Try all stable candidates first before trying unstable ones

compiler/rustc_session/src/options.rs

@@ -1233,6 +1233,8 @@ options! {
         and set the maximum total size of a const allocation for which this is allowed (default: never)"),
     perf_stats: bool = (false, parse_bool, [UNTRACKED],
         "print some performance-related statistics (default: no)"),
+    pick_stable_methods_before_any_unstable: bool = (true, parse_bool, [TRACKED],
+        "try to pick stable methods first before picking any unstable methods (default: yes)"),
     plt: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "whether to use the PLT when calling into shared libraries;
         only has effect for PIC code on systems with ELF binaries