Implement --unpretty mir

[jonas-schievink]

This allows obtaining a textual MIR dump for individual items or all items in the crate.

I haven't added any tests since I'm too lazy this is an unstable debugging option, but I'll add one if required.

MIR for a single function can now be dumped using rustc -Zunstable-options --unpretty mir=my_function and no longer requires the use of in-source #[rustc_mir] attributes.

Blocks rust-lang/rust-playpen#154 (if MIR dump support from the playpen is even wanted).

Example output:

fn main() {
    let x = Some(0);
    x.unwrap_or_else(|| 1);
}

MIR for expr || 1 (id=16)
fn(arg0: [closure@test.rs:3:22: 3:26]) -> i32 {
    let mut tmp0: ();

    bb0: {
        return = const 1;
        goto -> bb1;
    }

    bb1: {
        return;
    }
}
MIR for fn main::main (id=4)
fn() -> () {
    let var0: core::option::Option<i32>; // x
    let mut tmp0: ();
    let mut tmp1: i32;
    let mut tmp2: core::option::Option<i32>;
    let mut tmp3: [closure@test.rs:3:22: 3:26];

    bb0: {
        var0 = core::option::Option::Some(const 0);
        tmp2 = var0;
        tmp3 = [closure@test.rs:3:22: 3:26];
        tmp1 = core::option::Option<T>::unwrap_or_else(tmp2, tmp3) -> bb2;
    }

    bb1: {
        return;
    }

    bb2: {
        drop(tmp1) -> bb3;
    }

    bb3: {
        return = ();
        goto -> bb1;
    }
}

[rust-highfive]

r? @arielb1

(rust_highfive has picked a reviewer for you, use r? to override)

[nagisa]

Nit: I personally think the MIR pretty writer should be adapted to somehow work on MIR maps by e.g. outputting

fn main::main() -> ... {
...
}

for functions and

λ main::main::1(... {
}

for closures as well as improving the way closures are printed.

[jonas-schievink]

Agreed. That would require passing both the HIR and MIR map, as well as some custom formatting, so I'll leave that to another PR.

[arielb1]

r? @nrc

[nrc]

@bors: r+

[bors]

📌 Commit 67aa7b2 has been approved by nrc

[bors]

⌛ Testing commit 67aa7b2 with merge 7ce4afb...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-mac-ios-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt