add test that we do not merge neighboring SRW

tests/fail/stacked_borrows/disable_mut_does_not_merge_srw.rs

@@ -0,0 +1,17 @@
+// This tests demonstrates the effect of 'Disabling' mutable references on reads, rather than
+// removing them from the stack -- the latter would 'merge' neighboring SRW groups which we would
+// like to avoid.
+fn main() {
+    unsafe {
+        let mut mem = 0;
+        let base = &mut mem as *mut i32; // the base pointer we build the rest of the stack on
+        let mutref = &mut *base;
+        let raw = mutref as *mut i32;
+        // in the stack, we now have [base, mutref, raw]
+        let _val = *base;
+        // now mutref is disabled
+        *base = 1;
+        // this should pop raw from the stack, since it is in a different SRW group
+        let _val = *raw; //~ERROR: that tag does not exist in the borrow stack
+    }
+}

tests/fail/stacked_borrows/disable_mut_does_not_merge_srw.stderr

@@ -0,0 +1,28 @@
+error: Undefined Behavior: attempting a read access using <TAG> at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+  --> $DIR/disable_mut_does_not_merge_srw.rs:LL:CC
+   |
+LL |         let _val = *raw;
+   |                    ^^^^
+   |                    |
+   |                    attempting a read access using <TAG> at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+   |                    this error occurs as part of an access at ALLOC[0x0..0x4]
+   |
+   = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
+   = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
+help: <TAG> was created by a retag at offsets [0x0..0x4]
+  --> $DIR/disable_mut_does_not_merge_srw.rs:LL:CC
+   |
+LL |         let raw = mutref as *mut i32;
+   |                   ^^^^^^
+help: <TAG> was later invalidated at offsets [0x0..0x4]
+  --> $DIR/disable_mut_does_not_merge_srw.rs:LL:CC
+   |
+LL |         *base = 1;
+   |         ^^^^^^^^^
+   = note: backtrace:
+   = note: inside `main` at $DIR/disable_mut_does_not_merge_srw.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to previous error
+