Add tests showing how we duplicate allocations when we shouldn't

tests/ui/consts/auxiliary/const_mut_refs_crate.rs

@@ -0,0 +1,23 @@
+// This is a support file for ../const-mut-refs-crate.rs
+
+// This is to test that static inners from an external
+// crate like this one, still preserves the alloc.
+// That is, the address from the standpoint of rustc+llvm
+// is the same.
+// The need for this test originated from the GH issue
+// https://github.com/rust-lang/rust/issues/57349
+
+// See also ../const-mut-refs-crate.rs for more details
+// about this test.
+
+#![feature(const_mut_refs)]
+
+// if we used immutable references here, then promotion would
+// turn the `&42` into a promoted, which gets duplicated arbitrarily.
+pub static mut FOO: &'static mut i32 = &mut 42;
+pub static mut BAR: &'static mut i32 = unsafe { FOO };
+
+pub mod inner {
+    pub static INNER_MOD_FOO: &'static i32 = &43;
+    pub static INNER_MOD_BAR: &'static i32 = INNER_MOD_FOO;
+}

tests/ui/consts/const-mut-refs-crate.rs

@@ -0,0 +1,35 @@
+//@ run-pass
+//@ aux-build:const_mut_refs_crate.rs
+
+#![feature(const_mut_refs)]
+
+//! Regression test for https://github.com/rust-lang/rust/issues/79738
+//! Show how we are duplicationg allocations, even though static items that
+//! copy their value from another static should not also be duplicating
+//! memory behind references.
+
+extern crate const_mut_refs_crate as other;
+
+use other::{
+    inner::{INNER_MOD_BAR, INNER_MOD_FOO},
+    BAR, FOO,
+};
+
+pub static LOCAL_FOO: &'static i32 = &41;
+pub static LOCAL_BAR: &'static i32 = LOCAL_FOO;
+pub static mut COPY_OF_REMOTE_FOO: &'static mut i32 = unsafe { FOO };
+
+static DOUBLE_REF: &&i32 = &&99;
+static ONE_STEP_ABOVE: &i32 = *DOUBLE_REF;
+
+pub fn main() {
+    unsafe {
+        assert_ne!(FOO as *const i32, BAR as *const i32);
+        assert_eq!(INNER_MOD_FOO as *const i32, INNER_MOD_BAR as *const i32);
+        assert_eq!(LOCAL_FOO as *const i32, LOCAL_BAR as *const i32);
+        assert_eq!(*DOUBLE_REF as *const i32, ONE_STEP_ABOVE as *const i32);
+
+        // bug!
+        assert_ne!(FOO as *const i32, COPY_OF_REMOTE_FOO as *const i32);
+    }
+}