Add test for new attribute extern_html_root_url

Author: GuillaumeGomez

src/test/rustdoc-ui/extern_html_root_url.rs

@@ -0,0 +1,18 @@
+#![crate_type = "lib"]
+
+#![doc(extern_html_root_url)]
+//~^ ERROR
+#![doc(extern_html_root_url = "a")]
+//~^ ERROR
+#![doc(extern_html_root_url(a))]
+//~^ ERROR
+#![doc(extern_html_root_url(a()))]
+//~^ ERROR
+#![doc(extern_html_root_url(a = 1))]
+//~^ ERROR
+#![doc(extern_html_root_url(a = ""))]
+//~^ ERROR
+#![doc(extern_html_root_url(a = "1"))] // This one is supposed to work.
+#![doc(extern_html_root_url(b = "2", c = "3"))] // This one is supposed to work.
+
+pub fn foo() {}

src/test/rustdoc-ui/extern_html_root_url.stderr

@@ -0,0 +1,38 @@
+error: extern_html_root_url only accepts this form: `extern_html_root_url(crate = "url")`
+  --> $DIR/extern_html_root_url.rs:3:8
+   |
+LL | #![doc(extern_html_root_url)]
+   |        ^^^^^^^^^^^^^^^^^^^^
+
+error: extern_html_root_url only accepts this form: `extern_html_root_url(crate = "url")`
+  --> $DIR/extern_html_root_url.rs:5:8
+   |
+LL | #![doc(extern_html_root_url = "a")]
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: extern_html_root_url() only accepts `crate_name = "url"`
+  --> $DIR/extern_html_root_url.rs:7:29
+   |
+LL | #![doc(extern_html_root_url(a))]
+   |                             ^
+
+error: extern_html_root_url() only accepts `crate_name = "url"`
+  --> $DIR/extern_html_root_url.rs:9:29
+   |
+LL | #![doc(extern_html_root_url(a()))]
+   |                             ^^^
+
+error: extern_html_root_url() only accepts `crate_name = "url"`
+  --> $DIR/extern_html_root_url.rs:11:29
+   |
+LL | #![doc(extern_html_root_url(a = 1))]
+   |                             ^^^^^
+
+error: URL cannot be empty
+  --> $DIR/extern_html_root_url.rs:13:29
+   |
+LL | #![doc(extern_html_root_url(a = ""))]
+   |                             ^^^^^^
+
+error: aborting due to 6 previous errors
+

src/test/rustdoc/extern_html_root_url.rs

@@ -0,0 +1,6 @@
+#![crate_name = "foo"]
+#![doc(extern_html_root_url(std = "https://doc.rust-lang.org/nightly"))]
+
+// @has 'foo/index.html'
+// @has - '//a[@class="mod"]/@href' 'https://doc.rust-lang.org/nightly/std/index.html'
+pub use std as other;