Document features (#1257)

Signed-off-by: Filip Krawczyk <71193923+fmkra@users.noreply.github.com>
Co-authored-by: maciektr <mtratnowiecki@gmail.com>

website/docs/reference/conditional-compilation.md

@@ -51,3 +51,64 @@ Example values:
 
 - `'lib'`
 - `'starknet-contract'`
+
+## Features
+
+Features in Scarb provide a way to conditionally compile specific parts of the code during the build process.
+
+### `[features]` section
+
+A package defines a set of named features in the `[features]` section of `Scarb.toml` file. Each defined feature can list other features that should be enabled with it.
+
+For example, a package supporting various hash functions might define features like this:
+
+```toml
+[features]
+poseidon = []
+pedersen = []
+keccak = []
+```
+
+With these features set, conditional compilation (`cfg`) attributes can be used to selectively include code to support requested features during compile time. For instance:
+
+```rust
+// Conditionally include a module
+#[cfg(feature: 'poseidon')]
+mod poseidon;
+
+// Conditionally define a function
+#[cfg(feature: 'pedersen')]
+fn hash_pedersen(value: felt252) -> felt252 {
+  // ...
+}
+```
+
+To enable specific features, use the `--features` flag followed by a comma-separated list of features. For example, to build with only the `poseidon` and `pedersen` features enabled:
+
+```
+scarb build --features poseidon,pedersen
+```
+
+Enabling all features can be done with the `--all-features` flag.
+
+### `default` features
+
+By default, all features are disabled unless explicitly enabled. However, this behaviour can be changed by specifying a default feature in the `[features]` section, like so:
+
+```toml
+[features]
+default = ["poseidon", "pedersen"]
+poseidon = []
+pedersen = []
+keccak = []
+```
+
+When the package is built, the default feature is enabled which in turn enables the listed features.
+
+To disable the default feature, use the `--no-default-features` flag.
+
+For example, in the provided scenario:
+
+- Running `scarb build` would enable `poseidon` and `pedersen` features.
+- `scarb build --features keccak` would enable `poseidon`, `pedersen`, and `keccak` features.
+- `scarb build --no-default-features --features keccak` would enable only the `keccak` feature.

website/docs/reference/manifest.md

@@ -320,3 +320,7 @@ Each field can accept any valid toml value including a table.
 ## `[workspace]`
 
 See [Workspaces](./workspaces) page.
+
+## `[features]`
+
+See [Features](./conditional-compilation#features) page.