Test case illustrating some destruction code extent stuff.

src/test/mir-opt/end_region_destruction_extents_1.rs

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+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags: -Z identify_regions -Z span_free_formats -Z emit-end-regions
+// ignore-tidy-linelength
+
+// A scenario with significant destruction code extents (which have
+// suffix "dce" in current `-Z identify_regions` rendering).
+
+#![feature(generic_param_attrs)]
+#![feature(dropck_eyepatch)]
+
+fn main() {
+    // Since the second param to `D1` is may_dangle, it is legal for
+    // the region of that parameter to end before the drop code for D1
+    // is executed.
+    (D1(&S1("ex1"), &S1("dang1"))).0;
+}
+
+#[derive(Debug)]
+struct S1(&'static str);
+
+#[derive(Debug)]
+struct D1<'a, 'b>(&'a S1, &'b S1);
+
+// The `#[may_dangle]` means that references of type `&'b _` may be
+// invalid during the execution of this destructor; i.e. in this case
+// the destructor code is not allowed to read or write `*self.1`, while
+// it can read/write `*self.0`.
+unsafe impl<'a, #[may_dangle] 'b> Drop for D1<'a, 'b> {
+    fn drop(&mut self) {
+        println!("D1({:?}, _)", self.0);
+    }
+}
+
+// Notes on the MIR output below:
+//
+// 1. The `EndRegion('10s)` is allowed to precede the `drop(_3)`
+//    solely because of the #[may_dangle] mentioned above.
+//
+// 2. Regarding the occurrence of `EndRegion('12ds)` *after* `StorageDead(_6)`
+//    (where we have borrows `&'12ds _6`): Eventually:
+//
+//    i. this code should be rejected (by mir-borrowck), or
+//
+//    ii. the MIR code generation should be changed so that the
+//        EndRegion('12ds)` precedes `StorageDead(_6)` in the
+//        control-flow.  (Note: arielb1 views drop+storagedead as one
+//        unit, and does not see this option as a useful avenue to
+//        explore.), or
+//
+//    iii. the presence of EndRegion should be made irrelevant by a
+//        transformation encoding the effects of rvalue-promotion.
+//        This may be the simplest and most-likely option; note in
+//        particular that `StorageDead(_6)` goes away below in
+//        rustc.node4.QualifyAndPromoteConstants.after.mir
+
+// END RUST SOURCE
+
+// START rustc.node4.QualifyAndPromoteConstants.before.mir
+// fn main() -> () {
+//     let mut _0: ();
+//     let mut _1: &'12ds S1;
+//     let mut _2: &'12ds S1;
+//     let mut _3: D1<'12ds, '10s>;
+//     let mut _4: &'12ds S1;
+//     let mut _5: &'12ds S1;
+//     let mut _6: S1;
+//     let mut _7: &'10s S1;
+//     let mut _8: &'10s S1;
+//     let mut _9: S1;
+//
+//     bb0: {
+//         StorageLive(_2);
+//         StorageLive(_3);
+//         StorageLive(_4);
+//         StorageLive(_5);
+//         StorageLive(_6);
+//         _6 = S1::{{constructor}}(const "ex1",);
+//         _5 = &'12ds _6;
+//         _4 = &'12ds (*_5);
+//         StorageLive(_7);
+//         StorageLive(_8);
+//         StorageLive(_9);
+//         _9 = S1::{{constructor}}(const "dang1",);
+//         _8 = &'10s _9;
+//         _7 = &'10s (*_8);
+//         _3 = D1<'12ds, '10s>::{{constructor}}(_4, _7);
+//         EndRegion('10s);
+//         StorageDead(_7);
+//         StorageDead(_4);
+//         _2 = (_3.0: &'12ds S1);
+//         _1 = _2;
+//         StorageDead(_2);
+//         drop(_3) -> bb1;
+//     }
+//
+//     bb1: {
+//         StorageDead(_3);
+//         StorageDead(_8);
+//         StorageDead(_9);
+//         StorageDead(_5);
+//         StorageDead(_6);
+//         EndRegion('12ds);
+//         _0 = ();
+//         return;
+//     }
+// }
+// END rustc.node4.QualifyAndPromoteConstants.before.mir
+
+// START rustc.node4.QualifyAndPromoteConstants.after.mir
+// fn main() -> () {
+//     let mut _0: ();
+//     let mut _1: &'12ds S1;
+//     let mut _2: &'12ds S1;
+//     let mut _3: D1<'12ds, '10s>;
+//     let mut _4: &'12ds S1;
+//     let mut _5: &'12ds S1;
+//     let mut _6: S1;
+//     let mut _7: &'10s S1;
+//     let mut _8: &'10s S1;
+//     let mut _9: S1;
+//
+//     bb0: {
+//         StorageLive(_2);
+//         StorageLive(_3);
+//         StorageLive(_4);
+//         StorageLive(_5);
+//         _5 = promoted1;
+//         _4 = &'12ds (*_5);
+//         StorageLive(_7);
+//         StorageLive(_8);
+//         _8 = promoted0;
+//         _7 = &'10s (*_8);
+//         _3 = D1<'12ds, '10s>::{{constructor}}(_4, _7);
+//         EndRegion('10s);
+//         StorageDead(_7);
+//         StorageDead(_4);
+//         _2 = (_3.0: &'12ds S1);
+//         _1 = _2;
+//         StorageDead(_2);
+//         drop(_3) -> bb1;
+//     }
+//
+//     bb1: {
+//         StorageDead(_3);
+//         StorageDead(_8);
+//         StorageDead(_5);
+//         EndRegion('12ds);
+//         _0 = ();
+//         return;
+//     }
+// }
+// END rustc.node4.QualifyAndPromoteConstants.after.mir