Downloading a single package is much more painful on master than on 0.31

[gnzlbg]

Problem

Downloading a single package in cargo 0.31 was as simple as:

let package_id: PackageId;
let package = source.source.download(&package_id)?;

With cargo master one needs to:

let package_ids: [PackageId];  
let source: Box<Source + 'a>;
let config: Config;
let sources = {
    let mut s = SourceMap::new();
    s.insert(source);
    s
};

let package_set = PackageSet::new(&package_ids, sources, config)?;
let mut downloads = package_set.enable_download()?;
let package = if let Some(package) = downloads.start(package_ids[0])? {
    package
} else {
    downloads.wait()?;
    downloads.start(package_ids[0])?.expect("started download did not finish after wait")
};

This feels like a big API regression.

[alexcrichton]

FWIW get_one was added as a convenience to do all this

[gnzlbg]

Thanks! get_one cuts it down to:

let package_ids: [PackageId];  
let source: Box<Source + 'a>;
let config: Config;
let sources = {
    let mut s = SourceMap::new();
    s.insert(source);
    s
};

let package_set = PackageSet::new(&package_ids, sources, config)?;
let package = package_set.get_one(package_ids[0])?;

[dwijnand]

I feel bad for the libraries removal thing. I'd be happy to try and land some improvements to the API if you have any recommendations...

[gnzlbg]

@dwijnand I pretty much agree with you that we should be able to do that via the cargo metadata somehow. I've opened an issue about that in the cargo-metadata bindings (oli-obk/cargo_metadata#60 ) and it might make more sense to improve the bindings to support that, also improving cargo metadata as required.

We can revisit adding a better API to cargo for that if the cargo metadata approach ends up being too complicated.

[dwijnand]

Good to hear!

But I actually was thinking for the single download API. Perhaps source.source could have a download method that does the necessary packageset/sourcemap setup to download a package.

[gnzlbg]

That would help!

[dwijnand]

So I was looking back at this, trying to understand what cargo API rust-semverver used to use that's now gone. And I'm confused because source.download(&package_id) still exists:

cargo/src/cargo/core/source/mod.rs

Lines 50 to 52 in 23fa400

	/// The download method fetches the full package for each name and
	/// version specified.
	fn download(&mut self, package: PackageId) -> CargoResult<MaybePackage>;

So going back to the outset, why doesn't that work any more?

[ehuss]

@dwijnand It was changed to return a MaybePackage, so it doesn't actually download. PackageSet can be used to download now.