If I define a dependency like foo = { version = "1.0.0", optional = true },
will it be available when I do "cargo run"? Can I check if it is enabled in the code?
if cfg!(feature = "foo") {}
Doesn't seem to be working, like the feature is missing all the time.
Moving answer to 60258216 here:
Optional dependencies do double as features: https://stackoverflow.com/a/39759592/8182118
They will not be enabled by default unless they're listed in the default feature, though you can enable the feature using cargo run --features foo.
For clarity and forward compatibility you may want to create an actual feature which enables the dependency though, that way if you need to "fluff up" the feature in the future and that requires new optional dependencies it's much easier.
In the code, both #[cfg] and cfg! should work depending whether you want to check for it at compile-time or runtime.
It's not hard to test either:
[package]
name = "testx"
version = "0.1.0"
edition = "2018"
[features]
default = ["boolinator"]
magic = ["boolinator"]
empty = []
[dependencies]
boolinator = { version = "*", optional = true }
and a main.rs of:
fn main() {
# macro and attributes would work the same here
if cfg!(feature = "boolinator") {
println!("Hello, bool!");
} else {
println!("Hello, world!");
}
}
you get
$ cargo run -q
Hello, bool!
$ cargo run -q --no-default-features
Hello, world!
$ cargo run -q --no-default-features --features boolinator
Hello, bool!
$ cargo run -q --no-default-features --features magic
Hello, bool!
$ cargo run -q --no-default-features --features empty
Hello, world!
See also https://github.com/rust-lang/edition-guide/issues/96
Related
Is there a standard way to determine what features are available for a given crate?
I'm trying to read Postgres timezones, and this says to use the crate postgres = "0.17.0-alpha.1" crate's with-time or with-chrono features.
When I try this in my Cargo.toml:
[dependencies]
postgres = { version = "0.17.0-alpha.1", features = ["with-time"] }
I get this error:
error: failed to select a version for `postgres`.
... required by package `mypackage v0.1.0 (/Users/me/repos/mypackage)`
versions that meet the requirements `^0.17.0-alpha.1` are: 0.17.0, 0.17.0-alpha.2, 0.17.0-alpha.1
the package `mypackage` depends on `postgres`, with features: `with-time` but `postgres` does not have these features.
Furthermore, the crate page for postgres 0.17.0 says nothing about these features, so I don't even know if they're supposed to be supported or not.
It seems like there would be something on docs.rs about it?
Crates uploaded to crates.io (and thus to docs.rs) will show what feature flags exist. crates.io issue #465 suggests placing the feature list on the crate's page as well.
Beyond that, the only guaranteed way to see what features are available is to look at the Cargo.toml for the crate. This generally means that you need to navigate to the project's repository, find the correct file for the version you are interested in, and read it.
You are primarily looking for the [features] section, but also for any dependencies that are marked as optional = true, as optional dependencies count as an implicit feature flag.
Unfortunately, there's no requirement that the crate author puts any documentation about what each feature flag does. Good crates will document their feature flags in one or more locations:
As comments in Cargo.toml
Their README
Their documentation
See also:
How do you enable a Rust "crate feature"?
For the postgres crate, we can start at crates.io, then click "repository" to go to the repository. We then find the right tag (postgres-v0.17.0), then read the Cargo.toml:
[features]
with-bit-vec-0_6 = ["tokio-postgres/with-bit-vec-0_6"]
with-chrono-0_4 = ["tokio-postgres/with-chrono-0_4"]
with-eui48-0_4 = ["tokio-postgres/with-eui48-0_4"]
with-geo-types-0_4 = ["tokio-postgres/with-geo-types-0_4"]
with-serde_json-1 = ["tokio-postgres/with-serde_json-1"]
with-uuid-0_8 = ["tokio-postgres/with-uuid-0_8"]
You can use the cargo-feature crate to view and manage your dependencies' features:
> cargo install cargo-feature --locked
> cargo feature postgres
Avaliable features for `postgres`
array-impls = ["tokio-postgres/array-impls"]
with-bit-vec-0_6 = ["tokio-postgres/with-bit-vec-0_6"]
with-chrono-0_4 = ["tokio-postgres/with-chrono-0_4"]
with-eui48-0_4 = ["tokio-postgres/with-eui48-0_4"]
with-eui48-1 = ["tokio-postgres/with-eui48-1"]
with-geo-types-0_6 = ["tokio-postgres/with-geo-types-0_6"]
with-geo-types-0_7 = ["tokio-postgres/with-geo-types-0_7"]
with-serde_json-1 = ["tokio-postgres/with-serde_json-1"]
with-smol_str-01 = ["tokio-postgres/with-smol_str-01"]
with-time-0_2 = ["tokio-postgres/with-time-0_2"]
with-time-0_3 = ["tokio-postgres/with-time-0_3"]
with-uuid-0_8 = ["tokio-postgres/with-uuid-0_8"]
with-uuid-1 = ["tokio-postgres/with-uuid-1"]
Note: this only works if the crate is already in Cargo.toml as a dependency.
The list of features are also displayed when using cargo add:
> cargo add postgres
Updating crates.io index
Adding postgres v0.19.4 to dependencies.
Features:
- array-impls
- with-bit-vec-0_6
- with-chrono-0_4
- with-eui48-0_4
- with-eui48-1
- with-geo-types-0_6
- with-geo-types-0_7
- with-serde_json-1
- with-smol_str-01
- with-time-0_2
- with-time-0_3
- with-uuid-0_8
- with-uuid-1
It seems like there would be something on docs.rs about it?
Other people thought so too! This was added to docs.rs in late 2020. You can view the list of features from the crate's documentation by navigating to "Feature flags" on the top bar:
You still won't see the list of features of postgres 0.17.0 though since old documentation isn't regenerated when new functionality is added to the site, but any recently published versions will have it available.
Note: this is only available on docs.rs and not generated when running cargo doc.
The feature-set of a dependency can be retrieved programmatically via the cargo-metadata crate:
use cargo_metadata::MetadataCommand;
fn main() {
let metadata = MetadataCommand::new()
.exec()
.expect("failed to get metadata");
let dependency = metadata.packages
.iter()
.find(|package| package.name == "postgres")
.expect("failed to find dependency");
println!("{:#?}", dependency.features);
}
{
"with-time-0_3": [
"tokio-postgres/with-time-0_3",
],
"with-smol_str-01": [
"tokio-postgres/with-smol_str-01",
],
"with-chrono-0_4": [
"tokio-postgres/with-chrono-0_4",
],
"with-uuid-0_8": [
"tokio-postgres/with-uuid-0_8",
],
"with-uuid-1": [
"tokio-postgres/with-uuid-1",
],
"array-impls": [
"tokio-postgres/array-impls",
],
"with-eui48-1": [
"tokio-postgres/with-eui48-1",
],
"with-bit-vec-0_6": [
"tokio-postgres/with-bit-vec-0_6",
],
"with-geo-types-0_6": [
"tokio-postgres/with-geo-types-0_6",
],
"with-geo-types-0_7": [
"tokio-postgres/with-geo-types-0_7",
],
"with-eui48-0_4": [
"tokio-postgres/with-eui48-0_4",
],
"with-serde_json-1": [
"tokio-postgres/with-serde_json-1",
],
"with-time-0_2": [
"tokio-postgres/with-time-0_2",
],
}
This is how other tools like cargo add and cargo feature provide dependency and feature information.
Hi I'm trying to explore Rust. What I'm trying to do is using glob module, when i build code with $cargo build and $cargo run it's succesfully builds and runs the executable but if i try it with $rustc main.rs it gives me
error[E0432]: unresolved import `glob`
--> src/main.rs:1:5
|
1 | use glob::glob;
| ^^^^ use of undeclared type or module `glob`
Any ideas?
Version : ╰─ rustc --version rustc 1.43.1 (8d69840ab 2020-05-04)
Code is here:
use glob::glob;
fn main() {
for entry in glob("/home/user/*.jpg").unwrap(){
match entry {
Ok(path) => println!("{:?}", path.display()),
Err(e) => println!("{:?}",e),
}
};
}
My toml
[package]
name = "test1"
version = "0.1.0"
authors = ["ieuD <example#mail.com>"]
edition = "2018"
# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
[dependencies]
glob = "0.3.0"
rustc on its own doesn't handle your dependencies, it just compiles stuff. When you run rustc on your file, it will start compiling it and encounter the unknown glob crate. Dependencies are handled by cargo via Cargo.toml.
Though you could only use rustc (see the answer here) it's a task that's considerably more difficult, that's why cargo is around.
I need to build the rustc compiler using a modified libc crate. I cloned the libc directory and made the changes, now how do I include the modified libc in my build?
This is my Cargo.toml
[patch.crates-io]
# Similar to Cargo above we want the RLS to use a vendored version of `rustfmt`
# that we're shipping as well (to ensure that the rustfmt in RLS and the
# `rustfmt` executable are the same exact version).
rustfmt-nightly = { path = "src/tools/rustfmt" }
# See comments in `src/tools/rustc-workspace-hack/README.md` for what's going on
# here
rustc-workspace-hack = { path = 'src/tools/rustc-workspace-hack' }
# See comments in `tools/rustc-std-workspace-core/README.md` for what's going on
# here
rustc-std-workspace-core = { path = 'src/tools/rustc-std-workspace-core' }
rustc-std-workspace-alloc = { path = 'src/tools/rustc-std-workspace-alloc' }
rustc-std-workspace-std = { path = 'src/tools/rustc-std-workspace-std' }
libc = {path = "../libc"}
[patch."https://github.com/rust-lang/rust-clippy"]
clippy_lints = { path = "src/tools/clippy/clippy_lints" }
[dependencies]
# libc = {verion = "0.2", default-features= false, path = "../libc"}
This is the error I get:
mahto#hydlnxeng27:/local/mnt/workspace/mahto/rust$ ./x.py build --config config.toml src/libstd 2>&1 | tee build.log
Updating only changed submodules
Submodules updated in 0.04 seconds
error: failed to parse manifest at `/local/mnt/workspace/mahto/rust/Cargo.toml`
Caused by:
virtual manifests do not specify [dependencies]
failed to run: /local/mnt/workspace/mahto/rust/build/x86_64-unknown-linux-gnu/stage0/bin/cargo build --manifest-path /local/mnt/workspace/mahto/rust/src/bootstrap/Cargo.toml
Build completed unsuccessfully in 0:00:00
After commenting-out the dependencies section in Cargo.toml, I get this new error:
error[E0433]: failed to resolve: unresolved import
error: aborting due to previous error
For more information about this error, try `rustc --explain E0433`.
error: could not compile `libc`.
I try to build a project using rust-lang recently (this is my first rust project, and my boss is supporting me to use a new technology in my company). But, I suddenly get some red lines on my Cargo.toml :
could not compile `serde_derive`.
error: could not compile `async-trait`.
To learn more, run the command again with --verbose.
error: could not compile `rand_chacha`.
To learn more, run the command again with --verbose.
error: could not compile `proc-macro-hack`.
To learn more, run the command again with --verbose.
error: could not compile `diesel_derives`.
To learn more, run the command again with --verbose.
I run a command Cargo Run and my project is running well, but these red lines prevent me to tracking an error on my other code in my project (So if there is an error in code, it won't display as there are still exists some errors in another file, it is Cargo.toml)
I am using cargo 1.43.0-nightly (bda50510d 2020-03-02), rustc 1.43.0-nightly (c20d7eecb 2020-03-11), and vs code 1.43 version.
This is my Cargo.toml :
[package]
name = "app_base"
version = "0.1.0"
authors = ["yonathan"]
edition = "2018"
# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
[dependencies]
rocket = "0.4.3"
rocket_codegen = "0.4.3"
rocket_contrib = "0.4.3"
rocket_http = "0.4.3"
cookie = "0.11.2"
rocket-json-response = "0.5.10"
diesel = { version = "1.4.3", features = ["postgres"] }
dotenv = "0.15.0"
postgres = { version = "0.17.2", features = ["with-chrono-0_4"] }
serde = { version = "1.0", features = ["derive"] }
serde_json = "1.0"
json-gettext = "3.1.7"
debug-helper = "0.3.8"
serializers = "0.2.3"
rocket_cors = { git = "https://github.com/lawliet89/rocket_cors", branch = "master" }
chrono = "0.4"
As this is an RLS (rust language server) bug [which is closed apparently, see here], a temporary alternative can be using the rust-analyzer extension [website here].
To install the extension, you can launch VSCode, click on the Extensions tab on the left and search for rust-analyzer in the Marketplace.
Please note that as mentioned on the website [as of 04/01/2020], this project is still in ALPHA, which means it may be prone to breakages.
Is there a standard way to determine what features are available for a given crate?
I'm trying to read Postgres timezones, and this says to use the crate postgres = "0.17.0-alpha.1" crate's with-time or with-chrono features.
When I try this in my Cargo.toml:
[dependencies]
postgres = { version = "0.17.0-alpha.1", features = ["with-time"] }
I get this error:
error: failed to select a version for `postgres`.
... required by package `mypackage v0.1.0 (/Users/me/repos/mypackage)`
versions that meet the requirements `^0.17.0-alpha.1` are: 0.17.0, 0.17.0-alpha.2, 0.17.0-alpha.1
the package `mypackage` depends on `postgres`, with features: `with-time` but `postgres` does not have these features.
Furthermore, the crate page for postgres 0.17.0 says nothing about these features, so I don't even know if they're supposed to be supported or not.
It seems like there would be something on docs.rs about it?
Crates uploaded to crates.io (and thus to docs.rs) will show what feature flags exist. crates.io issue #465 suggests placing the feature list on the crate's page as well.
Beyond that, the only guaranteed way to see what features are available is to look at the Cargo.toml for the crate. This generally means that you need to navigate to the project's repository, find the correct file for the version you are interested in, and read it.
You are primarily looking for the [features] section, but also for any dependencies that are marked as optional = true, as optional dependencies count as an implicit feature flag.
Unfortunately, there's no requirement that the crate author puts any documentation about what each feature flag does. Good crates will document their feature flags in one or more locations:
As comments in Cargo.toml
Their README
Their documentation
See also:
How do you enable a Rust "crate feature"?
For the postgres crate, we can start at crates.io, then click "repository" to go to the repository. We then find the right tag (postgres-v0.17.0), then read the Cargo.toml:
[features]
with-bit-vec-0_6 = ["tokio-postgres/with-bit-vec-0_6"]
with-chrono-0_4 = ["tokio-postgres/with-chrono-0_4"]
with-eui48-0_4 = ["tokio-postgres/with-eui48-0_4"]
with-geo-types-0_4 = ["tokio-postgres/with-geo-types-0_4"]
with-serde_json-1 = ["tokio-postgres/with-serde_json-1"]
with-uuid-0_8 = ["tokio-postgres/with-uuid-0_8"]
You can use the cargo-feature crate to view and manage your dependencies' features:
> cargo install cargo-feature --locked
> cargo feature postgres
Avaliable features for `postgres`
array-impls = ["tokio-postgres/array-impls"]
with-bit-vec-0_6 = ["tokio-postgres/with-bit-vec-0_6"]
with-chrono-0_4 = ["tokio-postgres/with-chrono-0_4"]
with-eui48-0_4 = ["tokio-postgres/with-eui48-0_4"]
with-eui48-1 = ["tokio-postgres/with-eui48-1"]
with-geo-types-0_6 = ["tokio-postgres/with-geo-types-0_6"]
with-geo-types-0_7 = ["tokio-postgres/with-geo-types-0_7"]
with-serde_json-1 = ["tokio-postgres/with-serde_json-1"]
with-smol_str-01 = ["tokio-postgres/with-smol_str-01"]
with-time-0_2 = ["tokio-postgres/with-time-0_2"]
with-time-0_3 = ["tokio-postgres/with-time-0_3"]
with-uuid-0_8 = ["tokio-postgres/with-uuid-0_8"]
with-uuid-1 = ["tokio-postgres/with-uuid-1"]
Note: this only works if the crate is already in Cargo.toml as a dependency.
The list of features are also displayed when using cargo add:
> cargo add postgres
Updating crates.io index
Adding postgres v0.19.4 to dependencies.
Features:
- array-impls
- with-bit-vec-0_6
- with-chrono-0_4
- with-eui48-0_4
- with-eui48-1
- with-geo-types-0_6
- with-geo-types-0_7
- with-serde_json-1
- with-smol_str-01
- with-time-0_2
- with-time-0_3
- with-uuid-0_8
- with-uuid-1
It seems like there would be something on docs.rs about it?
Other people thought so too! This was added to docs.rs in late 2020. You can view the list of features from the crate's documentation by navigating to "Feature flags" on the top bar:
You still won't see the list of features of postgres 0.17.0 though since old documentation isn't regenerated when new functionality is added to the site, but any recently published versions will have it available.
Note: this is only available on docs.rs and not generated when running cargo doc.
The feature-set of a dependency can be retrieved programmatically via the cargo-metadata crate:
use cargo_metadata::MetadataCommand;
fn main() {
let metadata = MetadataCommand::new()
.exec()
.expect("failed to get metadata");
let dependency = metadata.packages
.iter()
.find(|package| package.name == "postgres")
.expect("failed to find dependency");
println!("{:#?}", dependency.features);
}
{
"with-time-0_3": [
"tokio-postgres/with-time-0_3",
],
"with-smol_str-01": [
"tokio-postgres/with-smol_str-01",
],
"with-chrono-0_4": [
"tokio-postgres/with-chrono-0_4",
],
"with-uuid-0_8": [
"tokio-postgres/with-uuid-0_8",
],
"with-uuid-1": [
"tokio-postgres/with-uuid-1",
],
"array-impls": [
"tokio-postgres/array-impls",
],
"with-eui48-1": [
"tokio-postgres/with-eui48-1",
],
"with-bit-vec-0_6": [
"tokio-postgres/with-bit-vec-0_6",
],
"with-geo-types-0_6": [
"tokio-postgres/with-geo-types-0_6",
],
"with-geo-types-0_7": [
"tokio-postgres/with-geo-types-0_7",
],
"with-eui48-0_4": [
"tokio-postgres/with-eui48-0_4",
],
"with-serde_json-1": [
"tokio-postgres/with-serde_json-1",
],
"with-time-0_2": [
"tokio-postgres/with-time-0_2",
],
}
This is how other tools like cargo add and cargo feature provide dependency and feature information.