options: Add -frust-edition flag and possible values

Co-authored-by: liushuyu <liushuyu011@gmail.com>

gcc/rust/lang.opt

@@ -83,6 +83,22 @@ frust-cfg=
 Rust Joined RejectNegative
 -frust-cfg=<name>             Set a config expansion option
 
+frust-edition=
+Rust Joined RejectNegative Enum(frust_edition) Var(flag_rust_edition)
+-frust-edition=[2015|2018|2021]             Choose which edition to use when compiling rust code
+
+Enum
+Name(frust_edition) Type(int) UnknownError(unknown rust edition %qs)
+
+EnumValue
+Enum(frust_edition) String(2015) Value(0)
+
+EnumValue
+Enum(frust_edition) String(2018) Value(1)
+
+EnumValue
+Enum(frust_edition) String(2021) Value(2)
+
 o
 Rust Joined Separate
 ; Documented in common.opt

gcc/rust/rust-session-manager.cc

@@ -373,6 +373,10 @@ Session::handle_option (
 	break;
       }
 
+    case OPT_frust_edition_:
+      options.set_edition (flag_rust_edition);
+      break;
+
     default:
       break;
     }

gcc/rust/rust-session-manager.h

@@ -184,6 +184,13 @@ struct CompileOptions
   bool enable_test = false;
   bool debug_assertions = false;
   bool proc_macro = false;
+  enum Edition
+  {
+    E2015 = 0,
+    E2018,
+    E2021,
+  } edition
+    = E2015;
 
   bool dump_option_enabled (DumpOption option) const
   {
@@ -211,6 +218,11 @@ struct CompileOptions
     crate_name = std::move (name);
     return true;
   }
+
+  void set_edition (int raw_edition)
+  {
+    edition = static_cast<Edition> (raw_edition);
+  }
 };
 
 /* Defines a compiler session. This is for a single compiler invocation, so