rustc overflowed its stack: extern crate inside const inside external trait impl

[CAD97]

...but only when the crate is introduced solely through edition2018 extern prelude and not when an extern crate statement exists at root level.

Minimized reproduction (on playground)

use serde::de::{Deserialize, Deserializer};

pub struct S;

impl<'de> Deserialize<'de> for S {
    fn deserialize<D>(_: D) -> Result<Self, D::Error>
    where
        D: Deserializer<'de>,
    {
        const _: () = {
            extern crate serde;
        };
        todo!()
    }
}

(Reason for this code shape: deriving serde's traits puts the impl within a const and uses extern crate to get serde at a known name.)

Edits that make this not overflow the stack:

• Add extern crate serde; at root level.
• Inline the Deserialize trait definition.
• Removing the const around the extern crate statement.
• Moving the const declaration to root scope.

Meta

rustc --version --verbose:

rustc 1.44.0-nightly (94d346360 2020-04-09)
binary: rustc
commit-hash: 94d346360da50f159e0dc777dc9bc3c5b6b51a00
commit-date: 2020-04-09
host: x86_64-pc-windows-msvc
release: 1.44.0-nightly
LLVM version: 9.0

This overflows the stack back to 1.31.0, the first stable release with edition 2018 support.

cargo check is enough to get the stack overflow.

Error output

thread 'rustc' has overflowed its stack
fatal runtime error: stack overflow

[CAD97]

Here's a mcve that works without serde, using a single manifest:

Cargo.toml

[package]
name = "playground"
version = "0.0.0"
edition = "2018"

lib.rs

pub trait Trait {
    fn method();
}

main.rs

use playground::Trait;

struct Struct;

impl Trait for Struct {
    fn method() {
        const _: () = {
            extern crate playground;
        };
    }
}

fn main() {}

Removing the main function from main.rs will cause an error without causing a stack overflow, so whatever causes the stack overflow runs after that check.

[petrochenkov]

Looks like a duplicate of #55779.