When documenting private items in a binary, ignore warnings about links to private items

Previously, rustdoc would warn about linking to items in a binary, even
though cargo unconditionally documents private items in a binary.
This changes cargo to silence the warning, since it's only relevant in
cases where the private items might not be documented.

src/cargo/ops/cargo_compile.rs

@@ -646,6 +646,12 @@ pub fn create_bcx<'a, 'cfg>(
             if rustdoc_document_private_items || unit.target.is_bin() {
                 let mut args = extra_args.take().unwrap_or_default();
                 args.push("--document-private-items".into());
+                if unit.target.is_bin() {
+                    // This warning only makes sense if it's possible to document private items
+                    // sometimes and ignore them at other times. But cargo consistently passes
+                    // `--document-private-items`, so the warning isn't useful.
+                    args.push("-Arustdoc::private-intra-doc-links".into());
+                }
                 extra_args = Some(args);
             }
 

tests/testsuite/doc.rs

@@ -2767,3 +2767,24 @@ fn doc_check_cfg_features() {
         )
         .run();
 }
+
+#[cargo_test]
+fn link_to_private_item() {
+    let main = r#"
+    //! [bar]
+    #[allow(dead_code)]
+    fn bar() {}
+    "#;
+    let p = project().file("src/lib.rs", main).build();
+    p.cargo("doc")
+        .with_stderr_contains("[..] documentation for `foo` links to private item `bar`")
+        .run();
+    // Check that binaries don't emit a private_intra_doc_links warning.
+    fs::rename(p.root().join("src/lib.rs"), p.root().join("src/main.rs")).unwrap();
+    p.cargo("doc")
+        .with_stderr(
+            "[DOCUMENTING] foo [..]\n\
+             [FINISHED] [..]",
+        )
+        .run();
+}