Add long explanation for E0464

The test is copied from `src/test/ui/crate-loading/crateresolve1.rs` and
its auxiliary tests. I added it to the `compile_fail` code example check
exemption list since it's hard if not impossible to reproduce this error
in a standalone code example.

Author: camelid

compiler/rustc_error_codes/src/error_codes.rs

@@ -237,6 +237,7 @@ E0455: include_str!("./error_codes/E0455.md"),
 E0458: include_str!("./error_codes/E0458.md"),
 E0459: include_str!("./error_codes/E0459.md"),
 E0463: include_str!("./error_codes/E0463.md"),
+E0464: include_str!("./error_codes/E0464.md"),
 E0466: include_str!("./error_codes/E0466.md"),
 E0468: include_str!("./error_codes/E0468.md"),
 E0469: include_str!("./error_codes/E0469.md"),
@@ -586,7 +587,6 @@ E0785: include_str!("./error_codes/E0785.md"),
     E0460, // found possibly newer version of crate `..`
     E0461, // couldn't find crate `..` with expected target triple ..
     E0462, // found staticlib `..` instead of rlib or dylib
-    E0464, // multiple matching crates for `..`
     E0465, // multiple .. candidates for `..` found
 //  E0467, removed
 //  E0470, removed

compiler/rustc_error_codes/src/error_codes/E0464.md

@@ -0,0 +1,6 @@
+The compiler found multiple library files with the requested crate name.
+
+This error can occur in several different cases -- for example, when using
+`extern crate` or passing `--extern` options without crate paths. It can also be
+caused by caching issues with the build directory, in which case `cargo clean`
+may help.

src/test/ui/crate-loading/crateresolve1.stderr

@@ -11,3 +11,4 @@ LL | extern crate crateresolve1;
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0464`.

src/test/ui/crate-loading/crateresolve2.stderr

@@ -11,3 +11,4 @@ LL | extern crate crateresolve2;
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0464`.

src/test/ui/error-codes/E0464.rs

@@ -0,0 +1,12 @@
+// aux-build:crateresolve1-1.rs
+// aux-build:crateresolve1-2.rs
+// aux-build:crateresolve1-3.rs
+
+// normalize-stderr-test: "\.so" -> ".dylib"
+// normalize-stderr-test: "\.dll" -> ".dylib"
+
+extern crate crateresolve1;
+//~^ ERROR multiple matching crates for `crateresolve1`
+
+fn main() {
+}

src/test/ui/error-codes/E0464.stderr

@@ -0,0 +1,14 @@
+error[E0464]: multiple matching crates for `crateresolve1`
+  --> $DIR/E0464.rs:8:1
+   |
+LL | extern crate crateresolve1;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: candidates:
+           crate `crateresolve1`: $TEST_BUILD_DIR/error-codes/E0464/auxiliary/libcrateresolve1-1.dylib
+           crate `crateresolve1`: $TEST_BUILD_DIR/error-codes/E0464/auxiliary/libcrateresolve1-2.dylib
+           crate `crateresolve1`: $TEST_BUILD_DIR/error-codes/E0464/auxiliary/libcrateresolve1-3.dylib
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0464`.

src/test/ui/error-codes/auxiliary/crateresolve1-1.rs

@@ -0,0 +1,5 @@
+// compile-flags:-C extra-filename=-1
+#![crate_name = "crateresolve1"]
+#![crate_type = "lib"]
+
+pub fn f() -> isize { 10 }

src/test/ui/error-codes/auxiliary/crateresolve1-2.rs

@@ -0,0 +1,5 @@
+// compile-flags:-C extra-filename=-2
+#![crate_name = "crateresolve1"]
+#![crate_type = "lib"]
+
+pub fn f() -> isize { 20 }

src/test/ui/error-codes/auxiliary/crateresolve1-3.rs

@@ -0,0 +1,5 @@
+// compile-flags:-C extra-filename=-3
+#![crate_name = "crateresolve1"]
+#![crate_type = "lib"]
+
+pub fn f() -> isize { 30 }

src/tools/tidy/src/error_codes_check.rs

@@ -15,7 +15,7 @@ const EXEMPTED_FROM_TEST: &[&str] = &[
 ];
 
 // Some error codes don't have any tests apparently...
-const IGNORE_EXPLANATION_CHECK: &[&str] = &["E0570", "E0601", "E0602", "E0729"];
+const IGNORE_EXPLANATION_CHECK: &[&str] = &["E0464", "E0570", "E0601", "E0602", "E0729"];
 
 // If the file path contains any of these, we don't want to try to extract error codes from it.
 //