Auto merge of #35168 - scottcarr:deaggregation, r=nikomatsakis

[MIR] Deaggregate structs to enable further optimizations

Currently, we generate MIR like:

```
tmp0 = ...;
tmp1 = ...;
tmp3 = Foo { a: ..., b: ... };
```

This PR implements "deaggregation," i.e.:

```
tmp3.0 = ...
tmp3.1 = ...
```

Currently, the code only deaggregates structs, not enums.  My understanding is that we do not have MIR to set the discriminant of an enum.

1 files changed, 41 insertions, 0 deletions

diff --git a/src/test/mir-opt/deaggregator_test.rs b/src/test/mir-opt/deaggregator_test.rs
new file mode 100644
index 00000000000..e57a9674cf6
--- /dev/null
+++ b/src/test/mir-opt/deaggregator_test.rs
@@ -0,0 +1,41 @@
+// Copyright 2016 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.
+
+struct Baz {
+    x: usize,
+    y: f32,
+    z: bool,
+}
+
+fn bar(a: usize) -> Baz {
+    Baz { x: a, y: 0.0, z: false }
+}
+
+fn main() {}
+
+// END RUST SOURCE
+// START rustc.node13.Deaggregator.before.mir
+// bb0: {
+//     var0 = arg0;                     // scope 0 at main.rs:8:8: 8:9
+//     tmp0 = var0;                     // scope 1 at main.rs:9:14: 9:15
+//     return = Baz { x: tmp0, y: const F32(0), z: const false }; // scope ...
+//     goto -> bb1;                     // scope 1 at main.rs:8:1: 10:2
+// }
+// END rustc.node13.Deaggregator.before.mir
+// START rustc.node13.Deaggregator.after.mir
+// bb0: {
+//     var0 = arg0;                     // scope 0 at main.rs:8:8: 8:9
+//     tmp0 = var0;                     // scope 1 at main.rs:9:14: 9:15
+//     (return.0: usize) = tmp0;        // scope 1 at main.rs:9:5: 9:34
+//     (return.1: f32) = const F32(0);  // scope 1 at main.rs:9:5: 9:34
+//     (return.2: bool) = const false;  // scope 1 at main.rs:9:5: 9:34
+//     goto -> bb1;                     // scope 1 at main.rs:8:1: 10:2
+// }
+// END rustc.node13.Deaggregator.after.mir
\ No newline at end of file