A tool for transforming Rust code using rules
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README.md

Rerast

Latest Version Build Status

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.

Installation

Ensuring you have the latest nightly toolchain installed (rustup update nightly) simply run:

cargo +nightly install rerast

Usage

Basic operations can be performed entirely from the command line

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

Alternatively you can put your rule in a Rust file

fn rule1(a: i32) {
  replace!(a + 1 => a - 1);
}

then use

cargo +nightly 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:

cargo +nightly 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.:

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

Here's a more complex example

use std::rc::Rc;
fn rule1<T>(r: Rc<T>) {
  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:

cargo +nightly 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.

More examples

See the Rerast Cookbook for more examples.

Groups

Authors

See Cargo.toml

Contributing

See CONTRIBUTING.md

Code of conduct

This project defers to the Rust code of conduct. 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.