add codegen test

tests/codegen/vecdeque-drain.rs

@@ -0,0 +1,68 @@
+// Check that draining at the front or back doesn't copy memory.
+
+// compile-flags: -O
+// ignore-debug: the debug assertions get in the way
+
+#![crate_type = "lib"]
+
+use std::collections::VecDeque;
+
+// CHECK-LABEL: @clear
+// CHECK-NOT: call
+// CHECK-NOT: br
+// CHECK: getelementptr inbounds
+// CHECK-NEXT: call void @llvm.memset
+// CHECK-NEXT: ret void
+#[no_mangle]
+pub fn clear(v: &mut VecDeque<i32>) {
+    v.drain(..);
+}
+
+// CHECK-LABEL: @truncate
+// CHECK-NOT: call
+// CHECK: br
+// CHECK-NOT: call
+// CHECK: br
+// CHECK-NOT: call
+// CHECK: br
+// CHECK-NOT: call
+// CHECK: br
+// CHECK-NOT: call
+// CHECK-NOT: br
+// CHECK: ret void
+#[no_mangle]
+pub fn truncate(v: &mut VecDeque<i32>, n: usize) {
+    if n < v.len() {
+        v.drain(n..);
+    }
+}
+
+// CHECK-LABEL: @advance
+// CHECK-NOT: call
+// CHECK: br
+// CHECK-NOT: call
+// CHECK: br
+// CHECK-NOT: call
+// CHECK: br
+// CHECK-NOT: call
+// CHECK: br
+// CHECK-NOT: call
+// CHECK: br
+// CHECK-NOT: call
+// CHECK-NOT: br
+// CHECK: ret void
+#[no_mangle]
+pub fn advance(v: &mut VecDeque<i32>, n: usize) {
+    if n < v.len() {
+        v.drain(..n);
+    } else {
+        v.clear();
+    }
+}
+
+// CHECK-LABEL: @remove
+// CHECK: call
+#[no_mangle]
+pub fn remove(v: &mut VecDeque<i32>, a: usize, b: usize) {
+    v.drain(a..b);
+}