ICE： NLL bug?

[ZhangHanDong]

#![feature(nll)]

use std::cell::RefCell;

fn main() {
    let s = RefCell::new(Some(10));
    if let Some(n) = *(s.borrow_mut()) {
        println!("num: {}", n);
    }
}

I thought the code should be compiled with nll feature ,

but got the following errors:

Compiling playground v0.0.1 (file:///playground)
thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value', libcore/option.rs:335:21
note: Run with `RUST_BACKTRACE=1` for a backtrace.

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.26.0-nightly (06fa27d7c 2018-04-01) running on x86_64-unknown-linux-gnu

note: compiler flags: -C codegen-units=1 -C debuginfo=2 --crate-type bin

note: some of the compiler flags provided by cargo are hidden

error: Could not compile `playground`.

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

play

the following code is good:

#![feature(nll)]

use std::cell::RefCell;

fn main() {
    let s = RefCell::new(Some(10));
    if let Some(n) = *(s.borrow_mut()) {
        println!("num: {}", n);
    }
    ;
}

[nikomatsakis]

Maybe fixed by @spastorino in #49808

[spastorino]

#49808 fixes this issue.

The output you get is ...

[santiago@archlinux tmp]$ rustc +stage1 test.rs 
error[E0597]: `s` does not live long enough
  --> test.rs:7:24
   |
7  |     if let Some(n) = *(s.borrow_mut()) {
   |                       -^--------------
   |                       ||
   |                       |borrowed value does not live long enough
   |                       borrow may end up in a temporary, created here
...
10 | }
   | -
   | |
   | borrowed value only lives until here
   | temporary later dropped here, potentially using the reference

error: aborting due to previous error

For more information about this error, try `rustc --explain E0597`.

[ZhangHanDong]

@spastorino

Why is it still error with NLL ?

[spastorino]

@ZhangHanDong I was just paying attention to the thing not ICEing :). Seems like there's something wrong /cc @nikomatsakis

[spastorino]

@ZhangHanDong @nikomatsakis explained here the bug #46413 (comment) so besides from the ICE there's something else going on :)

[ZhangHanDong]

@nikomatsakis

Why is it still error with NLL in #49808?
Can you help me explain the puzzle？

[KiChjang]

@ZhangHanDong I'm inclined to believe that this is the drop check rule in action again, because if you add a semicolon right after your if-let statement, the code compiles:

#![feature(nll)]

use std::cell::RefCell;

fn main() {
    let s = RefCell::new(Some(10));
    if let Some(n) = *(s.borrow_mut()) {
        println!("num: {}", n);
    };
}

In other words, the BorrowMut that the borrow_mut call generates does not strictly outlive the n variable binding (they are dropped at the same time). See further details here.

[KiChjang]

Closing as a duplicate of #22321.

[pnkfelix]

(it is more specifically a duplicate of #21114)

[pnkfelix]

(or really, at this point I might argue its a duplicate of #46413 ...)