added tests

Author: Kivooeo

tests/ui/explain/error-with-no-explanation.rs

@@ -0,0 +1,3 @@
+// It's a valid error with no added explanation
+//@ compile-flags: --explain E9999
+//~? ERROR: E9999 is not a valid error code

tests/ui/explain/error-with-no-explanation.stderr

@@ -0,0 +1,2 @@
+error: E9999 is not a valid error code
+

tests/ui/explain/invalid-error-code.rs

@@ -1,4 +1,2 @@
-// Check that we don't crash on error codes exceeding our internal limit.
-// issue: <https://github.com/rust-lang/rust/issues/140647>
-//@ compile-flags: --explain E10000
-//~? ERROR: E10000 is not a valid error code
+//@ compile-flags: --explain error_code
+//~? ERROR: error_code is not a valid error code

tests/ui/explain/invalid-error-code.stderr

@@ -1,2 +1,2 @@
-error: E10000 is not a valid error code
+error: error_code is not a valid error code
 

tests/ui/explain/no-E-prefix.rs

@@ -0,0 +1,2 @@
+//@ compile-flags: --explain 425
+//@ check-pass

tests/ui/explain/no-E-prefix.stdout

@@ -0,0 +1,57 @@
+An unresolved name was used.
+
+Erroneous code examples:
+
+```
+something_that_doesnt_exist::foo;
+// error: unresolved name `something_that_doesnt_exist::foo`
+
+// or:
+
+trait Foo {
+    fn bar() {
+        Self; // error: unresolved name `Self`
+    }
+}
+
+// or:
+
+let x = unknown_variable;  // error: unresolved name `unknown_variable`
+```
+
+Please verify that the name wasn't misspelled and ensure that the
+identifier being referred to is valid for the given situation. Example:
+
+```
+enum something_that_does_exist {
+    Foo,
+}
+```
+
+Or:
+
+```
+mod something_that_does_exist {
+    pub static foo : i32 = 0i32;
+}
+
+something_that_does_exist::foo; // ok!
+```
+
+Or:
+
+```
+let unknown_variable = 12u32;
+let x = unknown_variable; // ok!
+```
+
+If the item is not defined in the current module, it must be imported using a
+`use` statement, like so:
+
+```
+use foo::bar;
+bar();
+```
+
+If the item you are importing is not defined in some super-module of the
+current module, then it must also be declared as public (e.g., `pub fn`).

tests/ui/explain/overflow-error-code.rs

@@ -0,0 +1,4 @@
+// Check that we don't crash on error codes exceeding our internal limit.
+// issue: <https://github.com/rust-lang/rust/issues/140647>
+//@ compile-flags: --explain E10000
+//~? ERROR: E10000 is not a valid error code

tests/ui/explain/overflow-error-code.stderr

@@ -0,0 +1,2 @@
+error: E10000 is not a valid error code
+