Fix End-user description not implemented for field access on `TyClosure

[vramana]

• Add Tests

[nikomatsakis]

I think we are not quite ready for a PR here. I think we will want to edit the caller of describe_field_from_ty -- the end goal is to render the whole lvalue (*_0.0) as just the name of the local variable.

[nikomatsakis]

What is the issue number again?

[nikomatsakis]

ah #45698

[arielb1]

Use self.mir.lvalue_ty(proj.base).to_ty(tcx)

[arielb1]

this looks like it would print .local. I think it's probably a better idea to ditch the prefix/middle/suffix scheme and just push things from the match arms.

[alexcrichton]

r? @nikomatsakis

[arielb1]

So r=me with one of the tests (e.g. src/compile-fail/borrowck/borrowck-multiple-captures.rs) adjusted to also test MIR borrowck (see e.g. src/test/compile-fail/borrowck/borrowck-use-uninitialized-in-cast-trait.rs to how this is done).

[vramana]

@arielb1 Sure

[bors]

☔ The latest upstream changes (presumably #45436) made this pull request unmergeable. Please resolve the merge conflicts.

[nikomatsakis]

Can you add a nested test and convert to use the helper I suggested?

[nikomatsakis]

I saw that the code in ppaux does self.tcx.hir.name(local_def.var_id()) -- maybe we should do that. I think in particular this could be a Def::Upvar for nested closures. Maybe try this example:

// Field from upvar
{
    let mut x = 0;
       || { || {
            let y = &mut x;
            &mut x; //[ast]~ ERROR cannot borrow `**x` as mutable more than once at a time
                    //[mir]~^ ERROR cannot borrow `**x` as mutable more than once at a time (Ast)
                    //[mir]~| ERROR cannot borrow `(**x)` as mutable more than once at a time (Mir)
            *y = 1;
        } };
}

[nikomatsakis]

I just realized that the Mir has a vector of upvar_decls:

/// A closure capture, with its name and mode.
#[derive(Clone, Debug, RustcEncodable, RustcDecodable)]
pub struct UpvarDecl {
    pub debug_name: Name,

    /// If true, the capture is behind a reference.
    pub by_ref: bool
}

This might be a nicer thing to use than freevars for getting the names of upvars. Moreover, it would tell us the upvar is by reference, which should allow us to eliminate those extra *.

[arielb1]

@nikomatsakis

I'll r+ this PR anyway - we need a followup to fix the deref situation, but it's "pre-existing" #46003

@bors r+

[bors]

📌 Commit 5144d32 has been approved by arielb1

[arielb1]

@bors p=1

I'll like to land this today so I can have some sense of scale of the MIR borrowck situation

[bors]

⌛ Testing commit 5144d32 with merge fa26421...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: arielb1
Pushing fa26421 to master...