MIR visitor

The MIR visitor is a convenient tool for traversing the MIR and either looking for things or making changes to it. The visitor traits are defined in the rustc_middle::mir::visit module – there are two of them, generated via a single macro: Visitor (which operates on a &Mir and gives back shared references) and MutVisitor (which operates on a &mut Mir and gives back mutable references).

To implement a visitor, you have to create a type that represents your visitor. Typically, this type wants to "hang on" to whatever state you will need while processing MIR:

struct MyVisitor<...> {
    tcx: TyCtxt<'tcx>,
    ...
}

and you then implement the Visitor or MutVisitor trait for that type:

impl<'tcx> MutVisitor<'tcx> for MyVisitor {
    fn visit_foo(&mut self, ...) {
        ...
        self.super_foo(...);
    }
}

As shown above, within the impl, you can override any of the visit_foo methods (e.g., visit_terminator) in order to write some code that will execute whenever a foo is found. If you want to recursively walk the contents of the foo, you then invoke the super_foo method. (NB. You never want to override super_foo.)

A very simple example of a visitor can be found in LocalFinder. By implementing visit_local method, this visitor identifies local variables that can be candidates for reordering.

Traversal

In addition the visitor, the rustc_middle::mir::traversal module contains useful functions for walking the MIR CFG in different standard orders (e.g. pre-order, reverse post-order, and so forth).