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https://github.com/google/rerast

A tool for transforming Rust code using rules
https://github.com/google/rerast

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A tool for transforming Rust code using rules

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# Rerast



Rerast is a search/replace tool for Rust code using rules. A rule consists of a

search pattern, a replacement and possibly some placeholders that can appear in

both the search pattern and the replacement. Matching is done on syntax, not on

text, so formatting doesn't matter. Placeholders are typed and must match the

type found in the code for the rule to apply.



Rerast is deprecated. We suggest using the [Structured Search

Replace](https://rust-analyzer.github.io/manual.html#structural-seach-and-replace) feature available

in rust-analyzer. It is available either in vscode or from the command line (and possibly also vim).

If you are missing any particular feature that Rerast supported (or didn't), please comment on [this

issue](https://github.com/rust-analyzer/rust-analyzer/issues/3186).



If you'd like to still use Rerast, we suggest using Rerast version 0.1.88 with Rust version

nightly-2020-02-27. There are a few newer version of Rerast, but there are some broken features in

the newer versions.



## Installation



```sh

rustup toolchain add nightly-2020-02-27

rustup component add --toolchain nightly-2020-02-27 rustc-dev

cargo +nightly-2020-02-27 install --version 0.1.88 rerast

```



## Usage



Basic operations can be performed entirely from the command line

```sh

cargo +nightly-2020-02-27 rerast --placeholders 'a: i32' --search 'a + 1' --replace_with 'a - 1' --diff

```



Alternatively you can put your rule in a Rust file

```rust

fn rule1(a: i32) {

  replace!(a + 1 => a - 1);

}

```

then use



```sh

cargo +nightly-2020-02-27 rerast --rules_file=my_rules.rs

```

Putting your rules in a file is required if you want to apply multiple rules at once.



If you'd like to actually update your files, that can be done as follows:



```sh

cargo +nightly-2020-02-27 rerast --placeholders 'a: i32' --search 'a + 1' --replace_with 'a - 1' --force --backup

```



You can control which compilation roots rerast will inject the rule into using the `--file` argument, e.g.:



```sh

cargo +nightly-2020-02-27 rerast --rules_file=my_rules.rs --targets tests --file tests/testsuite/main.rs --diff

```



Here's a more complex example



```rust

use std::rc::Rc;

fn rule1(r: Rc) {

  replace!(r.clone() => Rc::clone(&r))

}

```



Here we're replacing calls to the clone() method on an Rc with the more explicit way of cloning

an Rc - via Rc::clone.



"r" is a placeholder which will match any expression of the type specified. The name of the function

"rule1" is not currently used for anything. In future it may be possible to selectively

enable/disable rules by specifying their name, so it's probably a good idea to put a slightly

descriptive name here. Similarly, comments placed before the function may in the future be displayed

to users when the rule matches. This is not yet implemented.



A function can contain multiple invocations of the replace! macro, with earlier rules taking precedence.

This is useful if you want to do several replacements that make use of the same placeholders or if you want

to handle certain special patterns first, ahead of a more general match.



Besides replace! there are several other replacement macros that can be used:



* replace\_pattern! - this replaces patterns. e.g. &Some(a). Such a pattern might appear in a match

  arm or if let. Irrefutable patterns (those that are guaranteed to always match) can also be

  matched within let statements and function arguments.

* replace\_type! - this replaces types. It's currently a bit limited in that it doesn't support

  placeholders. Also note, if your type is just a trait you should consider using

  replace\_trait\_ref! instead, since trait references can appear in contexts where types cannot -

  specifically generic bounds and where clauses.

* replace\_trait\_ref! - this replaces references to the named trait



Replacing statements is currently disabled pending a good use-case.



## Matching macro invocations



Macro invocations can be matched so long as they expand to code that can be matched. Note however

that a macro invocation will not match against the equivalent code, nor the invocation of a

different, but identical macro. This is intentional. When verifying a match, we check that the same

sequence of expansions was followed. Also note, that if a macro expands to something different every

time it is invoked, it will never match. println! is an example of such a macro, since it generates

a constant that is referenced from the expanded code and every invocation references a different

constant.



## Order of operations



Suppose you're replacing foo(a, b) with a && !b. Depending on what the placeholders end up matching

and what context the entire expression is in, there may be need for extra parenthesis. For example

if the matched code was !foo(x == 1, y == 2), if we didn't add any parenthesis, we'd end up with !x

== 1 && !y == 2 which clearly isn't correct. Rerast detects this and adds parenthesis as needed in

order to preserve the order or precedence found in the replacement. This would give !(x == 1 && !(y

== 2)).



## Formatting of code



No reformatting of code is currently done. Unmatched code will not be affected. Replacement code is

produced by copying the replacement code from the rule and splicing in any matched patterns. In

future, we may adjust identation for multi-line replacements. Running rustfmt afterwards is probably

a good idea since some identation and line lengths may not be ideal.



## Recursive and overlapping matches



The first matched rule wins. When some code is matched, no later rules will be applied to that

code. However, code matched to placeholders will be searched for further matches to all rules.



## Automatically determining a rule from a source code change



If you're about to make a change multiple times throughout your source code and you're using git,

you can commit (or stage) your changes, make one edit then run:



```sh

cargo +nightly-2020-02-27 rerast --replay_git --diff

```



This will locate the changed expression in your project (of which there should be only one) then try

to determine a rule that would have produced this change. It will print the rule, then apply it to

your project. If you are happy with the changes, you can run again with --force to apply them, or

you could copy the printed rule into a .rs file and apply it with --rules_file.



* The rule produced will use placeholders to the maximum extent possible. i.e. wherever a

  subexpression is found in both the old and the new code, it will be replaced with a placeholder.

* This only works for changed expressions at the moment, not for statements, types, patterns etc.

* Your code must be able to compile both with and without the change.



## Limitations



* Use statements are not yet updated, so depending on your rule, may need to be updated after the

  rule is applied. This should eventually be fixed, there just wasn't time before release and it's

  kind of tricky.

* Your code must be able to compile for this to work.

* The replacement code must also compile.  This means rerast is better at replacing a deprecated API

  usage with its non-deprecated equivalent than dealing with breaking changes.  Often the best

  workaround is to create a new API temporarily.

* Code within rustdoc is not yet processed and matched.

* Conditional code that disabled with a cfg attribute isn't matched. It's suggested to enable all

  features if possible when running so that as much code can be checked as possible.

* replace_type! doesn't yet support placeholders.

* Probably many bugs and missing features. Please feel free to file bugs / feature requests.



## Known issues



* If you have integration tests (a "tests" directory) in your project, you might

  be no matches. Not sure why. This started from nightly-2020-04-10. You might

  be able to work around this issue by passing `--targets ''` to cargo rerast.

  Unfortunately then you won't get matches in non-integration tests (i.e.

  cfg(test)). Alternatively you could install an older version of rust and the

  corresponding rerast version.

* Using `?` in the replacement is currently broken. This broke I think in

  February 2020. Something changed with spans.



## More examples

See the [Rerast Cookbook](COOKBOOK.md) for more examples.



## Groups

* [Users group](https://groups.google.com/forum/#!forum/rerast-users)

* [Developers group](https://groups.google.com/forum/#!forum/rerast-dev)



## Questions?

Feel free to just file an issue on github.



## Authors



See Cargo.toml



## Contributing



See [CONTRIBUTING.md](CONTRIBUTING.md)



## Code of conduct



This project defers to the [Rust code of conduct](https://www.rust-lang.org/en-US/conduct.html). If

you feel someone is not adhering to the code of conduct in relation to this project, please contact

David Lattimore. My email address is in Cargo.toml.



## Disclaimer



This is not an official Google product. It's released by Google only because the (original) author

happens to work there.