Rollup merge of #62213 - QuietMisdreavus:cfg-doctest, r=GuillaumeGomez

rustdoc: set cfg(doctest) when collecting doctests

Note: This PR builds on top of https://github.com/rust-lang/rust/pull/61199; only the last commit is specific to this PR.

As discussed in https://github.com/rust-lang/rust/pull/61199, we want the ability to isolate items to only when rustdoc is collecting doctests, but we can't use `cfg(test)` because of libcore's `#![cfg(not(test))]`. This PR proposes a new cfg flag, `cfg(doctest)`, specific to this situation, rather than reusing an existing flag. I've isolated it behind a feature gate so that we can contain the effects to nightly only. (A stable workaround that can be used in lieu of `#[cfg(doctest)]` is `#[cfg(rustdoc)] #[doc(hidden)]`, at least once https://github.com/rust-lang/rust/pull/61351 lands.)

Tracking issue: https://github.com/rust-lang/rust/issues/62210

src/test/rustdoc-ui/cfg-test.rs

@@ -1,10 +1,12 @@
 // build-pass (FIXME(62277): could be check-pass?)
-// compile-flags:--test
+// compile-flags:--test --test-args --test-threads=1
 // normalize-stdout-test: "src/test/rustdoc-ui" -> "$$DIR"
 
 // Crates like core have doctests gated on `cfg(not(test))` so we need to make
 // sure `cfg(test)` is not active when running `rustdoc --test`.
 
+#![feature(cfg_doctest)]
+
 /// this doctest will be ignored:
 ///
 /// ```
@@ -20,3 +22,11 @@ pub struct Foo;
 /// ```
 #[cfg(not(test))]
 pub struct Foo;
+
+/// this doctest will be tested, but will not appear in documentation:
+///
+/// ```
+/// assert!(true)
+/// ```
+#[cfg(doctest)]
+pub struct Bar;

src/test/rustdoc-ui/cfg-test.stdout

@@ -1,6 +1,7 @@
 
-running 1 test
-test $DIR/cfg-test.rs - Foo (line 18) ... ok
+running 2 tests
+test $DIR/cfg-test.rs - Bar (line 28) ... ok
+test $DIR/cfg-test.rs - Foo (line 20) ... ok
 
-test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
+test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
 

src/test/rustdoc/cfg-doctest.rs

@@ -0,0 +1,8 @@
+#![feature(cfg_doctest)]
+
+// @!has cfg_doctest/struct.SomeStruct.html
+// @!has cfg_doctest/index.html '//a/@href' 'struct.SomeStruct.html'
+
+/// Sneaky, this isn't actually part of docs.
+#[cfg(doctest)]
+pub struct SomeStruct;

src/test/ui/feature-gate/feature-gate-cfg_doctest.rs

@@ -0,0 +1,4 @@
+#[cfg(doctest)] //~ ERROR
+pub struct SomeStruct;
+
+fn main() {}

src/test/ui/feature-gate/feature-gate-cfg_doctest.stderr

@@ -0,0 +1,12 @@
+error[E0658]: `cfg(doctest)` is experimental and subject to change
+  --> $DIR/feature-gate-cfg_doctest.rs:1:7
+   |
+LL | #[cfg(doctest)]
+   |       ^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/62210
+   = help: add #![feature(cfg_doctest)] to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.