Add span label to E0384 for MIR borrowck

[KiChjang]

Corresponds to report_illegal_reassignment.

Part of #44596.

[rust-highfive]

r? @nikomatsakis

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[KiChjang]

r? @pnkfelix

[pnkfelix]

@KiChjang can you look into porting the test, in the same manner as discussed at #44811 (comment)

[pnkfelix]

Does the source info for the declaration always correspond to the assigned span when reported by AST borrowck?

In particular, I'm thinking of cases like:

let declare_without_init: i32;
match result {
    Ok(_) => declare_without_init = 1,
    Err(_) => declare_without_init = 2,
}
...
declare_without_init = 3;

where the AST-borrowck today, for better or worse, tags the assignment in the first match arm as the "first assignment"

[KiChjang]

I don't actually know, I've only looked around the source code and used my own deduction skills to do what makes sense in order to retrieve a span. Perhaps @nikomatsakis may know more about this?

[KiChjang]

I just compiled the example you gave with my patch -- it actually does the wrongest thing, which is displaying the span of let declare_without_init: i32;. It would seem like mir.local_decls would not provide the location in which a variable is assigned.

[KiChjang]

I believe this can be re-reviewed. r? @pnkfelix

[arielb1]

I think your revision tags are not working:

[00:51:10] ---- [compile-fail] compile-fail/borrowck/borrowck-match-binding-is-assignment.rs stdout ----
[00:51:10] 	
[00:51:10] error in revision `mir`: /checkout/src/test/compile-fail/borrowck/borrowck-match-binding-is-assignment.rs:29: unexpected "error": '29:13: 29:19: re-assignment of immutable variable `x` [E0384]'
[00:51:10] 
[00:51:10] error in revision `mir`: /checkout/src/test/compile-fail/borrowck/borrowck-match-binding-is-assignment.rs:37: unexpected "error": '37:13: 37:19: re-assignment of immutable variable `x` [E0384]'
[00:51:10] 
[00:51:10] error in revision `mir`: /checkout/src/test/compile-fail/borrowck/borrowck-match-binding-is-assignment.rs:45: unexpected "error": '45:13: 45:19: re-assignment of immutable variable `x` [E0384]'
[00:51:10] 
[00:51:10] error in revision `mir`: /checkout/src/test/compile-fail/borrowck/borrowck-match-binding-is-assignment.rs:53: unexpected "error": '53:13: 53:19: re-assignment of immutable variable `x` [E0384]'
[00:51:10] 
[00:51:10] error in revision `mir`: /checkout/src/test/compile-fail/borrowck/borrowck-match-binding-is-assignment.rs:61: unexpected "error": '61:13: 61:19: re-assignment of immutable variable `x` [E0384]'
[00:51:10] 
[00:51:10] error in revision `mir`: /checkout/src/test/compile-fail/borrowck/borrowck-match-binding-is-assignment.rs:29: expected error not found: re-assignment of immutable variable `x` (Mir)
[00:51:10] 
[00:51:10] error in revision `mir`: /checkout/src/test/compile-fail/borrowck/borrowck-match-binding-is-assignment.rs:29: expected error not found: re-assignment of immutable variable `x` (Ast)
[00:51:10] 
[00:51:10] error in revision `mir`: /checkout/src/test/compile-fail/borrowck/borrowck-match-binding-is-assignment.rs:37: expected error not found: re-assignment of immutable variable `x` (Mir)
[00:51:10] 
[00:51:10] error in revision `mir`: /checkout/src/test/compile-fail/borrowck/borrowck-match-binding-is-assignment.rs:37: expected error not found: re-assignment of immutable variable `x` (Ast)
[00:51:10] 
[00:51:10] error in revision `mir`: /checkout/src/test/compile-fail/borrowck/borrowck-match-binding-is-assignment.rs:45: expected error not found: re-assignment of immutable variable `x` (Mir)
[00:51:10] 
[00:51:10] error in revision `mir`: /checkout/src/test/compile-fail/borrowck/borrowck-match-binding-is-assignment.rs:45: expected error not found: re-assignment of immutable variable `x` (Ast)
[00:51:10] 
[00:51:10] error in revision `mir`: /checkout/src/test/compile-fail/borrowck/borrowck-match-binding-is-assignment.rs:53: expected error not found: re-assignment of immutable variable `x` (Mir)
[00:51:10] 
[00:51:10] error in revision `mir`: /checkout/src/test/compile-fail/borrowck/borrowck-match-binding-is-assignment.rs:53: expected error not found: re-assignment of immutable variable `x` (Ast)
[00:51:10] 
[00:51:10] error in revision `mir`: /checkout/src/test/compile-fail/borrowck/borrowck-match-binding-is-assignment.rs:61: expected error not found: re-assignment of immutable variable `x` (Mir)
[00:51:10] 
[00:51:10] error in revision `mir`: /checkout/src/test/compile-fail/borrowck/borrowck-match-binding-is-assignment.rs:61: expected error not found: re-assignment of immutable variable `x` (Ast)
[00:51:10] 

[arielb1]

I think you want to use [mir]compile-flags (not compfile) here. I think that's why you're getting the travis error.

[KiChjang]

Ack, this is so embarrassing!

[arielb1]

[00:14:00] error[E0308]: mismatched types
[00:14:00]    --> /checkout/src/librustc_mir/borrow_check.rs:594:75
[00:14:00]     |
[00:14:00] 594 |                 self.report_illegal_reassignment(context, (lvalue, span), assignment_span);
[00:14:00]     |                                                                           ^^^^^^^^^^^^^^^ expected struct `syntax_pos::Span`, found reference
[00:14:00]     |
[00:14:00]     = note: expected type `syntax_pos::Span`
[00:14:00]                found type `&rustc::mir::Statement<'_>`

[pnkfelix]

@bors r+

[bors]

📌 Commit 6d4989b has been approved by pnkfelix

[bors]

⌛️ Testing commit 6d4989b with merge d887369...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: pnkfelix
Pushing d887369 to master...