Flowistry: Powerful IDE Tools for Rust

Flowistry is a VSCode extension that helps you understand Rust programs. Flowistry uses dataflow analysis and pointer analysis to analyze Rust programs at a deeper level than just types can offer (e.g. as you can already find in rust-analyzer).

Flowistry is alpha software (see Limitations). I'm seeking early adopters to try it out and provide feedback! If you have questions or issues, please file a Github issue, join our Discord, or DM @wcrichton on Twitter.

Currently, Flowistry's capabilities are:

1. Backward slicing

A backward slice identifies every piece of code that affects a value of interest. For example, let's say you're debugging an assertion failure on x. Then you could compute the backward slice of x to rule out lines of code that don't influence its value, as shown here:

2. Forward slicing

A forward slice identifiers every piece of code that is affected by a value of interest. For example, let's say you have a program that times a calculation, and you want to comment out all the code related to timing. You could compute a forward slice of the timer:

3. Function effects

A function's effects are either inputs that it mutates, or values that it returns. The function effects panel helps identify lines of code that either mutate arguments or that could return values. Selecting an effect then shows the backward slice of that effect. Lines that are outside of the slice are grayed out. Lines that are unique to the slice are highlighted.

Installation

You can install Flowistry from the Visual Studio Marketplace or the Open VSX Registry. In VSCode:

• Go to the Extensions pane by clicking this button in the left margin:
• Search for "Flowistry" and then click "Install".
• Open a Rust workspace and wait for the tool to finish installing.

Alternatively, you can install it from source:

# Install flowistry binaries
git clone https://github.com/willcrichton/flowistry
cd flowistry
cargo install --path .

# Install vscode extension
cd ide
npm install
npm run build
ln -s $(pwd) ~/.vscode/extensions/flowistry

Usage

Flowistry has five commands:

• Flowistry: Backward Highlight: given a selected variable, this command highlights the backward slice of the variable.
• Flowistry: Backward Select: same as above, but this puts the slice in your selection rather than highlighting it.
• Flowistry: Forward Highlight and Flowistry: Forward Select: same as above, but for forward slices than backward slices.
• Flowistry: Effects: given your cursor is within a particular function, this command opens the effects panel for that function.

You can invoke these commands either through the context menu, by right-clicking and opening the "Flowistry" sub-menu. Or you can open the Command Palette (⇧⌘P on Mac) and type the name of the command.

Limitations

Flowistry is early-stage software. It inevitably has bugs in both the UI and the analysis. The UI may be unintuitive. The analysis, even if correct, may also be unintuitive.

Flowistry does not support all of Rust's features. Specifically:

• Raw pointers: Flowistry uses lifetimes to determine what a reference can point-to. However, raw pointers can point to anything and aren't tracked by the compiler. So Flowistry cannot detect the flow of information through a raw pointer. It can, however, detect information flow through a typed wrapper around a raw pointer!
• Some forms of interior mutability: as a corollary to above, Flowistry does not track the aliases of data wrapped in types like Rc<T>. For example, in the program:

  let x = Rc::new(RefCell::new(1));
  let y = x.clone();
  *x.borrow_mut() = 2;
  

  Flowistry can detect that x is modified (because borrow_mut uses lifetimes to relate the RefMut to RefCell), but not that y is modified (because nothing statically says that y aliases x).