Add test.

tests/mir-opt/pre-codegen/deref_nested_borrows.rs

@@ -0,0 +1,22 @@
+// EMIT_MIR_FOR_EACH_PANIC_STRATEGY
+
+fn src(x: &&u8) -> bool {
+    // CHECK-LABEL: fn src(
+    // CHECK: _0 = const true;
+    let y = **x;
+    unsafe { unknown() };
+    **x == y
+}
+
+#[inline(never)]
+unsafe fn unknown() {
+    // CHECK-LABEL: fn unknown(
+}
+
+fn main() {
+    // CHECK-LABEL: fn main(
+    src(&&0);
+}
+
+// EMIT_MIR deref_nested_borrows.src.GVN.diff
+// EMIT_MIR deref_nested_borrows.src.PreCodegen.after.mir

tests/mir-opt/pre-codegen/deref_nested_borrows.src.GVN.panic-abort.diff

@@ -0,0 +1,43 @@
+- // MIR for `src` before GVN
++ // MIR for `src` after GVN
+
+  fn src(_1: &&u8) -> bool {
+      debug x => _1;
+      let mut _0: bool;
+      let _2: u8;
+      let _3: ();
+      let mut _4: u8;
+      let mut _5: u8;
+      let mut _6: &u8;
+      let mut _7: &u8;
+      scope 1 {
+          debug y => _2;
+      }
+
+      bb0: {
+-         StorageLive(_2);
+-         _6 = deref_copy (*_1);
++         nop;
++         _6 = copy (*_1);
+          _2 = copy (*_6);
+          _3 = unknown() -> [return: bb1, unwind unreachable];
+      }
+
+      bb1: {
+          StorageLive(_4);
+-         _7 = deref_copy (*_1);
+-         _4 = copy (*_7);
++         _7 = copy _6;
++         _4 = copy _2;
+          StorageLive(_5);
+          _5 = copy _2;
+-         _0 = Eq(move _4, move _5);
++         _0 = const true;
+          StorageDead(_5);
+          StorageDead(_4);
+-         StorageDead(_2);
++         nop;
+          return;
+      }
+  }
+

tests/mir-opt/pre-codegen/deref_nested_borrows.src.GVN.panic-unwind.diff

@@ -0,0 +1,43 @@
+- // MIR for `src` before GVN
++ // MIR for `src` after GVN
+
+  fn src(_1: &&u8) -> bool {
+      debug x => _1;
+      let mut _0: bool;
+      let _2: u8;
+      let _3: ();
+      let mut _4: u8;
+      let mut _5: u8;
+      let mut _6: &u8;
+      let mut _7: &u8;
+      scope 1 {
+          debug y => _2;
+      }
+
+      bb0: {
+-         StorageLive(_2);
+-         _6 = deref_copy (*_1);
++         nop;
++         _6 = copy (*_1);
+          _2 = copy (*_6);
+          _3 = unknown() -> [return: bb1, unwind continue];
+      }
+
+      bb1: {
+          StorageLive(_4);
+-         _7 = deref_copy (*_1);
+-         _4 = copy (*_7);
++         _7 = copy _6;
++         _4 = copy _2;
+          StorageLive(_5);
+          _5 = copy _2;
+-         _0 = Eq(move _4, move _5);
++         _0 = const true;
+          StorageDead(_5);
+          StorageDead(_4);
+-         StorageDead(_2);
++         nop;
+          return;
+      }
+  }
+

tests/mir-opt/pre-codegen/deref_nested_borrows.src.PreCodegen.after.panic-abort.mir

@@ -0,0 +1,23 @@
+// MIR for `src` after PreCodegen
+
+fn src(_1: &&u8) -> bool {
+    debug x => _1;
+    let mut _0: bool;
+    let mut _2: &u8;
+    let _3: u8;
+    let _4: ();
+    scope 1 {
+        debug y => _3;
+    }
+
+    bb0: {
+        _2 = copy (*_1);
+        _3 = copy (*_2);
+        _4 = unknown() -> [return: bb1, unwind unreachable];
+    }
+
+    bb1: {
+        _0 = const true;
+        return;
+    }
+}

tests/mir-opt/pre-codegen/deref_nested_borrows.src.PreCodegen.after.panic-unwind.mir

@@ -0,0 +1,23 @@
+// MIR for `src` after PreCodegen
+
+fn src(_1: &&u8) -> bool {
+    debug x => _1;
+    let mut _0: bool;
+    let mut _2: &u8;
+    let _3: u8;
+    let _4: ();
+    scope 1 {
+        debug y => _3;
+    }
+
+    bb0: {
+        _2 = copy (*_1);
+        _3 = copy (*_2);
+        _4 = unknown() -> [return: bb1, unwind continue];
+    }
+
+    bb1: {
+        _0 = const true;
+        return;
+    }
+}