diff --git a/CMakeLists.txt b/CMakeLists.txt
index 1feb0fc95412df800bd48cdf377e6c5e73fc02cc..78e796ca22ccbae2e33e1d94c70f5bf73f1f7b34 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -219,6 +219,7 @@ install(FILES "${CMAKE_SOURCE_DIR}/std/mem.zig" DESTINATION "${ZIG_STD_DEST}")
 install(FILES "${CMAKE_SOURCE_DIR}/std/list.zig" DESTINATION "${ZIG_STD_DEST}")
 install(FILES "${CMAKE_SOURCE_DIR}/std/hash_map.zig" DESTINATION "${ZIG_STD_DEST}")
 install(FILES "${CMAKE_SOURCE_DIR}/std/empty.zig" DESTINATION "${ZIG_STD_DEST}")
+install(FILES "${CMAKE_SOURCE_DIR}/std/debug.zig" DESTINATION "${ZIG_STD_DEST}")
 
 add_executable(run_tests ${TEST_SOURCES})
 target_link_libraries(run_tests)
diff --git a/std/debug.zig b/std/debug.zig
new file mode 100644
index 0000000000000000000000000000000000000000..1a289b645e52db5b6d54d6cafc3ac39ca0503abc
--- /dev/null
+++ b/std/debug.zig
@@ -0,0 +1,16 @@
+const io = @import("io.zig");
+
+pub fn assert(b: bool) {
+    if (!b) unreachable{}
+}
+
+pub fn print_stack_trace() {
+    var maybe_fp: ?&const u8 = @frame_address();
+    while (true) {
+        const fp = maybe_fp ?? break;
+        const return_address = *(&const usize)(usize(fp) + @sizeof(usize));
+        %%io.stderr.print_u64(return_address);
+        %%io.stderr.printf("\n");
+        maybe_fp = *(&const ?&const u8)(fp);
+    }
+}
diff --git a/std/hash_map.zig b/std/hash_map.zig
index 86cd3daee82474edaef2e06d685e70f438402b8a..f5147fdf2f02ec15311cb81968613e999f0110d7 100644
--- a/std/hash_map.zig
+++ b/std/hash_map.zig
@@ -1,4 +1,4 @@
-const assert = @import("index.zig").assert;
+const assert = @import("debug.zig").assert;
 const math = @import("math.zig");
 const mem = @import("mem.zig");
 const Allocator = mem.Allocator;
diff --git a/std/index.zig b/std/index.zig
index 5f2ac7fd8c7cc1090e56500776194f30b5b02857..b22a34e59acfd013d712d45cbef6e902042cf5ff 100644
--- a/std/index.zig
+++ b/std/index.zig
@@ -8,13 +8,10 @@ pub const net = @import("net.zig");
 pub const list = @import("list.zig");
 pub const hash_map = @import("hash_map.zig");
 pub const mem = @import("mem.zig");
+pub const debug = @import("debug.zig");
 pub const linux = switch(@compile_var("os")) {
     linux => @import("linux.zig"),
     else => null_import,
 };
 
-pub fn assert(b: bool) {
-    if (!b) unreachable{}
-}
-
 const null_import = @import("empty.zig");
diff --git a/std/list.zig b/std/list.zig
index 923a17734568c668417927838839876659c6ba1b..31300a071eddd4e48f7aa68f07d995314417aaf6 100644
--- a/std/list.zig
+++ b/std/list.zig
@@ -1,4 +1,4 @@
-const assert = @import("index.zig").assert;
+const assert = @import("debug.zig").assert;
 const mem = @import("mem.zig");
 const Allocator = mem.Allocator;
 
diff --git a/std/mem.zig b/std/mem.zig
index a21706b92188b4503e4d423d2129f57b41b99f28..d2a1ab049f62d945d403c7fb16a8164f0552cfa8 100644
--- a/std/mem.zig
+++ b/std/mem.zig
@@ -1,3 +1,5 @@
+const assert = @import("debug.zig").assert;
+
 pub error NoMem;
 
 pub type Context = u8;
@@ -8,3 +10,9 @@ pub struct Allocator {
     context: ?&Context,
 }
 
+/// Copy all of source into dest at position 0.
+/// dest.len must be >= source.len.
+pub fn copy(T)(dest: []T, source: []T) {
+    assert(dest.len >= source.len);
+    @memcpy(dest.ptr, source.ptr, @sizeof(T) * source.len);
+}
diff --git a/std/net.zig b/std/net.zig
index cf0c12672e7fb0378d29b1f1c259c158af388364..f98b6fbadf22dcdb5252ca75bdf44d836411a774 100644
--- a/std/net.zig
+++ b/std/net.zig
@@ -1,6 +1,6 @@
 const linux = @import("linux.zig");
 const errno = @import("errno.zig");
-const assert = @import("index.zig").assert;
+const assert = @import("debug.zig").assert;
 
 pub error SigInterrupt;
 pub error Unexpected;
diff --git a/std/str.zig b/std/str.zig
index 529c8ae7a0c90df36eb62d7a542bd650f34f1f20..06879e9be5978465f0373123fc38e0ea7c7e9237 100644
--- a/std/str.zig
+++ b/std/str.zig
@@ -1,4 +1,4 @@
-const assert = @import("index.zig").assert;
+const assert = @import("debug.zig").assert;
 
 pub const eql = slice_eql(u8);
 
diff --git a/test/self_hosted.zig b/test/self_hosted.zig
index 0a9ef689724202a4bdaf055af580f870ec12c1e6..748d4ccc0b5bd5d3fe75486e86036026631b19ad 100644
--- a/test/self_hosted.zig
+++ b/test/self_hosted.zig
@@ -1,5 +1,5 @@
 const std = @import("std");
-const assert = std.assert;
+const assert = std.debug.assert;
 const str = std.str;
 const cstr = std.cstr;
 const other = @import("other.zig");