I have two features: feature_1 and feature_2:
[features]
default = ["feature_1"]
feature_1 = []
feature_2 = []
I want to let the user choose only one of them at a time because choosing both at the same time will lead to a duplicating of some important code and by some other reasons. How can I do that?
Fundamentally, you can't. Cargo features are additive, and features can be enabled by any crate in the dependency tree. There's an implicit assumption on the part of Cargo that it is always valid to enable additional features.
What's more, features aren't simply requests, they are demands. If one crate demands feature_1, and another crate demands feature_2, you've got two crates that cannot possibly work together. Cargo (and Rust itself) go to some lengths to try and make such situations difficult to create.
The best solution is to change how your crate is written such that both features can be enabled simultaneously. If that's really impossible, the best you can do is prevent compilation from succeeding. This can be done either with a build script for your crate that detects incompatible features and fails, or by placing non-compiling code in your crate that is only enabled when incompatible features are enabled.
For what it's worth, I opted to use this approach:
#[cfg(all(feature = "feature_1", feature = "feature_2"))]
compile_error!("Feature 1 and 2 are mutually exclusive and cannot be enabled together");
I hope it'll help others looking for a solution to the same problem.
Related
I want to extract all nightly features used by a crate for some research purposes.
Just to be clear, "nightly features" means codes like #![feature(...)]", but not crate provided optional features.
I tried using regex, and it works, but not accurate enough. cfg defined features may not be detected: #![cfg_attr(target_os = "macos", feature(...))].
So I turn to rustc compiler, I wish to modify some source code and print out what features crate uses. Now I'm still trying to find out how rustc deals with nightly features.
I hope someone can give me some tips and help about the process of rustc dealing with nightly features. I'm almost lost in codes.
Access to enabled features is done via rustc_session::Session::features. You can add code after they are set in rustc_interface, or look at how they're computed.
If you are at a later stage, you probably have access to the Session of the compiled crate. For example, TyCtxt has a sess() method (but also a features() method for direct access using the query system), and InferCtxt has a tcx member to access the TyCtxt.
On crates.io we can easily see the direct dependencies of a crate by just clicking on the Dependencies tab. Is there a way to also easily see the sub-dependencies of a crate? Perhaps in a tree-like view, similar to what cargo tree would display. Or at least the number of all (sub)dependencies.
I think that can be helpful, for example, when we need to decide which crate to use among alternatives. By having an indicator of the total number of (sub)dependencies, we would have a better idea on how "heavy" a library actually is. I think that can be especially useful for a language like Rust where the build speed seems to heavily depend on the number of dependencies.
For example, I use barcoders crate:
barcoders = {version = "0.10.0", features = ["image",]}
Is it possible to specify which version of image this dependency should use?
Something like
barcoders = {version = "0.10.0", features = ["image=0.22.3",]}
Because it uses image crate version 0.18.0 and in my project I use latest 0.22.3.
Does it mean that there's only 2 ways to resolve that:
I downgrade version in my package
Barcoders dependency get updated
No, there is no way to specify the version for a dependency's (optional) dependency. This makes sense, as your dependency run their tests only against the version they specify in their Cargo.toml. In this case, as it appears everything you're doing uses open source, you could fork barcoders, update the dependency, run the test suite and if it passes, use your fork. It would also be polite to open an issue in that case.
If barcoders wasn't open-source, so you couldn't fork it, your best bet would be to switch to the version of image that barcoders uses. If your crate is a library, it may be annoying to expose a public interface that uses outdated libraries, but that's life. The "proper" solution to this problem is to wait until image has a 1.0 release, which is basically a forward compatibility promise, then barcoders can specify image = "^1" (i.e. >=1.0.0 <2.0.0). I mention this "solution" only because you appear to have commit privileges on barcoders, in fact you solved your own problem by updating the image dependency in barcoders.
As one of the comments points out, this version compatibility issue is less fragile that it at first seems. So long as types from different versions of some dependency crate don't cross api boundaries, your project can include any number of versions of that dependency simultaneously. Working with multiple versions of libraries took some work from the rust team on name mangling, which you can read about here
No, you can't, and shouldn't, and shouldn't worry.
Libraries were developed at a single point in time, used dependencies with a certain API. The dependency is likely to change some of that between major versions (changing the type a function returns, exposing different patterns, or whatever). This may make it unable to compile anymore. To really update something, you might need to change parts of the code that is using the dependency in the first place.
This is open source world, so you can do so and publish a pull request in the original crate to update. It might be appreciated, but don't underestimate the care that needs to be taken to not break other people's crates yourself when doing so.
Or make your own fork of the crate that updates it just for you.
But you are probably just worried seeing duplicates of the same crate with different versions during compilation. Cargo indeed compiles with different versions, so all calls to the dependended crate will receive what the developer expected when he/she wrote it. This is not a problem, in performance, or amount of instructions that end up in the binary. Just stop worrying.
By forking and playing with some code, I noticed that Cargo can download and bundle multiple versions of the same crate in the same project (native-tls 0.1.5 and 0.2.1, for example). I have wasted so much time by looking at the documentation of the wrong version.
I have looked for some information about this behaviour but I was not able to find anything. Is this documented somewhere?
Is there an easy way to determine/detect the version used by the code you're working on (current edited file)? Or can we tell Cargo to show some warnings/prevent the build if two versions the same crate are required?
It was a conscious decision when designing Rust to allow multiple versions of the same crate.
You have probably heard of Dependency Hell before, which occurs when 2 (or more) dependencies A and B have a common dependency C but each require a version which is incompatible with the other.
Rust was designed to ensure that this would not be an issue.
In general, cargo will attempt to find a common version which satisfies all requirements. As long as crate authors use SemVer correctly, and the requirements give enough leeway, a single version of the dependency can be computed and used successfully.
Sometimes, however, multiple versions of the same dependency are necessary, such as in your case since 0.1.x and 0.2.x are considered two different major versions. In this case, Rust has two features to allow the two versions to be used within the same binary:
a unique hash per version is appended to each symbol.
the type system considers the same type Foo from two versions of C to be different types.
There is a limitation, of course. If a function of A returns an instance of C::Foo and you attempt to pass it to a function of B, the compiler will refuse it (it considers the two types to be different). This is an intractable problem1.
Anytime the dependency on C is internal, or the use of C is isolated, it otherwise works automatically. As your experience shows, it is so seamless that the user may not even realize it is happening.
1 See the dtolnay trick that crate authors can use to allow some types to be interchangeable.
Cargo can indeed link multiple versions of some crate, but only one of those versions can be a direct dependency. The others are indirect references.
The direct reference is always the version referenced by Cargo.toml and on top-level of Cargo.lock (while the indirect references are in the dependencies subsections).
I am not sure how much it is documented, unfortunately.
I want to add a relatively small feature to crate A which necessitates depending on a second crate B. A's maintainer reasonably objects about introducing a whole new dependency for a small, although potentially useful, feature.
What are the best ways of handling this? The options I can think of are:
make a new crate with the new feature (with dependencies on both A and B)
use Cargo "features" somehow so users have to opt in to the extra dependency.
1 seems suboptimal since it's a small feature that's pretty tied in to A, and it seems annoying to have to specially depend on a third crate, but 2 seems almost as bad...
Is there some way to compile my minor feature if and only if a crate explicitly depends on both A and B?
This is exactly the case optional features solve:
Cargo supports features to allow expression of:
conditional compilation options (usable through cfg attributes);
optional dependencies, which enhance a package, but are not required; and
clusters of optional dependencies, such as postgres, that would include the postgres package, the postgres-macros package, and
possibly other packages (such as development-time mocking libraries,
debugging tools, etc.).
Unfortunately, you seem to have already dismissed this option without any explanation as to why it's "almost bad", so... I guess the only option you have left is to fork the crate and apply your changes.