Inconsistent borrow check in async function

[Nugine]

I tried this code:

use std::collections::HashMap;

fn f1(map: &mut HashMap<i32, String>, k: i32) -> &String {
    if let Some(s) = map.get(&k) {
        return s;
    }
    map.insert(k, k.to_string());
    map.get(&k).unwrap()
}

async fn f2(map: &mut HashMap<i32, String>, k: i32) -> &String {
    if let Some(s) = map.get(&k) {
        return s;
    }
    map.insert(k, k.to_string());
    map.get(&k).unwrap()
}

f2 seems to be correct but can not be compiled.

   Compiling playground v0.0.1 (/playground)
error[E0502]: cannot borrow `*map` as mutable because it is also borrowed as immutable
  --> src/lib.rs:15:5
   |
12 |     if let Some(s) = map.get(&k) {
   |                      --- immutable borrow occurs here
13 |         return s;
   |                - returning this value requires that `*map` is borrowed for `'1`
14 |     }
15 |     map.insert(k, k.to_string());
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ mutable borrow occurs here
16 |     map.get(&k).unwrap()
17 | }
   | - return type of generator is &'1 std::string::String

error: aborting due to previous error

For more information about this error, try `rustc --explain E0502`.
error: could not compile `playground`.

To learn more, run the command again with --verbose.

Meta

According to the playground, the problem exists on 1.44.1 and 1.46.0-nightly (2020-07-04 0cd7ff7ddfb75a38dca8).

Other versions may be the same.

[SNCPlay42]

If f2 is commented out then f1 errors the same way (playground):

   Compiling playground v0.0.1 (/playground)
error[E0502]: cannot borrow `*map` as mutable because it is also borrowed as immutable
 --> src/lib.rs:7:5
  |
3 | fn f1(map: &mut HashMap<i32, String>, k: i32) -> &String {
  |            - let's call the lifetime of this reference `'1`
4 |     if let Some(s) = map.get(&k) {
  |                      --- immutable borrow occurs here
5 |         return s;
  |                - returning this value requires that `*map` is borrowed for `'1`
6 |     }
7 |     map.insert(k, k.to_string());
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ mutable borrow occurs here

error: aborting due to previous error

For more information about this error, try `rustc --explain E0502`.
error: could not compile `playground`.

To learn more, run the command again with --verbose.

In rustc 1.42.0 both errors are reported simultaneously (godbolt).

I'm not sure whether this is a bug or not - rustc need not report everything wrong with a crate in one go, but if it can report these simultaneously then it probably should.

[SNCPlay42]

cargo bisect-rustc results for when the compiler stopped reporting both errors simultaneously:

searched nightlies: from nightly-2020-02-06 to nightly-2020-07-05
regressed nightly: nightly-2020-02-16
searched commits: from 433aae9 to 61d9231
regressed commit: 19288dd

[Nugine]

I have found the answer.
http://smallcultfollowing.com/babysteps/blog/2018/06/15/mir-based-borrow-check-nll-status-update/
https://nikomatsakis.github.io/rust-belt-rust-2019/#87

How can I work around it without Polonius?
Just use Entry API.

[tmandry]

Thanks for digging into this, @SNCPlay42! This doesn't appear to be async-related, so removing that tag.

[nikomatsakis]

If @SNCPlay42's bisection is correct (thanks!), then this was triggered by #67681 (cc @matthewjasper). It is indeed true that the code example is not expected to compile, though you could make it compile by either using entry API or adding something like

if map.contains(..) {
   if let Some(...) = ... {
      ...
   }
}

I'm not sure exactly what caused this to occur, but it makes sense that #67681 might be related since that affected how we manage opaque type inference (and the async fn implicitly uses an opaque return type).

[matthewjasper]

The error is being suppressed because rustc_typeck::check_crate is (effectively) aborting compilation early.

[spastorino]

Prioritized as P-medium as per Zulip discussion.