rustdoc lint missing_doc_code_examples not working on stable

[JesHansen]

I wanted to try out the missing_doc_code_examples rustdoc lint, but it seems to
be bugged.

I tried following the example given in the book.

Steps to reproduce:

Create a new crate:

cargo new testmissingdocs

Then edit the src/main.rs file:

#![deny(missing_doc_code_examples)]

//! Top level crate documentation

fn main() {
    println!("Hello, world!");
}

/// This function has no code examples.
pub fn add(i: i32, j: i32) -> i32 {
    i + j
}

I would now expect to get a linting error when building the crate, since the add
function is public, but does not have a code example. However, cargo build as
well as cargo doc and cargo test --doc completes without any errors.

[ehuss]

The lint only works on nightly, the docs should probably be updated.

[jyn514]

These docs live in https://github.com/rust-lang/rust/blob/master/src/doc/rustdoc/src/lints.md. To fix this, add 'and is nightly-only' to the first sentence.

[JesHansen]

I created PR #76212 to update the documentation. Thanks for clarifying. :-)