feature call

src/SUMMARY.md

@@ -4,4 +4,5 @@
   - [Sum Variables](./essentials/sum.md)
   - [Requiring modules](./essentials/modules.md)
   - [Package names](./essentials/package-names.md)
-  - [Tag your Crate](./essentials/crates-tag.md)
+  - [Tag your Crate](./essentials/crates-tag.md)
+  - [Feature Attribute](./essentials/crates-tag.md)

src/essentials/feature-attribute.md

@@ -0,0 +1,18 @@
+In Rust, the #! syntax is used for attributes that apply to the entire crate or module. These are known as "inner attributes." They are typically placed at the top of a file and are used to configure various aspects of the Rust compiler's behavior for that file or crate.
+
+#![feature(...)]: This attribute allows you to use unstable features in your Rust code from the nightly Rust compiler. It must be placed at the top of the file. 
+
+As an example, the #![feature(test)] attribute in Rust is used to enable the test crate, which includes benchmarking and testing utilities that are not yet stabilized.
+
+```rust,noplaypen
+#![feature(test)]
+extern crate bcrypt;
+extern crate test;
+
+use bcrypt::{hash, DEFAULT_COST};
+
+#[bench]
+fn bench_cost_4(b: &mut test::Bencher) {
+    b.iter(|| hash("hunter2", 4));
+}
+```