Auto merge of #46537 - pnkfelix:two-phase-borrows, r=arielb1

[MIR-borrowck] Two phase borrows

This adds limited support for two-phase borrows as described in
  http://smallcultfollowing.com/babysteps/blog/2017/03/01/nested-method-calls-via-two-phase-borrowing/

The support is off by default; you opt into it via the flag `-Z two-phase-borrows`

I have written "*limited* support" above because there are simple variants of the simple `v.push(v.len())` example that one would think should work but currently do not, such as the one documented in the test compile-fail/borrowck/two-phase-reservation-sharing-interference-2.rs

(To be clear, that test is not describing something that is unsound. It is just providing an explicit example of a limitation in the implementation given in this PR. I have ideas on how to fix, but I want to land the work that is in this PR first, so that I can stop repeatedly rebasing this branch.)

src/librustc/mir/mod.rs

@@ -1137,7 +1137,7 @@ impl<'tcx> Debug for Statement<'tcx> {
 
 /// A path to a value; something that can be evaluated without
 /// changing or disturbing program state.
-#[derive(Clone, PartialEq, RustcEncodable, RustcDecodable)]
+#[derive(Clone, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable)]
 pub enum Place<'tcx> {
     /// local variable
     Local(Local),
@@ -1151,7 +1151,7 @@ pub enum Place<'tcx> {
 
 /// The def-id of a static, along with its normalized type (which is
 /// stored to avoid requiring normalization when reading MIR).
-#[derive(Clone, PartialEq, RustcEncodable, RustcDecodable)]
+#[derive(Clone, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable)]
 pub struct Static<'tcx> {
     pub def_id: DefId,
     pub ty: Ty<'tcx>,

src/librustc/session/config.rs

@@ -1028,6 +1028,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "emit EndRegion as part of MIR; enable transforms that solely process EndRegion"),
     borrowck: Option<String> = (None, parse_opt_string, [UNTRACKED],
         "select which borrowck is used (`ast`, `mir`, or `compare`)"),
+    two_phase_borrows: bool = (false, parse_bool, [UNTRACKED],
+        "use two-phase reserved/active distinction for `&mut` borrows in MIR borrowck"),
     time_passes: bool = (false, parse_bool, [UNTRACKED],
         "measure time of each rustc pass"),
     count_llvm_insns: bool = (false, parse_bool,