Miri gives an error when resolving strings

[jyn514]

Given the following simple use of string_interner, miri gives the error trying to reborrow for SharedReadOnly, but parent tag <2253> does not have an appropriate item in the borrow stack.

I am not quite sure what this error means, but I am concerned about possible UB in string_interner. Here is a link to a similar issue in owning-ref: rust-lang/unsafe-code-guidelines#194

use string_interner::StringInterner;

fn main() {
    let mut strings = StringInterner::default();
    let str_ref = strings.get_or_intern("hello");
    let maybe_resolved = strings.resolve(str_ref);
    println!("{}", maybe_resolved.unwrap());
}

Full miri backtrace

error: Miri evaluation error: trying to reborrow for SharedReadOnly, but parent tag <2253> does not have an appropriate item in the borrow stack
    --> /home/joshua/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/liballoc/boxed.rs:1041:9
     |
1041 |         &**self
     |         ^^^^^^^ Miri evaluation error: trying to reborrow for SharedReadOnly, but parent tag <2253> does not have an appropriate item in the borrow stack
     |
     = note: inside call to `<std::boxed::Box<str> as std::convert::AsRef<str>>::as_ref` at /home/joshua/.cargo/registry/src/proxy-git.cwkhome.fun-1ecc6299db9ec823/string-interner-0.7.1/src/lib.rs:367:30
     = note: inside call to closure at /home/joshua/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/option.rs:449:29
     = note: inside call to `std::option::Option::<&std::boxed::Box<str>>::map::<&str, [closure@DefId(14:114 ~ string_interner[4f21]::{{impl}}[12]::resolve[0]::{{closure}}[0])]>` at /home/joshua/.cargo/registry/src/proxy-git.cwkhome.fun-1ecc6299db9ec823/string-interner-0.7.1/src/lib.rs:365:9
note: inside call to `string_interner::StringInterner::<string_interner::Sym>::resolve` at src/bin.rs:6:26
    --> src/bin.rs:6:26
     |
6    |     let maybe_resolved = strings.resolve(str_ref);
     |                          ^^^^^^^^^^^^^^^^^^^^^^^^
     = note: inside call to `main` at /home/joshua/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:61:34
     = note: inside call to closure at /home/joshua/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:48:73
     = note: inside call to closure at /home/joshua/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/sys_common/backtrace.rs:136:5
     = note: inside call to `std::sys_common::backtrace::__rust_begin_short_backtrace::<[closure@DefId(1:6021 ~ std[7e82]::rt[0]::lang_start_internal[0]::{{closure}}[0]::{{closure}}[0]) 0:&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /home/joshua/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:48:13
     = note: inside call to closure at /home/joshua/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panicking.rs:287:40
     = note: inside call to `std::panicking::try::do_call::<[closure@DefId(1:6020 ~ std[7e82]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /home/joshua/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panicking.rs:282:5
     = note: inside call to `std::panicking::try::<i32, [closure@DefId(1:6020 ~ std[7e82]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe]>` at /home/joshua/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panic.rs:395:9
     = note: inside call to `std::panic::catch_unwind::<[closure@DefId(1:6020 ~ std[7e82]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /home/joshua/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:47:25
     = note: inside call to `std::rt::lang_start_internal` at /home/joshua/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:61:5
     = note: inside call to `std::rt::lang_start::<()>`

[Robbepop]

That's interesting. Thank you for the info.
What does miri say about the pin branch of string_interner?
I feel that it is time for the next round of improvements for string_interner, also in safety perspectives.
The string_interner is using self-referential str for some internal optimizations. Maybe miri found some unsoundness in their usage. I haven't read through the whole issue you linked but it also might be a problem with stacked borrows and self referential structs.

[jyn514]

Interesting, miri does not give any error for the Pin branch.

[jyn514]

string_interner is using self-referential str

Yeah I don't think miri likes that very much: rust-lang/unsafe-code-guidelines#148

[Robbepop]

Interesting, miri does not give any error for the Pin branch.

That's also interesting because the pin branch is also self-referential, however by making use of the Pin utilities. We did not merge this into master because we had to remove certain APIs that did not work well but also the pin branch by now is pretty old.

[jyn514]

BTW you can test with miri locally like this:

$ rustup component add --toolchain nightly miri
$ cargo +nightly miri run

[jyn514]

Any progress on fixing this?

[jyn514]

Oops, I missed #17. I can confirm that PR fixes the miri evaluation error :)

[Robbepop]

This should have been resolved with v0.8. :)
Closed.