version of jemalloc, it will build rust bindings which link to that binary. Otherwise it will build its own vendored version of the jemalloc source.
Handling these packages will need to be done on a case-by-case basis with some additional setup glue in the package definitions, as necessary. - (generate-checksums rsrc src)
+ (generate-checksums rsrc "/dev/null")
This probably deserves a short comment to clarify the intent.
Do you mean commenting on the intent of `generate-checksums`
or the intent of the /dev/null parameter?
I mean the "/dev/null" argument, mainly. As far as I can tell, it looks like the generate-checksums procedure builds a file with checksums to satisfy some requirement of the cargo tool. That seems reasonable, but I'm not sure why we use "/dev/null" here.
Cargo expects the checksum package to include a checksum of each individual file as well as a checksum for the entire directory. The `generate-checksums` procedure doesn’t correctly handle the second parameter being a directory and raises an error. Luckily, cargo doesn’t seem to care about the contents of this checksum, as long as the declaration is there.
I didn’t want to change the `generate-checksums` procedure just yet since it’s used during building of rust-1.20, and doing so will require a full bootstrap of the compiler chain, which would have blocked me for a while. Finally, two more minor comments about code style which I don't think
you need to change but are good to know going forward:
- Instead of "system*", we prefer to use "invoke" (defined in (guix
build utils)) whenever possible. It throws an exception when an error
occurs, so it's harder to accidentally ignore errors.
- Using "and" and "or" statements for control flow is OK, especially
since you're using "system*". In fact, we do this in other parts of
Guix, too. However, I personally feel that forms like "if", "when",
and "unless" are clearer in some cases and should probably be
preferred, especially when using "invoke" instead of "system*”.
Thank you for the tips, I’ll keep these in mind going forward. Still pretty new to guile, its APIs, and guix’s utilities on top of that! :)
—Ivan
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