Enable MIR-borrowck-only output

[nikomatsakis]

Currently, if you pass -Zborrowck-mir, we output both the AST output (tagged with (Ast)) and the MIR output (tagged with (Mir)).

Example:

> cd src/test/compile-fail
> rustc -Zborrowck-mir borrowck/borrowck-unary-move.rs
error[E0505]: cannot move out of `x` because it is borrowed (Ast)
  --> borrowck/borrowck-unary-move.rs:17:10
   |
16 |     let y = &*x;
   |              -- borrow of `*x` occurs here
17 |     free(x); //[ast]~ ERROR cannot move out of `x` because it is borrowed
   |          ^ move out of `x` occurs here

error[E0505]: cannot move out of `x` because it is borrowed (Mir)
  --> borrowck/borrowck-unary-move.rs:17:10
   |
16 |     let y = &*x;
   |             --- borrow of `(*x)` occurs here
17 |     free(x); //[ast]~ ERROR cannot move out of `x` because it is borrowed
   |          ^ move out of `x` occurs here

error: aborting due to 2 previous errors

I would like to alter the switch to be one where there are three mores:

-Zborrowck=ast -- the default, emits only AST
-Zborrowck=mir -- emits only MIR errors (and doesn't write `(Mir)`)
-Zborrowck=compare -- emits both, like `-Zborrowck-mir` does today.

This will be useful as we progress towards our yearly goal of having a working "demo", since the demo isn't very good if people are seeing the old and new errors. It will also be less annoying for editing the unit tests, since we won't need three copies of each error. And if we want the current output when running by hand, it is still available.

[nikomatsakis]

Mentoring instructions

This is where the -Zborrowck-mir flag is defined:

rust/src/librustc/session/config.rs

Lines 976 to 977 in 5f1c37a

	borrowck_mir: bool = (false, parse_bool, [UNTRACKED],
	"implicitly treat functions as if they have `#[rustc_mir_borrowck]` attribute"),

That is part of a macro that ultimately defines the DebuggingOpts struct. We could change that line to something like this:

    borrowck: bool = (false, parse_opt_string, [UNTRACKED],
        "select which borrowck is used (`ast`, `mir`, or `compare`)"),

This will change from a borrowck_mir: bool field to a borrowck: Option<String> field. One problem is that this will also accept nonsense like -Zborrowck=foo. I think what we ought to do is to add an enum BorrowckMode comparable to PrintRequest and the others that exist already:

#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
enum BorrowckMode {
    Ast,
    Mir,
    Compare,
}

Then we will add a field in the Options struct by inserting a line around here like this:

/// Determines which borrow checker(s) to run. This is the parsed, sanitized
/// version of `debugging_opts.borrowck`, which is just a plain string.
borrowck_mode: BorrowckMode,

(The setup here is that there is an Options struct, which we just edited, which contains a debugging_opts: DebuggingOpts field that has the "raw" debugging options; that DebuggingOpts struct is the one that is auto-generated by the macro above.)

Then we can add some code around here that will parse the debugging_opts.borrowck data and create a BorrowckMode enum, or else report an error (using a call to early_error like so). The incremental field is sort of roughly the model we want, except that it doesn't report any errors:

• raw incremental field is parsed here by converting it into a PathBuf
• parsed version is added into the Options struct here

OK, once we've done that, we should get a few compilation errors because the field tcx.sess.opts.debugging_opts.borrowck_mir no longer exists. You can convert those to examine the tcx.sess.opts.borrowck_mode field. I think I would add some methods to BorrowckMode:

impl BorrowckMode {
    /// Should the AST-based borrow checker execute at all?
    pub fn use_ast(self) -> bool {
        match self {
            BorrowckMode::Ast => true,
            BorrowckMode::Compare => true,
            BorrowckMode::Mir => false,
        }
    }
    ...
}

and so on so that we can convert to tcx.sess.opts.borrowck_mode.use_ast() or whatever.

Finally, we have to make the AST-based borrow checker not execute (or at least not report errors) unless we are in the right mode. It's actually easier to just make it not report errors: this is because the AST-based borrowck is still used for giving "unused mut" warnings, so if we just skip it altogether we'll get a bunch of weird warnings. To make it not report errors, though, we just have to modify the functions in this file, having them check the origin and just do nothing if it is wrong.

[nikomatsakis]

Oh, one last thing. We have to edit all the tests! They can be changed from having -Zborrowck-mir at the top to -Zborrowck=mir, and then removing the extra, duplicate comments from the [mir] revision.

[est31]

I want to work on this!

[est31]

@LooMaclin there seems to be a conflict here!

[est31]

Especially, I'm almost done with my PR, the only thing that remains is updating the tests which is a bit time intensive...

[LooMaclin]

@est31 no problem. im stop working on that