rustdoc: set cfg(doctest) when collecting doctests

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

@@ -5,6 +5,8 @@
 // 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 - Foo (line 20) ... ok
+test $DIR/cfg-test.rs - Bar (line 28) ... 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`.