docs/test: add UI test and long-form error docs for E0461

compiler/rustc_error_codes/src/error_codes.rs

@@ -244,6 +244,7 @@ E0457: include_str!("./error_codes/E0457.md"),
 E0458: include_str!("./error_codes/E0458.md"),
 E0459: include_str!("./error_codes/E0459.md"),
 E0460: include_str!("./error_codes/E0460.md"),
+E0461: include_str!("./error_codes/E0461.md"),
 E0462: include_str!("./error_codes/E0462.md"),
 E0463: include_str!("./error_codes/E0463.md"),
 E0464: include_str!("./error_codes/E0464.md"),
@@ -595,7 +596,6 @@ E0791: include_str!("./error_codes/E0791.md"),
 //  E0421, // merged into 531
 //  E0427, // merged into 530
 //  E0456, // plugin `..` is not available for triple `..`
-    E0461, // couldn't find crate `..` with expected target triple ..
     E0465, // multiple .. candidates for `..` found
 //  E0467, // removed
 //  E0470, // removed

compiler/rustc_error_codes/src/error_codes/E0461.md

@@ -0,0 +1,30 @@
+Couldn't find crate `..` with expected target triple `..`.
+
+Example of erroneous code:
+
+`a.rs`
+```ignore (cannot-link-with-other-tests)
+#![crate_type = "lib"]
+
+fn foo() {}
+```
+
+`main.rs`
+```ignore (cannot-link-with-other-tests)
+extern crate a;
+
+fn main() {
+    a::foo();
+}
+```
+
+`a.rs` is then compiled with `--target powerpc-unknown-linux-gnu` and `b.rs`
+with `--target x86_64-unknown-linux-gnu`. `a.rs` is compiled into a binary
+format incompatible with `b.rs`; PowerPC and x86 are totally different
+architectures. This issue also extends to any difference in target triples, as
+`std` is operating-system specific.
+
+This error can be fixed by:
+ * Using [Cargo](../cargo/index.html), the Rust package manager, automatically
+   fixing this issue.
+ * Recompiling either crate so that they target a consistent target triple.