Add test for missing_docs lint on tuple fields

Author: GuillaumeGomez

src/test/rustdoc-ui/missing-doc-tuple-variant.rs

@@ -0,0 +1,15 @@
+// compile-flags: -Z unstable-options --check
+
+#![deny(missing_docs)]
+
+//! crate level doc
+
+/// Enum doc.
+pub enum Foo {
+    /// Variant doc.
+    Foo(String),
+    /// Variant Doc.
+    Bar(String, u32),
+    //~^ ERROR
+    //~^^ ERROR
+}

src/test/rustdoc-ui/missing-doc-tuple-variant.stderr

@@ -0,0 +1,20 @@
+error: missing documentation for a struct field
+  --> $DIR/missing-doc-tuple-variant.rs:12:9
+   |
+LL |     Bar(String, u32),
+   |         ^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/missing-doc-tuple-variant.rs:3:9
+   |
+LL | #![deny(missing_docs)]
+   |         ^^^^^^^^^^^^
+
+error: missing documentation for a struct field
+  --> $DIR/missing-doc-tuple-variant.rs:12:17
+   |
+LL |     Bar(String, u32),
+   |                 ^^^
+
+error: aborting due to 2 previous errors
+