Make conflicting borrow description more robust.

This commit improves the logic for place descriptions in conflicting
borrow errors so that borrows of union fields have better messages even
when the unions are embedded in other unions or structs.

src/test/ui/nll/issue-57100.rs

@@ -0,0 +1,69 @@
+#![allow(unused)]
+#![feature(nll)]
+
+// ignore-tidy-linelength
+
+// This tests the error messages for borrows of union fields when the unions are embedded in other
+// structs or unions.
+
+#[derive(Clone, Copy, Default)]
+struct Leaf {
+    l1_u8: u8,
+    l2_u8: u8,
+}
+
+#[derive(Clone, Copy)]
+union First {
+    f1_leaf: Leaf,
+    f2_leaf: Leaf,
+    f3_union: Second,
+}
+
+#[derive(Clone, Copy)]
+union Second {
+    s1_leaf: Leaf,
+    s2_leaf: Leaf,
+}
+
+struct Root {
+    r1_u8: u8,
+    r2_union: First,
+}
+
+// Borrow a different field of the nested union.
+fn nested_union() {
+    unsafe {
+        let mut r = Root {
+            r1_u8: 3,
+            r2_union: First { f3_union: Second { s2_leaf: Leaf { l1_u8: 8, l2_u8: 4 } } }
+        };
+
+        let mref = &mut r.r2_union.f3_union.s1_leaf.l1_u8;
+        //                                  ^^^^^^^
+        *mref = 22;
+        let nref = &r.r2_union.f3_union.s2_leaf.l1_u8;
+        //                              ^^^^^^^
+        //~^^ ERROR cannot borrow `r.r2_union.f3_union` (via `r.r2_union.f3_union.s2_leaf.l1_u8`) as immutable because it is also borrowed as mutable (via `r.r2_union.f3_union.s1_leaf.l1_u8`) [E0502]
+        println!("{} {}", mref, nref)
+    }
+}
+
+// Borrow a different field of the first union.
+fn first_union() {
+    unsafe {
+        let mut r = Root {
+            r1_u8: 3,
+            r2_union: First { f3_union: Second { s2_leaf: Leaf { l1_u8: 8, l2_u8: 4 } } }
+        };
+
+        let mref = &mut r.r2_union.f2_leaf.l1_u8;
+        //                         ^^^^^^^
+        *mref = 22;
+        let nref = &r.r2_union.f1_leaf.l1_u8;
+        //                     ^^^^^^^
+        //~^^ ERROR cannot borrow `r.r2_union` (via `r.r2_union.f1_leaf.l1_u8`) as immutable because it is also borrowed as mutable (via `r.r2_union.f2_leaf.l1_u8`) [E0502]
+        println!("{} {}", mref, nref)
+    }
+}
+
+fn main() {}

src/test/ui/nll/issue-57100.stderr

@@ -0,0 +1,27 @@
+error[E0502]: cannot borrow `r.r2_union.f3_union` (via `r.r2_union.f3_union.s2_leaf.l1_u8`) as immutable because it is also borrowed as mutable (via `r.r2_union.f3_union.s1_leaf.l1_u8`)
+  --> $DIR/issue-57100.rs:44:20
+   |
+LL |         let mref = &mut r.r2_union.f3_union.s1_leaf.l1_u8;
+   |                    -------------------------------------- mutable borrow occurs here (via `r.r2_union.f3_union.s1_leaf.l1_u8`)
+...
+LL |         let nref = &r.r2_union.f3_union.s2_leaf.l1_u8;
+   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ immutable borrow occurs here (via `r.r2_union.f3_union.s2_leaf.l1_u8`)
+...
+LL |         println!("{} {}", mref, nref)
+   |                           ---- mutable borrow later used here
+
+error[E0502]: cannot borrow `r.r2_union` (via `r.r2_union.f1_leaf.l1_u8`) as immutable because it is also borrowed as mutable (via `r.r2_union.f2_leaf.l1_u8`)
+  --> $DIR/issue-57100.rs:62:20
+   |
+LL |         let mref = &mut r.r2_union.f2_leaf.l1_u8;
+   |                    ----------------------------- mutable borrow occurs here (via `r.r2_union.f2_leaf.l1_u8`)
+...
+LL |         let nref = &r.r2_union.f1_leaf.l1_u8;
+   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^ immutable borrow occurs here (via `r.r2_union.f1_leaf.l1_u8`)
+...
+LL |         println!("{} {}", mref, nref)
+   |                           ---- mutable borrow later used here
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0502`.