turn rustc_box into an intrinsic

compiler/rustc_mir_build/src/errors.rs

@@ -1067,25 +1067,6 @@ pub(crate) enum MiscPatternSuggestion {
     },
 }
 
-#[derive(Diagnostic)]
-#[diag(mir_build_rustc_box_attribute_error)]
-pub(crate) struct RustcBoxAttributeError {
-    #[primary_span]
-    pub(crate) span: Span,
-    #[subdiagnostic]
-    pub(crate) reason: RustcBoxAttrReason,
-}
-
-#[derive(Subdiagnostic)]
-pub(crate) enum RustcBoxAttrReason {
-    #[note(mir_build_attributes)]
-    Attributes,
-    #[note(mir_build_not_box)]
-    NotBoxNew,
-    #[note(mir_build_missing_box)]
-    MissingBox,
-}
-
 #[derive(LintDiagnostic)]
 #[diag(mir_build_rust_2024_incompatible_pat)]
 pub(crate) struct Rust2024IncompatiblePat<'a> {