message: update debugging instructions in the README
diff: @@ -42,10 +42,22 @@ cargo run --bin miri tests/run-pass-fullmir/vecs.rs # Or whatever test you like.
  ## Debugging
  -You can get detailed, statement-by-statement traces by setting the `MIRI_LOG`
  -environment variable to `trace`. These traces are indented based on call stack
  -depth. You can get a much less verbose set of information with other logging
  -levels such as `warn`.
  +Since the heart of miri (the main interpreter engine) lives in rustc, tracing
  +the interpreter requires a version of rustc compiled with tracing. To this
  +end, you will have to compile your own rustc:
  +```
  +git clone https://github.com/rust-lang/rust/ rustc
  +cd rustc
  +cp config.toml.example config.toml
  +# Now edit `config.toml` and set `debug-assertions = true`
  +./x.py build
  +rustup toolchain link custom build/x86_64-unknown-linux-gnu/stage2
  +```
  +The `build` step can take 30 to 60 minutes.
  +
  +Now, in the miri directory, you can `rustup override set custom` and re-build
  +everything. Finally, if you now set `RUST_LOG=rustc_mir::interpret=trace` as
  +environment variable, you will get detailed step-by-step tracing information.
  ## Running miri on your own project('s test suite)
  