Improve slug name error

src/test/ui-fulldeps/session-diagnostic/enforce_slug_naming.rs

@@ -0,0 +1,24 @@
+// rustc-env:CARGO_CRATE_NAME=rustc_dummy
+
+#![feature(rustc_private)]
+#![crate_type = "lib"]
+
+extern crate rustc_span;
+use rustc_span::symbol::Ident;
+use rustc_span::Span;
+
+extern crate rustc_macros;
+use rustc_macros::{Diagnostic, LintDiagnostic, Subdiagnostic};
+
+extern crate rustc_middle;
+use rustc_middle::ty::Ty;
+
+extern crate rustc_errors;
+use rustc_errors::{Applicability, MultiSpan};
+
+extern crate rustc_session;
+
+#[derive(Diagnostic)]
+#[diag(compiletest_example, code = "E0123")]
+//~^ ERROR diagnostic slug and crate name do not match
+struct Hello {}

src/test/ui-fulldeps/session-diagnostic/enforce_slug_naming.stderr

@@ -0,0 +1,11 @@
+error: diagnostic slug and crate name do not match
+  --> $DIR/enforce_slug_naming.rs:22:8
+   |
+LL | #[diag(compiletest_example, code = "E0123")]
+   |        ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: slug is `compiletest_example` but the crate name is `rustc_dummy`
+   = help: expected a slug starting with `dummy_...`
+
+error: aborting due to previous error
+