"borrow of possibly uninitialized variable" instead of "use of moved value"

[stephaneyfx]

NLL combined with a struct field assignment gives an error mentioning the use/borrow of an uninitialized variable instead of the use/borrow of a moved variable.

Reproduction example

Simplified code based on @Yatekii's question on IRC
Playground

#![feature(nll)]

struct A {
    b: B,
}

#[derive(Clone)]
struct B;

fn foo(_: A) {}

fn bar(mut a: A) -> B {
    // Commenting the line below gives a move error instead of uninitialized
    // error.
    a.b = B;
    foo(a);
    a.b.clone()
}

fn main() {}

Expected behavior

The compiler gives an error about using a moved value, similar to the following:

Without NLL:

error[E0382]: use of moved value: `a.b`
  --> src/main.rs:17:5
   |
16 |     foo(a);
   |         - value moved here
17 |     a.b.clone()
   |     ^^^ value used here after move
   |
   = note: move occurs because `a` has type `A`, which does not implement the `Copy` trait

With NLL but without the field assignment:

error[E0382]: borrow of moved value: `a.b`
  --> src/main.rs:17:5
   |
16 |     foo(a);
   |         - value moved here
17 |     a.b.clone()
   |     ^^^ value borrowed here after move
   |
   = note: move occurs because `a` has type `A`, which does not implement the `Copy` trait

Actual behavior

The error mentions an uninitialized value instead of a moved value:

error[E0381]: borrow of possibly uninitialized variable: `a.b`
  --> src/main.rs:17:5
   |
17 |     a.b.clone()
   |     ^^^ use of possibly uninitialized `a.b`

[nikomatsakis]

I would classify this as a "minor diagnostic improvement". I'm moving to the Milestone and marking as E-needs-mentor.

[csmoe]

EDIT: Duplicate of #21232

---

this issue may caused by Place checking I guess:

rust/src/librustc_mir/borrow_check/mod.rs

Line 1620 in 71a1ef1

// 2. Move of `a.b.c`, use of `a.b.c.d` (without first reinitializing `a.b.c.d`)

rust/src/librustc_mir/borrow_check/mod.rs

Line 1631 in 71a1ef1

// 7. Move of `a.b.c` then reinit of `a.b.c.d`, use of `a.b.c.d`

the comments reason why the uninitialized lint emitted here: for place a.b, if a is moved, then the usage of projection a.b will be treated as uninitialized, let's take an experiment:
https://play.rust-lang.org/?gist=12d7880fc733e74b5bac20119465ada3&version=nightly&mode=debug&edition=2015

fn bar(mut a: A) -> B {
    a.b = B;
    foo(a);
    a.b
}

=>

error[E0381]: use of possibly uninitialized variable: `a.b`
  --> src/main.rs:16:5
   |
16 |     a.b
   |     ^^^ use of possibly uninitialized `a.b`

==========

instead of using place a.b, we return a:

```rust
fn too(mut a: A) -> A {
    a.b = B;
    foo(a);
    a
}

=>

error[E0382]: use of moved value: `a`
  --> src/main.rs:16:5
   |
15 |     foo(a);
   |         - value moved here
16 |     a
   |     ^ value used here after move
   |
   = note: move occurs because `a` has type `A`, which does not implement the `Copy` trait

[nikomatsakis]

Clearly not an RC blocker

[nikomatsakis]

Still occurs

[nikomatsakis]

Seems worth fixing to me for RC2 — this could be a "new user stumbling point".

[nikomatsakis]

Might be a good fit for @spastorino, who recently did #46590, which is touching the same code that produces this error. Thoughts @spastorino ?