Debug flag to bypass restriction of mutation in match guards.

Now, if you pass `-Z disable-ast-check-for-mutation-in-guard`, then we
will just allow you to mutably-borrow and assign in guards of `match`
arms.

This is wildly unsound with AST-borrowck. It is also unsound with
MIR-borrowck without further adjustments, which come in later in the
commit series on this Pull Request.

See also #24535 and rust-lang/rfcs#1006.

src/librustc/session/config.rs

@@ -1290,6 +1290,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         useful for profiling / PGO."),
     relro_level: Option<RelroLevel> = (None, parse_relro_level, [TRACKED],
         "choose which RELRO level to use"),
+    disable_ast_check_for_mutation_in_guard: bool = (false, parse_bool, [UNTRACKED],
+        "skip AST-based mutation-in-guard check (mir-borrowck provides more precise check)"),
     nll_subminimal_causes: bool = (false, parse_bool, [UNTRACKED],
         "when tracking region error causes, accept subminimal results for faster execution."),
     nll_facts: bool = (false, parse_bool, [UNTRACKED],

src/librustc/ty/context.rs

@@ -1344,6 +1344,12 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         self.on_disk_query_result_cache.serialize(self.global_tcx(), encoder)
     }
 
+    /// If true, we should use a naive AST walk to determine if match
+    /// guard could perform bad mutations (or mutable-borrows).
+    pub fn check_for_mutation_in_guard_via_ast_walk(self) -> bool {
+        !self.sess.opts.debugging_opts.disable_ast_check_for_mutation_in_guard
+    }
+
     /// If true, we should use the MIR-based borrowck (we may *also* use
     /// the AST-based borrowck).
     pub fn use_mir_borrowck(self) -> bool {

src/librustc_mir/hair/pattern/check_match.rs

@@ -181,7 +181,9 @@ impl<'a, 'tcx> MatchVisitor<'a, 'tcx> {
             // Second, if there is a guard on each arm, make sure it isn't
             // assigning or borrowing anything mutably.
             if let Some(ref guard) = arm.guard {
-                check_for_mutation_in_guard(self, &guard);
+                if self.tcx.check_for_mutation_in_guard_via_ast_walk() {
+                    check_for_mutation_in_guard(self, &guard);
+                }
             }
 
             // Third, perform some lints.