add OR_PATTERNS_BACK_COMPAT lint

test: add more cases

test: add comments

refine msg

compiler/rustc_lint_defs/src/lib.rs

@@ -267,6 +267,7 @@ pub enum BuiltinLintDiagnostics {
     LegacyDeriveHelpers(Span),
     ExternDepSpec(String, ExternDepSpec),
     ProcMacroBackCompat(String),
+    OrPatternsBackCompat(Span, String),
 }
 
 /// Lints that are buffered up early on in the `Session` before the