Auto merge of #44442 - Aaron1011:promote-static-ref, r=eddyb

Fix regression in promotion of rvalues referencing a static

This commit makes librustc_passes::consts::CheckCrateVisitor properly
mark expressions as promotable if they reference a static, as it's
perfectly fine for one static to reference another. It fixes a
regression that prevented a temporary rvalue from referencing a static
if it was itself declared within a static.

Prior to commit b8c05fe90b,
`region::ScopeTree` would only register a 'terminating scope' for function
bodies. Thus, while rvalues in a static that referenced a static would be marked
unpromotable, the lack of enclosing scope would cause
mem_categorization::MemCategorizationContext::cat_rvalue_node
to compute a 'temporary scope' of `ReStatic`. Since this had the same
effect as explicitly selecting a scope of `ReStatic`
due to the rvalue being marked by CheckCrateVisitor as promotable,
no issue occurred.

However, commit b8c05fe90b
made ScopeTree unconditionally register a 'terminating scope'
Since mem_categorization would now compute a non-static 'temporary scope', the
aforementioned rvalues would be erroneously marked as living for too
short a time.

By fixing the behavior of CheckCrateVisitor, this commit avoids changing
mem_categorization's behavior, while ensuring that temporary values in
statics are still allowed to reference other statics.

Fixes issue #44373

src/test/compile-fail/issue-44373.rs

@@ -0,0 +1,16 @@
+// 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.
+
+
+static FOO: u32 = 50;
+
+fn main() {
+    let _val: &'static [&'static u32] = &[&FOO]; //~ ERROR borrowed value does not live long enough
+}

src/test/run-pass/issue-44373.rs

@@ -0,0 +1,18 @@
+// 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.
+
+struct Foo(bool);
+
+struct Container(&'static [&'static Foo]);
+
+static FOO: Foo = Foo(true);
+static CONTAINER: Container = Container(&[&FOO]);
+
+fn main() {}