diff --git a/src/Compilation.zig b/src/Compilation.zig
index 4bd3218a35..6acbe0ac39 100644
--- a/src/Compilation.zig
+++ b/src/Compilation.zig
@@ -2702,6 +2702,72 @@ pub fn makeBinFileWritable(self: *Compilation) !void {
     return self.bin_file.makeWritable();
 }
 
+const Header = extern struct {
+    intern_pool: extern struct {
+        items_len: u32,
+        extra_len: u32,
+        limbs_len: u32,
+        string_bytes_len: u32,
+    },
+};
+
+/// Note that all state that is included in the cache hash namespace is *not*
+/// saved, such as the target and most CLI flags. A cache hit will only occur
+/// when subsequent compiler invocations use the same set of flags.
+pub fn saveState(comp: *Compilation) !void {
+    var bufs_list: [6]std.os.iovec_const = undefined;
+    var bufs_len: usize = 0;
+
+    const emit = comp.bin_file.options.emit orelse return;
+
+    if (comp.bin_file.options.module) |mod| {
+        const ip = &mod.intern_pool;
+        const header: Header = .{
+            .intern_pool = .{
+                .items_len = @intCast(ip.items.len),
+                .extra_len = @intCast(ip.extra.items.len),
+                .limbs_len = @intCast(ip.limbs.items.len),
+                .string_bytes_len = @intCast(ip.string_bytes.items.len),
+            },
+        };
+        addBuf(&bufs_list, &bufs_len, mem.asBytes(&header));
+        addBuf(&bufs_list, &bufs_len, mem.sliceAsBytes(ip.limbs.items));
+        addBuf(&bufs_list, &bufs_len, mem.sliceAsBytes(ip.extra.items));
+        addBuf(&bufs_list, &bufs_len, mem.sliceAsBytes(ip.items.items(.data)));
+        addBuf(&bufs_list, &bufs_len, mem.sliceAsBytes(ip.items.items(.tag)));
+        addBuf(&bufs_list, &bufs_len, ip.string_bytes.items);
+
+        // TODO: compilation errors
+        // TODO: files
+        // TODO: namespaces
+        // TODO: decls
+        // TODO: linker state
+    }
+    var basename_buf: [255]u8 = undefined;
+    const basename = std.fmt.bufPrint(&basename_buf, "{s}.zcs", .{
+        comp.bin_file.options.root_name,
+    }) catch o: {
+        basename_buf[basename_buf.len - 4 ..].* = ".zcs".*;
+        break :o &basename_buf;
+    };
+
+    // Using an atomic file prevents a crash or power failure from corrupting
+    // the previous incremental compilation state.
+    var af = try emit.directory.handle.atomicFile(basename, .{});
+    defer af.deinit();
+    try af.file.pwritevAll(bufs_list[0..bufs_len], 0);
+    try af.finish();
+}
+
+fn addBuf(bufs_list: []std.os.iovec_const, bufs_len: *usize, buf: []const u8) void {
+    const i = bufs_len.*;
+    bufs_len.* = i + 1;
+    bufs_list[i] = .{
+        .iov_base = buf.ptr,
+        .iov_len = buf.len,
+    };
+}
+
 /// This function is temporally single-threaded.
 pub fn totalErrorCount(self: *Compilation) u32 {
     var total: usize = self.failed_c_objects.count() +
diff --git a/src/main.zig b/src/main.zig
index 8970719509..04ae483a6e 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -594,6 +594,7 @@ const usage_build_generic =
     \\  --debug-log [scope]          Enable printing debug/info log messages for scope
     \\  --debug-compile-errors       Crash with helpful diagnostics at the first compile error
     \\  --debug-link-snapshot        Enable dumping of the linker's state in JSON format
+    \\  --debug-incremental          Enable experimental feature: incremental compilation
     \\
 ;
 
@@ -904,6 +905,7 @@ fn buildOutputType(
     var minor_subsystem_version: ?u32 = null;
     var wasi_exec_model: ?std.builtin.WasiExecModel = null;
     var enable_link_snapshots: bool = false;
+    var debug_incremental: bool = false;
     var install_name: ?[]const u8 = null;
     var hash_style: link.HashStyle = .both;
     var entitlements: ?[]const u8 = null;
@@ -1272,6 +1274,8 @@ fn buildOutputType(
                         } else {
                             enable_link_snapshots = true;
                         }
+                    } else if (mem.eql(u8, arg, "--debug-incremental")) {
+                        debug_incremental = true;
                     } else if (mem.eql(u8, arg, "--entitlements")) {
                         entitlements = args_iter.nextOrFatal();
                     } else if (mem.eql(u8, arg, "-fcompiler-rt")) {
@@ -3591,11 +3595,16 @@ fn buildOutputType(
     }
 
     updateModule(comp) catch |err| switch (err) {
-        error.SemanticAnalyzeFail => if (listen == .none) process.exit(1),
+        error.SemanticAnalyzeFail => {
+            assert(listen == .none);
+            saveState(comp, debug_incremental);
+            process.exit(1);
+        },
         else => |e| return e,
     };
     if (build_options.only_c) return cleanExit();
     try comp.makeBinFileExecutable();
+    saveState(comp, debug_incremental);
 
     if (test_exec_args.items.len == 0 and object_format == .c) default_exec_args: {
         // Default to using `zig run` to execute the produced .c code from `zig test`.
@@ -3658,6 +3667,14 @@ fn buildOutputType(
     return cleanExit();
 }
 
+fn saveState(comp: *Compilation, debug_incremental: bool) void {
+    if (debug_incremental) {
+        comp.saveState() catch |err| {
+            warn("unable to save incremental compilation state: {s}", .{@errorName(err)});
+        };
+    }
+}
+
 fn serve(
     comp: *Compilation,
     in: fs.File,