MIR Debugging

The -Z dump-mir flag can be used to dump a text representation of the MIR. The following optional flags, used in combination with -Z dump-mir, enable additional output formats, including:

  • -Z dump-mir-graphviz - dumps a .dot file that represents MIR as a control-flow graph
  • -Z dump-mir-dataflow - dumps a .dot file showing the dataflow state at each point in the control-flow graph
  • -Z dump-mir-spanview - dumps an .html file that highlights the source spans associated with MIR elements (including mouse-over actions to reveal elements obscured by overlaps, and tooltips to view the MIR statements). This flag takes an optional value: statement (the default), terminator, or block, to generate span highlights with different levels of granularity.

-Z dump-mir=F is a handy compiler option that will let you view the MIR for each function at each stage of compilation. -Z dump-mir takes a filter F which allows you to control which functions and which passes you are interested in. For example:

> rustc -Z dump-mir=foo ...

This will dump the MIR for any function whose name contains foo; it will dump the MIR both before and after every pass. Those files will be created in the mir_dump directory. There will likely be quite a lot of them!

> cat > foo.rs
fn main() {
    println!("Hello, world!");
> rustc -Z dump-mir=main foo.rs
> ls mir_dump/* | wc -l

The files have names like rustc.main.000-000.CleanEndRegions.after.mir. These names have a number of parts:

      ---- --- --- --------------- ----- either before or after
      |    |   |   name of the pass
      |    |   index of dump within the pass (usually 0, but some passes dump intermediate states)
      |    index of the pass
      def-path to the function etc being dumped

You can also make more selective filters. For example, main & CleanEndRegions will select for things that reference both main and the pass CleanEndRegions:

> rustc -Z dump-mir='main & CleanEndRegions' foo.rs
> ls mir_dump
rustc.main.000-000.CleanEndRegions.after.mir	rustc.main.000-000.CleanEndRegions.before.mir

Filters can also have | parts to combine multiple sets of &-filters. For example main & CleanEndRegions | main & NoLandingPads will select either main and CleanEndRegions or main and NoLandingPads:

> rustc -Z dump-mir='main & CleanEndRegions | main & NoLandingPads' foo.rs
> ls mir_dump

(Here, the main-promoted[0] files refer to the MIR for "promoted constants" that appeared within the main function.)

The -Z unpretty=mir-cfg flag can be used to create a graphviz MIR control-flow diagram for the whole crate:

A control-flow diagram

TODO: anything else?