Auto merge of #74945 - dingxiangfei2009:promote-static-ref-deref, r=oli-obk

[mir] Special treatment for dereferencing a borrow to a static definition

Fix #70584.

As suggested by @oli-obk in this [comment](https://github.com/rust-lang/rust/issues/70584#issuecomment-626009260), one can chase the definition of the local variable being de-referenced and check if it is a true static variable. If that is the case, `validate_place` will admit the promotion.

This is my first time to contribute to `rustc`, and I have two questions.
1. A generalization to some extent is applied to decide if the promotion is possible in the static context. In case that there are more projection operations preceding the de-referencing, `validate_place` recursively decent into inner projection operations. I have put thoughts into its correctness but I am not totally sure about it.
2. I have a hard time to find a good place for the test case. This patch has to do with MIR, but this test case would look out of place compared to other tests in `src/test/ui/mir` or `src/test/ui/borrowck` because it does not generate errors while others do. It is tentatively placed in `src/test/ui/statics` for now.

Thank you for any comments and suggestions!

src/test/ui/statics/static-promotion.rs

@@ -0,0 +1,34 @@
+// check-pass
+
+// Use of global static variables in literal values should be allowed for
+// promotion.
+// This test is to demonstrate the issue raised in
+// https://github.com/rust-lang/rust/issues/70584
+
+// Literal values were previously promoted into local static values when
+// other global static variables are used.
+
+struct A<T: 'static>(&'static T);
+struct B<T: 'static + ?Sized> {
+    x: &'static T,
+}
+static STR: &'static [u8] = b"hi";
+static C: A<B<B<[u8]>>> = {
+    A(&B {
+        x: &B { x: STR },
+    })
+};
+
+pub struct Slice(&'static [i32]);
+
+static CONTENT: i32 = 42;
+pub static CONTENT_MAP: Slice = Slice(&[CONTENT]);
+
+pub static FOO: (i32, i32) = (42, 43);
+pub static CONTENT_MAP2: Slice = Slice(&[FOO.0]);
+
+fn main() {
+    assert_eq!(b"hi", C.0.x.x);
+    assert_eq!(&[42], CONTENT_MAP.0);
+    assert_eq!(&[42], CONTENT_MAP2.0);
+}