This PR uses the `measureme` crate to profile the call stack of the
program being interpreted by Miri. This is accomplished by starting a
measureme 'event' when we enter a function call, and ending the event
when we exit the call. The `measureme` tooling can be used to produce a
call stack from the generated profile data.
Limitations:
* We currently record every single entry/exit. This might generate very
large profile outputs for programs with a large number of function
calls. In follow-up work, we might want to explore sampling (e.g. only
recording every N function calls).
* This does not integrate very well with Miri's concurrency support.
Each event we record starts when we push a frame, and ends when we pop
a frame. As a result, switching between virtual threads will cause
events from different threads to be interleaved. Additionally, the
recorded for a particular frame will include all of the work Miri does
before that frame completes, including executing another thread.
The `measureme` integration is off by default, and must be enabled via
`-Zmiri-measureme=<output_name>`
Move cargo-miri into separate package
So far, `cargo-miri` depended on the Miri library crate. That was mostly for convenience (to avoid having to deal with workspaces), but also because until recently `cargo-miri` needed access to the ["Miri default flags"](af044209b0/src/lib.rs (L75)). But with `MIRI_BE_RUSTC` that is no longer the case, so we can remove this fake dependency. :)
@oli-obk what do you think? Also I expect the rustc bootstrap integration will need updates, hints would be appreciated. :D
