Rustdoc shouldn't lint about missing code examples in derived traits

[toku-sa-n]

I tried this code:

#[deny(missing_doc_code_examples)]
/// Foo struct.
///
/// # Examples
///
/// ```
/// let f = Foo;
/// ```
#[derive(Copy, Clone, Debug, Default, Ord, PartialOrd, Eq, PartialEq, Hash)]
pub struct Foo;

I expected to see this happen: cargo doc succeeds.

Instead, this happened:

 Documenting foo v0.1.0 (/tmp/tmp.uwQktSx1yP/foo)
error: missing code example in this documentation
 --> src/lib.rs:9:16
  |
9 | #[derive(Copy, Clone, Debug, Default, Ord, PartialOrd, Eq, PartialEq, Hash)]
  |                ^^^^^
  |
note: the lint level is defined here
 --> src/lib.rs:1:8
  |
1 | #[deny(missing_doc_code_examples)]
  |        ^^^^^^^^^^^^^^^^^^^^^^^^^

error: missing code example in this documentation
 --> src/lib.rs:9:16
  |
9 | #[derive(Copy, Clone, Debug, Default, Ord, PartialOrd, Eq, PartialEq, Hash)]
  |                ^^^^^

error: missing code example in this documentation
 --> src/lib.rs:9:10
  |
9 | #[derive(Copy, Clone, Debug, Default, Ord, PartialOrd, Eq, PartialEq, Hash)]
  |          ^^^^

error: missing code example in this documentation
 --> src/lib.rs:9:30
  |
9 | #[derive(Copy, Clone, Debug, Default, Ord, PartialOrd, Eq, PartialEq, Hash)]
  |                              ^^^^^^^

error: missing code example in this documentation
 --> src/lib.rs:9:56
  |
9 | #[derive(Copy, Clone, Debug, Default, Ord, PartialOrd, Eq, PartialEq, Hash)]
  |                                                        ^^

error: missing code example in this documentation
 --> src/lib.rs:9:39
  |
9 | #[derive(Copy, Clone, Debug, Default, Ord, PartialOrd, Eq, PartialEq, Hash)]
  |                                       ^^^

error: missing code example in this documentation
 --> src/lib.rs:9:60
  |
9 | #[derive(Copy, Clone, Debug, Default, Ord, PartialOrd, Eq, PartialEq, Hash)]
  |                                                            ^^^^^^^^^

error: missing code example in this documentation
 --> src/lib.rs:9:44
  |
9 | #[derive(Copy, Clone, Debug, Default, Ord, PartialOrd, Eq, PartialEq, Hash)]
  |                                            ^^^^^^^^^^

error: missing code example in this documentation
 --> src/lib.rs:9:23
  |
9 | #[derive(Copy, Clone, Debug, Default, Ord, PartialOrd, Eq, PartialEq, Hash)]
  |                       ^^^^^

error: missing code example in this documentation
 --> src/lib.rs:9:71
  |
9 | #[derive(Copy, Clone, Debug, Default, Ord, PartialOrd, Eq, PartialEq, Hash)]
  |                                                                       ^^^^

error: aborting due to 10 previous errors

error: could not document `foo`

Caused by:
  process didn't exit successfully: `rustdoc --edition=2018 --crate-type lib --crate-name foo src/lib.rs -o /tmp/tmp.uwQktSx1yP/foo/target/doc --error-format=json --json=diagnostic-rendered-ansi -L dependency=/tmp/tmp.uwQktSx1yP/foo/target/debug/deps --crate-version 0.1.0` (exit code: 1)

Meta

rustc --version --verbose:

rustc 1.51.0-nightly (04caa632d 2021-01-30)
binary: rustc
commit-hash: 04caa632dd10c2bf64b69524c7f9c4c30a436877
commit-date: 2021-01-30
host: x86_64-unknown-linux-gnu
release: 1.51.0-nightly
LLVM version: 11.0.1

[hellow554]

searched nightlies: from nightly-2020-09-30 to nightly-2020-11-30
regressed nightly: nightly-2020-11-12
searched commits: from cf9cf7c to 5404efc
regressed commit: 38030ff

bisected with cargo-bisect-rustc v0.6.0

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc 2020-09-30 --end=2020-11-30 --script=./test.sh --access=github

so probably #78908?

[poliorcetics]

What is the correct option here ? Ignoring derived traits or putting default examples on the derive for those types ?

The first seems hard to do but I'm maybe missing knowledge to see it done.

The second means the examples won't be very relevant to the type on which they are derived. For Clone this is not much of a problem, but for thing like serve::Serialize I can see it confusing people.

[jyn514]

I think rustdoc shouldn't require examples for derived traits.

rust/src/librustdoc/passes/doc_test_lints.rs

Lines 77 to 80 in f9435f4

	let hir_id = cx.tcx.hir().local_def_id_to_hir_id(item.def_id.expect_local());
	let (level, source) =
	cx.tcx.lint_level_at_node(lint::builtin::MISSING_DOC_CODE_EXAMPLES, hir_id);
	level != lint::Level::Allow || matches!(source, LintLevelSource::Default)

A possible fix is to only call source_callsite() in render, and use the macro span everywhere else for the purpose of lints.