A crate attribute is an attribute (`#[...]`) that applies to the enclosing context (`#![...]`). This attribute must be added to the top of your *crate root*, thus the context is the crate itself: 

```
#![attribute_name]
#![attribute_name(arg1, ...)]
```

If you are creating

- a library — the crate root will be a file called `lib.rs`
- an application — the crate root would be the primary `.rs` file you build. In many cases, this will be called `main.rs`
- an integration test - the crate root is each file in `tests/`
- an example - the crate root is each file in `examples/`

---

[*The Rust Programming Language*](https://doc.rust-lang.org/book/appendix-02-operators.html#non-operator-symbols) and the [Rust Reference](https://doc.rust-lang.org/reference/attributes.html) talk a bit about attributes in general. [The Unstable Book](https://doc.rust-lang.org/unstable-book/) contains a list of feature flags and brief documentation on what they do.

There are many different crate attributes, but the `feature` crate attribute (`#![feature(feature1, feature2)]`) may only be used in a **nightly** version of the compiler. Unstable features are not allowed to be used in stable Rust versions.


I have a Swift project for which I&#39;m trying to import ObjC-based frameworks. The framework is located in a directory under the project&#39;s path and is referenced by the project in Xcode. It&#39;s also added to the &quot;Link Binary with Libraries&quot; in the project&#39;s &#39;Build Phases&#39; page.

However, for some reason, I can&#39;t seem to include the framework in the Bridging-Header file. I get the following error:

    BridgingHeader.h:5:9: error: &#39;Parse/Parse.h&#39; file not found
    #import &lt;Parse/Parse.h&gt;
            ^
    &lt;unknown&gt;:0: error: failed to import bridging header &#39;BridgingHeader.h&#39;

Things I&#39;ve checked:

1. The &quot;Install Objective-C Compatibility Header&quot; is set to &quot;YES&quot;.
2. The Header Search Paths includes the path to the frameworks.

I&#39;m sure I&#39;m missing something, so if anyone has any clues, that would be great.


Getting &quot;file not found&quot; in Bridging Header when importing Objective-C frameworks into Swift project

I want to use a existing native library from **another** Android project, so I just copied the NDK built library (**libcalculate.so**) to my new Android project. In my new Android project I created a folder `libs/armeabi/` and put **libcalculate.so** there. There is **no** jni/ folder. My testing device has ARM architecture.

In my java code I load the library by:

      static{
        System.loadLibrary(&quot;calculate&quot;);
      }

When I run my new android project, I got error:

    java.lang.UnsatisfiedLinkError:  ...
    nativeLibraryDirectories=[/vendor/lib, /system/lib]]] couldn&#39;t find &quot;libcalculate.so&quot;

So, as error says, the copied native library is not in /verdor/lib or /system/lib , how to resolve this problem in my case?

(I unziped the apk package, under lib/ there is libcalculate.so)

**====UPDATE=====**

I also tried to create a jni/ folder under project root, and add an Android.mk file under jni/. The content of Android.mk is:

    LOCAL_PATH := $(call my-dir)
    
    include $(CLEAR_VARS)
    LOCAL_MODULE    := libcalculate
    LOCAL_SRC_FILES := libcalculate.so
    include $(PREBUILT_SHARED_LIBRARY)

Then, under project root, I executed ndk-build . After that, the armeabi/ and armeabi-v7a/ directories are generated by ndk-build (with libcalculate.so inside the folder).

Then I run my maven build the project successfully. In the final apk package, there are:

    lib/armeabi/libcalculate.so
    lib/armeabi-v7a/libcalculate.so

But when I run my app, the same error throw:

    java.lang.UnsatisfiedLinkError:  ...
    nativeLibraryDirectories=[/vendor/lib, /system/lib]]] couldn&#39;t find &quot;libcalculate.so&quot;

System.loadLibrary(...) couldn&#39;t find native library in my case

In order to get a feel for how Rust works, I decided to look at a little terminal-based text editor called [Iota](https://github.com/gchp/iota). I cloned the repository and ran `cargo build` only to be told:

&gt; &lt;!-- language: none --&gt;
&gt;
&gt;     error: *if let* syntax is experimental
&gt; 
&gt;     help: add #![feature(if_let)] to the crate attributes to enable

Where am I supposed to add `#![feature(if_let)]` to the crate attributes?

What is a crate attribute and where do I add it?

In order to get a feel for how Rust works, I decided to look at a little terminal-based text editor called <a href="https://github.com/gchp/iota" target="_blank" rel="noopener noreferrer">Iota</a>. I cloned the repository and ran <code>cargo build</code> only to be told:
> 
>
> error: if let syntax is experimental
>
> help: add #![feature(if_let)] to the crate attributes to enable
Where am I supposed to add <code>#![feature(if_let)]</code> to the crate attributes?

&gt; **Note** This question contains syntax that predates Rust 1.0. The code is invalid, but the concepts are still relevant.

How do you create a global static array of strings in Rust?

For integers, this compiles:

    static ONE:u8 = 1;
    static TWO:u8 = 2;
    static ONETWO:[&amp;&#39;static u8, ..2] = [&amp;ONE, &amp;TWO];

But I can&#39;t get something similar for strings to compile:

    static STRHELLO:&amp;&#39;static str = &quot;Hello&quot;;
    static STRWORLD:&amp;&#39;static str = &quot;World&quot;;
    static ARR:[&amp;&#39;static str, ..2] = [STRHELLO,STRWORLD]; // Error: Cannot refer to the interior of another static

How to create a static array of strings?

The following Rust code compiles and runs without any issues.

    fn main() {
        let text = &quot;abc&quot;;
        println!(&quot;{}&quot;, text.split(&#39; &#39;).take(2).count());
    }

After that, I tried something like this .... but it didn&#39;t compile

    fn main() {
        let text = &quot;word1 word2 word3&quot;;
        println!(&quot;{}&quot;, to_words(text).take(2).count());
    }
    
    fn to_words(text: &amp;str) -&gt; &amp;Iterator&lt;Item = &amp;str&gt; {
        &amp;(text.split(&#39; &#39;))
    }
The main problem is that I&#39;m not sure what return type the function `to_words()` should have. The compiler says:

&lt;!-- language: none --&gt;

    error[E0599]: no method named `count` found for type `std::iter::Take&lt;std::iter::Iterator&lt;Item=&amp;str&gt;&gt;` in the current scope
     --&gt; src/main.rs:3:43
      |
    3 |     println!(&quot;{}&quot;, to_words(text).take(2).count());
      |                                           ^^^^^
      |
      = note: the method `count` exists but the following trait bounds were not satisfied:
              `std::iter::Iterator&lt;Item=&amp;str&gt; : std::marker::Sized`
              `std::iter::Take&lt;std::iter::Iterator&lt;Item=&amp;str&gt;&gt; : std::iter::Iterator`

What would be the correct code to make this run? .... and where is my knowledge gap?

What is the correct way to return an Iterator (or any other trait)?

Recent Rust changes have made &quot;trait objects&quot; more prominent to me, but I only have a nebulous grasp of what actually makes something into a trait object. One change in particular is the [upcoming change](https://github.com/rust-lang/rust/issues/19187) to allow trait objects to forward trait implementations to the inner type.

Given a trait `Foo`, I&#39;m pretty sure that `Box&lt;Foo&gt;` / `Box&lt;dyn Foo&gt;` is a trait object. Is `&amp;Foo` / `&amp;dyn Foo` also a trait object? What about other smart-pointer things like `Rc` or `Arc`? How could I make my own type that would count as a trait object?

The [reference](https://doc.rust-lang.org/0.12.0/reference.html) only mentions trait objects once, but nothing like a definition.

What makes something a &quot;trait object&quot;?

How I can create a HashMap literal in Rust? In Python I can do it so:

&lt;!-- language: python --&gt;
    hashmap = {
       &#39;element0&#39;: {
           &#39;name&#39;: &#39;My New Element&#39;,
           &#39;childs&#39;: {
               &#39;child0&#39;: {
                   &#39;name&#39;: &#39;Child For Element 0&#39;,
                   &#39;childs&#39;: {
                       ...
                   }
               }
           }
       },
       ...
    }

And in Go like this:

&lt;!-- language: go --&gt;
    type Node struct {
        name string
        childs map[string]Node
    }

    hashmap := map[string]Node {
        &quot;element0&quot;: Node{
            &quot;My New Element&quot;,
            map[string]Node {
                &#39;child0&#39;: Node{
                    &quot;Child For Element 0&quot;,
                    map[string]Node {}
                }
            }
        }
    }


How do I create a HashMap literal?

One uses this to send output to stdout:

    println!(&quot;some output&quot;)

I think there is no corresponding macro to do the same for stderr.

How to send output to stderr?

How do you access a Cargo package&#39;s metadata (e.g. version) from the Rust code in the package? In my case, I am building a command line tool that I&#39;d like to have a standard `--version` flag, and I&#39;d like the implementation to read the version of the package from `Cargo.toml` so I don&#39;t have to maintain it in two places. I can imagine there are other reasons someone might want to access Cargo metadata from the program as well.

How can a Rust program access metadata from its Cargo package?

Just having found Rust and having read the first two chapters of the documentation, I find the approach and the way they defined the language particularly interesting. So I decided to get my fingers wet and started out with Hello world... 

I did so on Windows 7 x64, btw.

	fn main() {
		println!(&quot;Hello, world!&quot;);
	}

Issuing `cargo build` and looking at the result in `targets\debug` I found the resulting `.exe` being 3MB. After some searching (documentation of cargo command line flags is hard to find...) I found `--release` option and created the release build. To my surprise, the .exe size has only become smaller by an insignificant amount: 2.99MB instead of 3MB.

So, confessing I am a newbie to Rust and its ecosystem, my expectation would have been that a Systems Programming language would produce something compact.

Can anyone elaborate on what Rust is compiling to, how it can be possible it produces such huge images from a 3 liner program? Is it compiling to a virtual machine? Is there a strip command I missed (debug info inside the release build?)? Anything else which might allow to understand what is going on?

Why are Rust executables so huge?

I have heard cargo has the ability to automatically recompile changed source files, but I&#39;m having a hard time figuring out how to tell it to do so.

For now, I am manually running `cargo build` or `cargo run` every time I want to type check my code. I would prefer to instead simply save the file and see the results in the neighboring terminal window.

In case you still have no idea what I&#39;m talking about, I&#39;m looking for the cargo equivalent of `sbt ~compile` or `sbt ~run`.

It seems strangely hard to find, so I&#39;m starting to wonder if it&#39;s really supported. It&#39;s possible someone had said cargo could detect changed files and recompile them when what he meant to say was that cargo could detect *unchanged* files and *avoid* recompiling them, like `make`.

How can I get cargo to recompile changed files automatically?

I have a Rust library that implements a lint plugin. I want to include `compiletest`, but not require it outside of testing. What is the correct way to specify that the dependency is for testing only?

How to define test-only dependencies?

I&#39;d like to update a package that I used `cargo install` to globally install packages, such as rustfmt or racer. I can&#39;t find a way to update an installed package without first deleting it (via `cargo uninstall`) and then running the install command again. Is there an update command?

Content Type	Original Author	Original Content on Stackoverflow
Question	Soham Chowdhury	View Question on Stackoverflow
Solution 1 - Rust	Shepmaster	View Answer on Stackoverflow

What is a crate attribute and where do I add it?

Rust Problem Overview

Rust Solutions

Solution 1 - Rust

System.loadLibrary(...) couldn't find native library in my case

Getting "file not found" in Bridging Header when importing Objective-C frameworks into Swift project

Attributions