diff --git a/src/lang/io/include/sourcemeta/core/io.h b/src/lang/io/include/sourcemeta/core/io.h
index b7c0c2533..ac607c8e8 100644
--- a/src/lang/io/include/sourcemeta/core/io.h
+++ b/src/lang/io/include/sourcemeta/core/io.h
@@ -100,6 +100,22 @@ auto read_file(const std::filesystem::path &path)
   return stream;
 }
 
+/// @ingroup io
+///
+/// Recursively mirror a directory tree using hard links for regular files.
+/// Directories are created, regular files are hard-linked. Both paths must
+/// reside on the same filesystem. The destination must not be inside the
+/// source tree, as that would cause infinite recursion.
+///
+/// ```cpp
+/// #include <sourcemeta/core/io.h>
+///
+/// sourcemeta::core::hardlink_directory("/source", "/destination");
+/// ```
+SOURCEMETA_CORE_IO_EXPORT
+auto hardlink_directory(const std::filesystem::path &source,
+                        const std::filesystem::path &destination) -> void;
+
 /// @ingroup io
 ///
 /// Flush an existing file to disk, beyond just to the operating system. For
diff --git a/src/lang/io/io.cc b/src/lang/io/io.cc
index 0c09bce83..745ddb56e 100644
--- a/src/lang/io/io.cc
+++ b/src/lang/io/io.cc
@@ -47,6 +47,25 @@ auto starts_with(const std::filesystem::path &path,
   return true;
 }
 
+auto hardlink_directory(const std::filesystem::path &source,
+                        const std::filesystem::path &destination) -> void {
+  assert(std::filesystem::is_directory(source));
+  assert(!std::filesystem::exists(destination) ||
+         std::filesystem::is_directory(destination));
+  assert(!starts_with(destination, source));
+  std::filesystem::create_directories(destination);
+  for (const auto &entry :
+       std::filesystem::recursive_directory_iterator{source}) {
+    const auto target{destination /
+                      std::filesystem::relative(entry.path(), source)};
+    if (entry.is_directory()) {
+      std::filesystem::create_directories(target);
+    } else if (entry.is_regular_file()) {
+      std::filesystem::create_hard_link(entry.path(), target);
+    }
+  }
+}
+
 auto flush(const std::filesystem::path &path) -> void {
 #if defined(_WIN32)
   HANDLE hFile =
diff --git a/test/io/CMakeLists.txt b/test/io/CMakeLists.txt
index 33d202a26..55500c59b 100644
--- a/test/io/CMakeLists.txt
+++ b/test/io/CMakeLists.txt
@@ -6,9 +6,11 @@ sourcemeta_googletest(NAMESPACE sourcemeta PROJECT core NAME io
     io_starts_with_test.cc
     io_read_file_test.cc
     io_fileview_test.cc
-    io_temporary_test.cc)
+    io_temporary_test.cc
+    io_hardlink_directory_test.cc)
 
 target_link_libraries(sourcemeta_core_io_unit
   PRIVATE sourcemeta::core::io)
 target_compile_definitions(sourcemeta_core_io_unit
-  PRIVATE STUBS_DIRECTORY="${CMAKE_CURRENT_SOURCE_DIR}/stubs")
+  PRIVATE STUBS_DIRECTORY="${CMAKE_CURRENT_SOURCE_DIR}/stubs"
+  PRIVATE BUILD_DIRECTORY="${CMAKE_CURRENT_BINARY_DIR}")
diff --git a/test/io/io_hardlink_directory_test.cc b/test/io/io_hardlink_directory_test.cc
new file mode 100644
index 000000000..95cc2adf9
--- /dev/null
+++ b/test/io/io_hardlink_directory_test.cc
@@ -0,0 +1,101 @@
+#include <gtest/gtest.h>
+
+#include <sourcemeta/core/io.h>
+
+#include <filesystem> // std::filesystem
+#include <fstream>    // std::ofstream, std::ifstream
+#include <string>     // std::string
+
+TEST(IO_hardlink_directory, mirrors_structure) {
+  const sourcemeta::core::TemporaryDirectory workspace{
+      std::filesystem::path{BUILD_DIRECTORY}, ".test-hardlink-"};
+  const auto source{workspace.path() / "source"};
+  const auto destination{workspace.path() / "destination"};
+
+  std::filesystem::create_directories(source / "subdir");
+  std::ofstream{source / "a.txt"} << "alpha";
+  std::ofstream{source / "subdir" / "b.txt"} << "beta";
+
+  sourcemeta::core::hardlink_directory(source, destination);
+
+  EXPECT_TRUE(std::filesystem::exists(destination / "a.txt"));
+  EXPECT_TRUE(std::filesystem::exists(destination / "subdir" / "b.txt"));
+  EXPECT_TRUE(std::filesystem::is_directory(destination / "subdir"));
+}
+
+TEST(IO_hardlink_directory, files_share_inodes) {
+  const sourcemeta::core::TemporaryDirectory workspace{
+      std::filesystem::path{BUILD_DIRECTORY}, ".test-hardlink-"};
+  const auto source{workspace.path() / "source"};
+  const auto destination{workspace.path() / "destination"};
+
+  std::filesystem::create_directory(source);
+  std::ofstream{source / "file.txt"} << "content";
+
+  sourcemeta::core::hardlink_directory(source, destination);
+
+  EXPECT_EQ(std::filesystem::hard_link_count(source / "file.txt"), 2);
+  EXPECT_EQ(std::filesystem::hard_link_count(destination / "file.txt"), 2);
+  EXPECT_TRUE(std::filesystem::equivalent(source / "file.txt",
+                                          destination / "file.txt"));
+}
+
+TEST(IO_hardlink_directory, unlink_then_write_independence) {
+  const sourcemeta::core::TemporaryDirectory workspace{
+      std::filesystem::path{BUILD_DIRECTORY}, ".test-hardlink-"};
+  const auto source{workspace.path() / "source"};
+  const auto destination{workspace.path() / "destination"};
+
+  std::filesystem::create_directory(source);
+  std::ofstream{source / "file.txt"} << "original";
+
+  sourcemeta::core::hardlink_directory(source, destination);
+
+  std::filesystem::remove(destination / "file.txt");
+  std::ofstream{destination / "file.txt"} << "modified";
+
+  std::ifstream source_stream{source / "file.txt"};
+  std::string source_content;
+  std::getline(source_stream, source_content);
+  EXPECT_EQ(source_content, "original");
+
+  std::ifstream destination_stream{destination / "file.txt"};
+  std::string destination_content;
+  std::getline(destination_stream, destination_content);
+  EXPECT_EQ(destination_content, "modified");
+}
+
+TEST(IO_hardlink_directory, empty_source) {
+  const sourcemeta::core::TemporaryDirectory workspace{
+      std::filesystem::path{BUILD_DIRECTORY}, ".test-hardlink-"};
+  const auto source{workspace.path() / "source"};
+  const auto destination{workspace.path() / "destination"};
+
+  std::filesystem::create_directory(source);
+
+  sourcemeta::core::hardlink_directory(source, destination);
+
+  EXPECT_TRUE(std::filesystem::exists(destination));
+  EXPECT_TRUE(std::filesystem::is_directory(destination));
+  EXPECT_TRUE(std::filesystem::is_empty(destination));
+}
+
+TEST(IO_hardlink_directory, deeply_nested) {
+  const sourcemeta::core::TemporaryDirectory workspace{
+      std::filesystem::path{BUILD_DIRECTORY}, ".test-hardlink-"};
+  const auto source{workspace.path() / "source"};
+  const auto destination{workspace.path() / "destination"};
+
+  std::filesystem::create_directories(source / "a" / "b" / "c");
+  std::ofstream{source / "a" / "b" / "c" / "deep.txt"} << "deep";
+
+  sourcemeta::core::hardlink_directory(source, destination);
+
+  EXPECT_TRUE(
+      std::filesystem::exists(destination / "a" / "b" / "c" / "deep.txt"));
+
+  std::ifstream stream{destination / "a" / "b" / "c" / "deep.txt"};
+  std::string content;
+  std::getline(stream, content);
+  EXPECT_EQ(content, "deep");
+}
diff --git a/test/io/io_temporary_test.cc b/test/io/io_temporary_test.cc
index 1d7917ede..46e8626b7 100644
--- a/test/io/io_temporary_test.cc
+++ b/test/io/io_temporary_test.cc
@@ -7,21 +7,21 @@
 #include <string>     // std::string
 
 TEST(IO_TemporaryDirectory, creates_directory) {
-  const auto parent{std::filesystem::temp_directory_path()};
+  const auto parent{std::filesystem::path{BUILD_DIRECTORY}};
   const sourcemeta::core::TemporaryDirectory temporary{parent, ".test-"};
   EXPECT_TRUE(std::filesystem::exists(temporary.path()));
   EXPECT_TRUE(std::filesystem::is_directory(temporary.path()));
 }
 
 TEST(IO_TemporaryDirectory, unique_names) {
-  const auto parent{std::filesystem::temp_directory_path()};
+  const auto parent{std::filesystem::path{BUILD_DIRECTORY}};
   const sourcemeta::core::TemporaryDirectory first{parent, ".test-"};
   const sourcemeta::core::TemporaryDirectory second{parent, ".test-"};
   EXPECT_NE(first.path(), second.path());
 }
 
 TEST(IO_TemporaryDirectory, custom_prefix) {
-  const auto parent{std::filesystem::temp_directory_path()};
+  const auto parent{std::filesystem::path{BUILD_DIRECTORY}};
   const sourcemeta::core::TemporaryDirectory temporary{parent,
                                                        ".my-test-prefix-"};
   const auto filename{temporary.path().filename().string()};
@@ -32,7 +32,7 @@ TEST(IO_TemporaryDirectory, removed_on_destruction) {
   std::filesystem::path path_copy;
   {
     const sourcemeta::core::TemporaryDirectory temporary{
-        std::filesystem::temp_directory_path(), ".test-"};
+        std::filesystem::path{BUILD_DIRECTORY}, ".test-"};
     path_copy = temporary.path();
     EXPECT_TRUE(std::filesystem::exists(path_copy));
   }
@@ -43,7 +43,7 @@ TEST(IO_TemporaryDirectory, removed_on_destruction_non_empty) {
   std::filesystem::path path_copy;
   {
     const sourcemeta::core::TemporaryDirectory temporary{
-        std::filesystem::temp_directory_path(), ".test-"};
+        std::filesystem::path{BUILD_DIRECTORY}, ".test-"};
     path_copy = temporary.path();
     std::filesystem::create_directory(path_copy / "subdir");
     std::ofstream{path_copy / "file.txt"} << "hello";
@@ -55,14 +55,14 @@ TEST(IO_TemporaryDirectory, removed_on_destruction_non_empty) {
 
 TEST(IO_TemporaryDirectory, inside_parent) {
   const auto parent{
-      std::filesystem::canonical(std::filesystem::temp_directory_path())};
+      std::filesystem::canonical(std::filesystem::path{BUILD_DIRECTORY})};
   const sourcemeta::core::TemporaryDirectory temporary{parent, ".test-"};
   EXPECT_EQ(temporary.path().parent_path(), parent);
 }
 
 TEST(IO_TemporaryDirectory, creates_nonexistent_parent) {
   const sourcemeta::core::TemporaryDirectory unique_parent{
-      std::filesystem::temp_directory_path(), ".test-parent-"};
+      std::filesystem::path{BUILD_DIRECTORY}, ".test-parent-"};
   const auto parent{unique_parent.path() / "nonexistent"};
   EXPECT_FALSE(std::filesystem::exists(parent));
   const sourcemeta::core::TemporaryDirectory temporary{parent, ".test-"};
@@ -72,7 +72,7 @@ TEST(IO_TemporaryDirectory, creates_nonexistent_parent) {
 
 TEST(IO_TemporaryDirectory, creates_nested_nonexistent_parents) {
   const sourcemeta::core::TemporaryDirectory unique_parent{
-      std::filesystem::temp_directory_path(), ".test-parent-"};
+      std::filesystem::path{BUILD_DIRECTORY}, ".test-parent-"};
   const auto parent{unique_parent.path() / "nested_a" / "nested_b"};
   EXPECT_FALSE(std::filesystem::exists(parent));
   const sourcemeta::core::TemporaryDirectory temporary{parent, ".test-"};
@@ -82,7 +82,7 @@ TEST(IO_TemporaryDirectory, creates_nested_nonexistent_parents) {
 
 TEST(IO_TemporaryDirectory, parent_is_a_file) {
   const sourcemeta::core::TemporaryDirectory unique_parent{
-      std::filesystem::temp_directory_path(), ".test-parent-"};
+      std::filesystem::path{BUILD_DIRECTORY}, ".test-parent-"};
   const auto file_path{unique_parent.path() / "test_parent_file"};
   std::ofstream{file_path};
   EXPECT_TRUE(std::filesystem::exists(file_path));