Add test for line-number setting

src/ci/docker/host-x86_64/x86_64-gnu-tools/Dockerfile

@@ -72,7 +72,7 @@ ENV PATH="/node-v14.4.0-linux-x64/bin:${PATH}"
 # https://github.com/puppeteer/puppeteer/issues/375
 #
 # We also specify the version in case we need to update it to go around cache limitations.
-RUN npm install -g browser-ui-test@0.4.3 --unsafe-perm=true
+RUN npm install -g browser-ui-test@0.4.5 --unsafe-perm=true
 
 ENV RUST_CONFIGURE_ARGS \
   --build=x86_64-unknown-linux-gnu \

src/test/rustdoc-gui/docblock-code-block-line-number.goml

@@ -0,0 +1,22 @@
+// Checks that the setting "line numbers" is working as expected.
+goto: file://|DOC_PATH|/test_docs/fn.foo.html
+
+// We check that without this setting, there is no line number displayed.
+assert-false: "pre.line-number"
+
+// We now set the setting to show the line numbers on code examples.
+local-storage: {"rustdoc-line-numbers": "true" }
+// We reload to make the line numbers appear.
+reload:
+
+// We wait for them to be added into the DOM by the JS...
+wait-for: "pre.line-number"
+// If the test didn't fail, it means that it was found!
+// Let's now check some CSS properties...
+assert-css: ("pre.line-number", {
+    "margin": "0px",
+    "padding": "13px 8px",
+    "text-align": "right"
+})
+// The first code block has two lines so let's check its `<pre>` elements lists both of them.
+assert-text: ("pre.line-number", "1\n2")

src/test/rustdoc-gui/src/test_docs/lib.rs

@@ -12,6 +12,7 @@ use std::fmt;
 ///
 /// ```
 /// println!("nothing fancy");
+/// println!("but with two lines!");
 /// ```
 ///
 /// A failing to compile one: