Borrowed Ref seems to outlive its block

[snoyberg]

This may be a misunderstanding on my part, apologies for the noise if so. In the code below, I believe the Ref is improperly living beyond its containing block, and the code in question should be a compile-time error instead of a runtime panic. Consider this code:

use std::cell::RefCell;

fn main() {
    let strings: RefCell<Vec<String>> = RefCell::new(Vec::new());
    let _hello: &str = {
        strings.borrow_mut().push("Hello".to_string());
        &strings.borrow()[0]
    };
    strings.borrow_mut();
}

The strings.borrow() call results in a Ref<Vec<String>>, which is then indexed to get a &str which is the result of the block. Importantly, note that the &str is borrowed from the Ref, and therefore should not extend lifetime beyond the enclosing block. Therefore, from my understanding, this should be a compile time error.

Instead, the Ref appears to continue to live. The evidence of this is that this program compiles successfully, but when run, the borrow_mut() call after the block panics since there is still an active borrow on the RefCell. A variation on the program above helps clarify the situation further. If I explicitly capture the result of strings.borrow(), like so:

use std::cell::RefCell;

fn main() {
    let strings: RefCell<Vec<String>> = RefCell::new(Vec::new());
    let _hello: &str = {
        strings.borrow_mut().push("Hello".to_string());
        let strings_ref = strings.borrow();
        &strings_ref[0]
    };
    strings.borrow_mut();
}

I get the expected error message about strings_ref not living long enough:

error[E0597]: `strings_ref` does not live long enough
 --> src\main.rs:8:10
  |
5 |     let _hello: &str = {
  |         ------ borrow later stored here
...
8 |         &strings_ref[0]
  |          ^^^^^^^^^^^ borrowed value does not live long enough
9 |     };
  |     - `strings_ref` dropped here while still borrowed

Meta

I tested with both stable 1.46.0 release:

>rustc --version --verbose
rustc 1.46.0 (04488afe3 2020-08-24)
binary: rustc
commit-hash: 04488afe34512aa4c33566eb16d8c912a3ae04f9
commit-date: 2020-08-24
host: x86_64-pc-windows-msvc
release: 1.46.0
LLVM version: 10.0

And with nightly for 2020-09-11:

>rustc +nightly --version --verbose
rustc 1.48.0-nightly (a1947b3f9 2020-09-10)
binary: rustc
commit-hash: a1947b3f9e2831e2060bc42f0c78e4a2bb67930a
commit-date: 2020-09-10
host: x86_64-pc-windows-msvc
release: 1.48.0-nightly
LLVM version: 11.0

The panics are slightly different between the two versions. On stable, the panic references a code location with std, while nightly references a code location within my own project. Stable:

>cargo run
   Compiling foo v0.1.0 (C:\Users\snoyb\Desktop\foo)
    Finished dev [unoptimized + debuginfo] target(s) in 0.50s
     Running `target\debug\foo.exe`
thread 'main' panicked at 'already borrowed: BorrowMutError', C:\Users\snoyb\.rustup\toolchains\stable-x86_64-pc-windows-msvc\lib/rustlib/src/rust\src\libcore\cell.rs:867:31
stack backtrace:
   0: backtrace::backtrace::trace_unsynchronized
             at C:\Users\runneradmin\.cargo\registry\src\github.com-1ecc6299db9ec823\backtrace-0.3.46\src\backtrace\mod.rs:66
   1: std::sys_common::backtrace::_print_fmt
             at /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9\/src\libstd\sys_common\backtrace.rs:78
   2: std::sys_common::backtrace::_print::{{impl}}::fmt
             at /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9\/src\libstd\sys_common\backtrace.rs:59
   3: core::fmt::write
             at /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9\/src\libcore\fmt\mod.rs:1076
   4: std::io::Write::write_fmt<std::sys::windows::stdio::Stderr>
             at /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9\/src\libstd\io\mod.rs:1537
   5: std::sys_common::backtrace::_print
             at /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9\/src\libstd\sys_common\backtrace.rs:62
   6: std::sys_common::backtrace::print
             at /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9\/src\libstd\sys_common\backtrace.rs:49
   7: std::panicking::default_hook::{{closure}}
             at /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9\/src\libstd\panicking.rs:198
   8: std::panicking::default_hook
             at /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9\/src\libstd\panicking.rs:217
   9: std::panicking::rust_panic_with_hook
             at /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9\/src\libstd\panicking.rs:526
  10: std::panicking::begin_panic_handler
             at /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9\/src\libstd\panicking.rs:437
  11: core::panicking::panic_fmt
             at /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9\/src\libcore\panicking.rs:85
  12: core::option::expect_none_failed
             at /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9\/src\libcore\option.rs:1269
  13: core::result::Result<core::cell::RefMut<alloc::vec::Vec<alloc::string::String>>, core::cell::BorrowMutError>::expect<core::cell::RefMut<alloc::vec::Vec<alloc::string::String>>,core::cell::BorrowMutError>
             at C:\Users\snoyb\.rustup\toolchains\stable-x86_64-pc-windows-msvc\lib\rustlib\src\rust\src\libcore\result.rs:963
  14: core::cell::RefCell<alloc::vec::Vec<alloc::string::String>>::borrow_mut<alloc::vec::Vec<alloc::string::String>>
             at C:\Users\snoyb\.rustup\toolchains\stable-x86_64-pc-windows-msvc\lib\rustlib\src\rust\src\libcore\cell.rs:867
  15: foo::main
             at .\src\main.rs:9
  16: std::rt::lang_start::{{closure}}<tuple<>>
             at C:\Users\snoyb\.rustup\toolchains\stable-x86_64-pc-windows-msvc\lib\rustlib\src\rust\src\libstd\rt.rs:67
  17: std::rt::lang_start_internal::{{closure}}
             at /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9\/src\libstd\rt.rs:52
  18: std::panicking::try::do_call
             at /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9\/src\libstd\panicking.rs:348
  19: std::panicking::try
             at /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9\/src\libstd\panicking.rs:325
  20: std::panic::catch_unwind
             at /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9\/src\libstd\panic.rs:394
  21: std::rt::lang_start_internal
             at /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9\/src\libstd\rt.rs:51
  22: std::rt::lang_start<tuple<>>
             at C:\Users\snoyb\.rustup\toolchains\stable-x86_64-pc-windows-msvc\lib\rustlib\src\rust\src\libstd\rt.rs:67
  23: main
  24: invoke_main
             at D:\agent\_work\9\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:78
  25: __scrt_common_main_seh
             at D:\agent\_work\9\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288
  26: BaseThreadInitThunk
  27: RtlUserThreadStart
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
error: process didn't exit successfully: `target\debug\foo.exe` (exit code: 101)

And nightly:

>cargo +nightly run
   Compiling foo v0.1.0 (C:\Users\snoyb\Desktop\foo)
    Finished dev [unoptimized + debuginfo] target(s) in 0.46s
     Running `target\debug\foo.exe`
thread 'main' panicked at 'already borrowed: BorrowMutError', src\main.rs:9:13
stack backtrace:
   0: std::panicking::begin_panic_handler
             at /rustc/a1947b3f9e2831e2060bc42f0c78e4a2bb67930a\/library\std\src\panicking.rs:483
   1: core::panicking::panic_fmt
             at /rustc/a1947b3f9e2831e2060bc42f0c78e4a2bb67930a\/library\core\src\panicking.rs:85
   2: core::option::expect_none_failed
             at /rustc/a1947b3f9e2831e2060bc42f0c78e4a2bb67930a\/library\core\src\option.rs:1221
   3: core::result::Result<core::cell::RefMut<alloc::vec::Vec<alloc::string::String>>, core::cell::BorrowMutError>::expect<core::cell::RefMut<alloc::vec::Vec<alloc::string::String>>,core::cell::BorrowMutError>
             at /rustc/a1947b3f9e2831e2060bc42f0c78e4a2bb67930a\library\core\src\result.rs:933
   4: core::cell::RefCell<alloc::vec::Vec<alloc::string::String>>::borrow_mut<alloc::vec::Vec<alloc::string::String>>
             at /rustc/a1947b3f9e2831e2060bc42f0c78e4a2bb67930a\library\core\src\cell.rs:869
   5: foo::main
             at .\src\main.rs:9
   6: core::ops::function::FnOnce::call_once<fn(),tuple<>>
             at /rustc/a1947b3f9e2831e2060bc42f0c78e4a2bb67930a\library\core\src\ops\function.rs:227
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
error: process didn't exit successfully: `target\debug\foo.exe` (exit code: 101)

[jonas-schievink]

Temporaries created in the trailing expression of a block can have their lifetime extended to cover the let bindings. This is documented here: https://doc.rust-lang.org/reference/destructors.html?highlight=temporary,life#temporary-lifetime-extension

[snoyberg]

Wow, thank you for the very fast explanation! Sorry for the noise.

[jonas-schievink]

No worries!