docs/test: add UI test and long-form error docs for E0519

compiler/rustc_error_codes/src/error_codes.rs

@@ -280,6 +280,7 @@ E0515: include_str!("./error_codes/E0515.md"),
 E0516: include_str!("./error_codes/E0516.md"),
 E0517: include_str!("./error_codes/E0517.md"),
 E0518: include_str!("./error_codes/E0518.md"),
+E0519: include_str!("./error_codes/E0519.md"),
 E0520: include_str!("./error_codes/E0520.md"),
 E0521: include_str!("./error_codes/E0521.md"),
 E0522: include_str!("./error_codes/E0522.md"),
@@ -616,7 +617,6 @@ E0791: include_str!("./error_codes/E0791.md"),
 //  E0489, // type/lifetime parameter not in scope here
     E0490, // a value of type `..` is borrowed for too long
     E0514, // metadata version mismatch
-    E0519, // local crate and dependency have same (crate-name, disambiguator)
     E0523, // two dependencies have same (crate-name, disambiguator) but different SVH
 //  E0526, // shuffle indices are not constant
 //  E0540, // multiple rustc_deprecated attributes

compiler/rustc_error_codes/src/error_codes/E0519.md

@@ -0,0 +1,40 @@
+The current crate is indistinguishable from one of its dependencies, in terms
+of metadata.
+
+Example of erroneous code:
+
+`a.rs`
+```ignore (cannot-link-with-other-tests)
+#![crate_name = "a"]
+#![crate_type = "lib"]
+
+pub fn foo() {}
+```
+
+`b.rs`
+```ignore (cannot-link-with-other-tests)
+#![crate_name = "a"]
+#![crate_type = "lib"]
+
+// error: the current crate is indistinguishable from one of its dependencies:
+//        it has the same crate-name `a` and was compiled with the same
+//        `-C metadata` arguments. This will result in symbol conflicts between
+//        the two.
+extern crate a;
+
+pub fn foo() {}
+
+fn bar() {
+    a::foo(); // is this calling the local crate or the dependency?
+}
+```
+
+The above example compiles two crates with exactly the same name and
+`crate_type` (plus any other metadata). This causes an error because it becomes
+impossible for the compiler to distinguish between symbols (`pub` item names).
+
+This error can be fixed by:
+ * Using [Cargo](../cargo/index.html), the Rust package manager, automatically
+   fixing this issue.
+ * Recompiling the crate with different metadata (different name/
+   `crate_type`).