# Rerast cookbook
Here we've got some examples of things you can do with rerast. If you've got any
more examples, please feel free to send pull requests.
### Replace ```try!(something)``` with ```something?```
```rust
fn rule1<T,E,X: From<E>>(r: Result<T,E>) -> Result<T,X> {
replace!(try!(r) => r?);
unreachable!()
}
```
This will change:
```rust
fn get_file_contents(filename: &Path) -> io::Result<String> {
let mut result = String::new();
try!(try!(File::open(filename)).read_to_string(&mut result));
Ok(result)
}
```
Into this:
```rust
fn get_file_contents(filename: &Path) -> io::Result<String> {
let mut result = String::new();
File::open(filename)?.read_to_string(&mut result)?;
Ok(result)
}
```
This rule also shows how to handle rules that have return statements in
them. i.e. specify a return type for your rule function. ```unreachable!()```
can then be used to avoid having to construct an actual value.
### Use accessors instead of direct field access
```rust
fn r(mut p: Point, x: i32, y: i32) {
replace!(Point{ x: x, y: y } => Point::new(x, y));
replace!(p.x = x => p.set_x(x));
replace!(&mut p.x => p.get_mut_x());
replace!(p.x => p.get_x());
}
```
This will change:
```rust
fn f1(point: Point, point_ref: &Point, mut_point_ref: &mut Point) {
let p2 = Point { x: 1, y: 2 };
process_i32(point.x);
mut_point_ref.x = 1;
let x = &mut mut_point_ref.x;
*x = 42;
process_i32(point_ref.x);
}
```
Into:
```rust
fn f1(point: Point, point_ref: &Point, mut_point_ref: &mut Point) {
let p2 = Point::new(1, 2);
process_i32(point.get_x());
mut_point_ref.set_x(1);
let x = mut_point_ref.get_mut_x();
*x = 42;
process_i32(point_ref.get_x());
}
```
### Remove the argument from a function
[cargo-issue]: https://github.com/rust-lang/cargo/issues/5746
[cargo-pr]: https://github.com/rust-lang/cargo/pull/5752
This is a real world example from the Cargo project ([issue][cargo-issue], [PR][cargo-pr]).
```rust
use cargotest::support::{ project, project_foo };
fn rule1(a: &'static str) {
replace!(project("foo") => project_foo());
replace!(project(a) => project_foo().at(a));
}
```
Changes:
```rust
let p = project("foo")
.file("Cargo.toml", &basic_bin_manifest("foo"))
.file("src/foo.rs", &main_file(r#""i am foo""#, &[]))
.build();
let _p2 = project("bar")
.file("Cargo.toml", &basic_bin_manifest("bar"))
.file("src/bar.rs", &main_file(r#""i am bar""#, &[]))
.build();
```
Into:
```rust
let p = project_foo()
.file("Cargo.toml", &basic_bin_manifest("foo"))
.file("src/foo.rs", &main_file(r#""i am foo""#, &[]))
.build();
let _p2 = project_foo().at("bar")
.file("Cargo.toml", &basic_bin_manifest("bar"))
.file("src/bar.rs", &main_file(r#""i am bar""#, &[]))
.build();
```
When run like this:
```terminal
cargo +nightly rerast --rules_file=rewrite.rs --force --targets tests --file tests/testsuite/main.rs
```
Afterwhich a simple search and replace can rename `project_foo` back to `project` and the argument
can be dropped.
### Replace await! macro with .await
Since await can only be used from within an async function, you'll need to
delcare your rule function async. This cannot be done from the command line, so
you'll need to create a rule file with the following contents:
```
async fn rule1<T: std::future::Future>(r: T) {
replace!(await!(r) => r.await)
}
```
### Replace yield
Since yield can only be used from within a generator, you'll need to delcare
your replace rule inside a generator. For example:
```
fn r1(x: i32) {
let _ = || {
replace!(yield x => yield x + 1);
};
}
```