Auto merge of #51833 - wesleywiser:faster_large_constant_arrays, r=oli-obk

Speed up compilation of large constant arrays

This is a different approach to #51672 as suggested by @oli-obk. Rather
than write each repeated value one-by-one, we write the first one and
then copy its value directly into the remaining memory.

With this change, the [toy program](c2f4744d2d/src/test/run-pass/mir_heavy_promoted.rs) goes from 63 seconds to 19 seconds on my machine.

Edit: Inlining `Size::bytes()` saves an additional 6 seconds dropping the total time to 13 seconds on my machine.

Edit2: Now down to 2.8 seconds.

r? @oli-obk

cc @nnethercote @eddyb

src/test/compile-fail/const-err4.rs

@@ -0,0 +1,24 @@
+// Copyright 2018 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.
+
+#[derive(Copy, Clone)]
+union Foo {
+    a: isize,
+    b: (),
+}
+
+enum Bar {
+    Boo = [unsafe { Foo { b: () }.a }; 4][3],
+    //~^ ERROR constant evaluation of enum discriminant resulted in non-integer
+}
+
+fn main() {
+    assert_ne!(Bar::Boo as isize, 0);
+}

src/test/run-pass/const-repeated-values.rs

@@ -0,0 +1,19 @@
+// Copyright 2018 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.
+
+const FOO: isize = 42;
+
+enum Bar {
+    Boo = *[&FOO; 4][3],
+}
+
+fn main() {
+    assert_eq!(Bar::Boo as isize, 42);
+}