Rollup merge of rust-lang#59940 - GuillaumeGomez:rustdoc-test, r=ollie27

Set cfg(test) when rustdoc is running with --test option

Following a [discussion on twitter](https://twitter.com/burntsushi5/status/1117091914199785473), I proposed this change. What do you think about it?

r? @QuietMisdreavus

cc @BurntSushi

src/librustdoc/config.rs

@@ -351,6 +351,9 @@ impl Options {
                             .unwrap_or_else(|| PathBuf::from("doc"));
         let mut cfgs = matches.opt_strs("cfg");
         cfgs.push("rustdoc".to_string());
+        if should_test {
+            cfgs.push("test".to_string());
+        }
 
         let extension_css = matches.opt_str("e").map(|s| PathBuf::from(&s));
 

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

@@ -0,0 +1,19 @@
+// compile-pass
+// compile-flags:--test
+// normalize-stdout-test: "src/test/rustdoc-ui" -> "$$DIR"
+
+/// this doctest will be ignored:
+///
+/// ```
+/// assert!(false);
+/// ```
+#[cfg(not(test))]
+pub struct Foo;
+
+/// this doctest will be tested:
+///
+/// ```
+/// assert!(true);
+/// ```
+#[cfg(test)]
+pub struct Foo;

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

@@ -0,0 +1,6 @@
+
+running 1 test
+test $DIR/cfg-test.rs - Foo (line 15) ... ok
+
+test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
+