serde causes 'rustc' stack overflow with in function struct definition

[xu-cheng]

Example to reproduce the bug.

• Cargo.toml

[dependencies]
serde = { version = "1.0", features = ["derive"] }

• src/lib.rs

use serde::{
    ser::{SerializeSeq, Serializer},
    Serialize,
};

pub struct Container<T> {
    inner: Vec<T>,
}

impl<T: Serialize + Clone> Serialize for Container<T> {
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: Serializer,
    {
        #[derive(Serialize)]
        struct Foo<T> {
            obj: T,
        }

        let mut seq = serializer.serialize_seq(Some(self.inner.len()))?;
        for k in self.inner.iter() {
            seq.serialize_element(&Foo { obj: k.clone() })?;
        }
        seq.end()
    }
}

• Compile result

❯ cargo +nightly check
    Checking test-rust v0.1.0 (/private/tmp/test-rust)

thread 'rustc' has overflowed its stack
fatal runtime error: stack overflow
error: could not compile `test-rust`

Caused by:
  process didn't exit successfully: `rustc --crate-name test_rust --edition=2018 src/lib.rs --error-format=json --json=diagnostic-rendered-ansi --crate-type lib --emit=dep-info,metadata -C embed-bitcode=no -C debuginfo=2 -C metadata=6743fa578220919d -C extra-filename=-6743fa578220919d --out-dir /private/tmp/test-rust/target/debug/deps -C incremental=/private/tmp/test-rust/target/debug/incremental -L dependency=/private/tmp/test-rust/target/debug/deps --extern serde=/private/tmp/test-rust/target/debug/deps/libserde-80a0b09a70478260.rmeta` (signal: 6, SIGABRT: process abort signal)

I am on macOS. This bug can be reproduced with both the stable (1.49.0 (e1884a8e3 2020-12-29)) and latest nightly (1.51.0-nightly (4253153db 2021-01-17)) rustc.

If we move Foo struct to the outside of the serialize function, it can compile successfully.

[xu-cheng]

Also, this may be a regression. Because the same code was able to be compiled one year ago.

[xu-cheng]

Using cargo expand, I reduce the example to the following:

use serde::{ser::Serializer, Serialize};

pub struct Container;

impl Serialize for Container {
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: Serializer,
    {
        const _: () = {
            extern crate serde as _serde;
        };

        todo!()
    }
}

❯ cargo +nightly check
    Checking test-rust v0.1.0 (/private/tmp/test-rust)
warning: unused variable: `serializer`
 --> src/lib.rs:6:28
  |
6 |     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
  |                            ^^^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_serializer`
  |
  = note: `#[warn(unused_variables)]` on by default


thread 'rustc' has overflowed its stack
fatal runtime error: stack overflow
error: could not compile `test-rust`

Caused by:
  process didn't exit successfully: `rustc --crate-name test_rust --edition=2018 src/lib.rs --error-format=json --json=diagnostic-rendered-ansi --crate-type lib --emit=dep-info,metadata -C embed-bitcode=no -C debuginfo=2 -C metadata=6743fa578220919d -C extra-filename=-6743fa578220919d --out-dir /private/tmp/test-rust/target/debug/deps -C incremental=/private/tmp/test-rust/target/debug/incremental -L dependency=/private/tmp/test-rust/target/debug/deps --extern serde=/private/tmp/test-rust/target/debug/deps/libserde-80a0b09a70478260.rmeta` (signal: 6, SIGABRT: process abort signal)

[dtolnay]

This is a rustc bug. rust-lang/rust#55779

[xu-cheng]

Thanks for the info. Should I close this issue? Or leave it until it is fixed by rustc?