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librustdoc: wrong span in "the main function of this doctest won't be run" diagnostic #157371
Description
Code
/// This part creates the doctest with a stray `;`.
///
/// ```
/// #[derive(Debug, PartialEq)]
/// struct Type {
/// left: i32,
/// right: i32,
/// }; // <- Stray `;`.
///
/// fn main () {
/// let x = Type {
/// left: 10,
/// right: 20,
/// };
/// assert_eq!(
/// x,
/// Type {
/// left: 10,
/// right: 20,
/// },
/// );
/// }
/// ```
pub fn add(left: u64, right: u64) -> u64 {
// This code is completely unrelated to the doctest.
left + right
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn it_works() {
let result = add(2, 2);
assert_eq!(result, 4);
}
}Current output
$ cargo test
Finished `test` profile [unoptimized + debuginfo] target(s) in 0.04s
Running unittests src/lib.rs (target/debug/deps/doctest_main_bug-eb79733dddb28b66)
running 1 test
test tests::it_works ... ok
test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
Doc-tests doctest_main_bug
warning: the `main` function of this doctest won't be run as it contains expressions at the top level, meaning that the whole doctest code will be wrapped in a function
--> src/lib.rs:25:1
|
25 | // This code is completely unrelated to the doctest.
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
warning: 1 warning emitted
running 1 test
test src/lib.rs - add (line 3) ... ok
test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.01s
all doctests ran in 1.12s; merged doctests compilation took 0.72sDesired output
The span must highlight the semicolon in the doctest, not an unrelated comment.
8 | };
9 | ^Rationale and extra context
- At minimum the diagnostic should point to the rogue
;, not at an unrelated code below. - Better, an expression consisting of semicolon alone at the root of the doctest may benefit from a more specific diagnostic. It is clear that a semicolon by itself isn't doing anything, and in my opinion it is more likely that someone with a C++ background accidentally put it after a struct than meant it as a genuine expression.
Other cases
Rust Version
Reproducible on both stable and nightly.
$ rustc --version --verbose
rustc 1.96.0 (ac68faa20 2026-05-25)
binary: rustc
commit-hash: ac68faa20c58cbccd01ee7208bf3b6e93a7d7f96
commit-date: 2026-05-25
host: aarch64-apple-darwin
release: 1.96.0
LLVM version: 22.1.2
$ rustc +nightly --version --verbose
rustc 1.98.0-nightly (d595fce01 2026-06-02)
binary: rustc
commit-hash: d595fce01043347bf7f80e85b76dcc41b59a3e6e
commit-date: 2026-06-02
host: aarch64-apple-darwin
release: 1.98.0-nightly
LLVM version: 22.1.6Anything else?
This seems to be some sort of a span miscalculation issue. If I add more lines before or inside fn add, the error always points to the same line.
Source code of the error message:
rust/src/librustdoc/doctest/make.rs
Lines 647 to 649 in 53509ca
Activity
Rollup merge of rust-lang#157740 - pbkx:issue-157371-rustdoc-doctest-…
Rollup merge of rust-lang#157740 - pbkx:issue-157371-rustdoc-doctest-…
Rollup merge of rust-lang#157740 - pbkx:issue-157371-rustdoc-doctest-…