Marking a doctest code block as edition2018 causes it not to run

[DevJac]

I have a fresh Rust library, started with cargo new --lib foo, and I have the following in lib.rs:

/// Adds 1
/// ```edition2018
/// assert_eq!(1 + 1, 999)
/// ```
pub fn add_1(n: i32) -> i32 {
    n + 1
}

I can cargo build and cargo test this library. The doctest is obviously never run because 1 + 1 == 999 is false. The output from cargo test confirms this, showing that zero test were run.

The doctest book lead me to believe the doctest should still work, even if marked as edition2018.

Cargo.toml is:

[package]
name = "foo"
version = "0.1.0"
authors = ["..."]
edition = "2018"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]

cargo version: cargo 1.38.0 (23ef9a4ef 2019-08-20)
rustc version: rustc 1.38.0 (625451e37 2019-09-23)

[ehuss]

That's really strange! It works on nightly...any nightly!

I think it is due to this code:

rust/src/librustdoc/html/markdown.rs

Lines 683 to 686 in c553e8e

	x if allow_error_code_check && x.starts_with("edition") => {
	// allow_error_code_check is true if we're on nightly, which
	// is needed for edition support
	data.edition = x[7..].parse::<Edition>().ok();

Was this maybe forgotten to be stabilized when editions were stabilized?

[DevJac]

This still happens with Rust 1.39.0. I created a minimal project that demonstrates this bug at https://github.com/DevJac/rust_edition2018_doctest_bug_demo .

[ehuss]

I posted PR #66238 to fix this.