diff --git a/.cargo/config.toml b/.cargo/config.toml
index 3d99f63..adea6df 100644
--- a/.cargo/config.toml
+++ b/.cargo/config.toml
@@ -1,3 +1,6 @@
+[alias]
+xtask = ["run", "-p", "xtask", "--bin", "xtask", "--"]
+
 [build]
 # This works both on local `cargo doc` and on docs.rs because we don't have any `normal` dependency.
 # In most cases, `package.metadata.docs.rs` is recommended.
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 9658366..114d0ee 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -92,7 +92,7 @@ jobs:
         uses: actions-rs/cargo@v1
         with:
           command: test
-          args: --workspace --no-fail-fast
+          args: --no-fail-fast
         env:
           RUST_BACKTRACE: full
 
diff --git a/Cargo.toml b/Cargo.toml
index 5348943..3ab81ff 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,3 +1,6 @@
+[workspace]
+members = ["xtask/"]
+
 [package]
 name = "ac-library-rs"
 version = "0.1.0"
diff --git a/xtask/Cargo.toml b/xtask/Cargo.toml
new file mode 100644
index 0000000..da156d7
--- /dev/null
+++ b/xtask/Cargo.toml
@@ -0,0 +1,20 @@
+[package]
+name = "xtask"
+version = "0.0.0"
+authors = ["rust-lang-ja developers"]
+edition = "2018"
+license = "CC0-1.0"
+repository = "https://github.com/rust-lang-ja/ac-library-rs"
+publish = false
+
+[dependencies]
+anyhow = "1.0.32"
+atty = "0.2.14"
+cargo_metadata = "0.11.2"
+duct = "0.13.4"
+proc-macro2 = { version = "1.0.21", features = ["span-locations"] }
+quote = "1.0.7"
+structopt = "0.3.17"
+syn = { version = "1.0.40", features = ["full"] }
+tempfile = "3.1.0"
+termcolor = "1.1.0"
diff --git a/xtask/src/commands.rs b/xtask/src/commands.rs
new file mode 100644
index 0000000..743c3e4
--- /dev/null
+++ b/xtask/src/commands.rs
@@ -0,0 +1 @@
+pub(crate) mod export;
diff --git a/xtask/src/commands/export.rs b/xtask/src/commands/export.rs
new file mode 100644
index 0000000..8b5ad32
--- /dev/null
+++ b/xtask/src/commands/export.rs
@@ -0,0 +1,150 @@
+use crate::shell::Shell;
+use anyhow::{anyhow, bail, Context as _};
+use cargo_metadata::{self as cm, MetadataCommand};
+use duct::cmd;
+use quote::ToTokens as _;
+use std::{
+    env,
+    io::{self, Write as _},
+    path::{Path, PathBuf},
+};
+use structopt::StructOpt;
+use syn::{Item, ItemMod};
+
+#[derive(StructOpt, Debug)]
+pub struct OptExport {
+    /// Save the output to the file
+    #[structopt(short, long, value_name("PATH"))]
+    output: Option<PathBuf>,
+}
+
+pub(crate) fn run(opt: OptExport, shell: &mut Shell) -> anyhow::Result<()> {
+    let OptExport { output } = opt;
+
+    let metadata = MetadataCommand::new()
+        .no_deps()
+        .exec()
+        .map_err(|err| match err {
+            cm::Error::CargoMetadata { stderr } => {
+                anyhow!("{}", stderr.trim_start_matches("error: "))
+            }
+            err => anyhow!("{}", err),
+        })?;
+
+    let cm::Target { src_path, .. } = metadata
+        .packages
+        .iter()
+        .filter(|p| p.manifest_path == metadata.workspace_root.join("Cargo.toml"))
+        .flat_map(|p| &p.targets)
+        .find(|cm::Target { kind, .. }| *kind == ["lib".to_owned()])
+        .with_context(|| "could find the library")?;
+
+    let code = std::fs::read_to_string(src_path)?;
+    let syn::File { items, .. } =
+        syn::parse_file(&code).with_context(|| format!("`{}` is broken", src_path.display()))?;
+
+    let mut acc = vec!["".to_owned(), "".to_owned()];
+
+    for item in items {
+        match item {
+            Item::Mod(ItemMod {
+                attrs,
+                vis,
+                ident,
+                content: None,
+                semi: Some(_),
+                ..
+            }) => {
+                let acc = &mut acc[1];
+                let path = src_path
+                    .with_file_name(ident.to_string())
+                    .with_extension("rs");
+                if !path.exists() {
+                    unimplemented!("is this `mod.rs`?: {}", ident);
+                }
+                let content = std::fs::read_to_string(&path)?;
+                let is_safe_to_indent = !syn::parse_file(&content)
+                    .map_err(|e| anyhow!("{:?}", e))
+                    .with_context(|| format!("could not parse `{}`", path.display()))?
+                    .into_token_stream()
+                    .into_iter()
+                    .any(|tt| {
+                        matches!(
+                            tt, proc_macro2::TokenTree::Literal(lit)
+                            if lit.span().start().line != lit.span().end().line
+                        )
+                    });
+
+                for attr in attrs {
+                    *acc += &attr.to_token_stream().to_string();
+                    *acc += "\n";
+                }
+                *acc += &vis.to_token_stream().to_string();
+                *acc += " mod ";
+                *acc += &ident.to_string();
+                *acc += " {\n";
+                if is_safe_to_indent {
+                    for line in content.lines() {
+                        *acc += "    ";
+                        *acc += line;
+                        *acc += "\n";
+                    }
+                } else {
+                    *acc += &content;
+                }
+                *acc += "}\n";
+            }
+            item => {
+                let acc = &mut acc[0];
+                *acc += &item.to_token_stream().to_string();
+                *acc += "\n";
+            }
+        }
+    }
+
+    let acc = rustfmt(&acc.join("\n"))?;
+
+    shell.status(
+        "Expanded",
+        format!("{} ({} B)", src_path.display(), acc.len()),
+    )?;
+
+    if let Some(output) = output {
+        std::fs::write(&output, acc)
+            .with_context(|| format!("could not write `{}`", output.display()))?;
+        shell.status("Wrote", output.display())?;
+    } else {
+        io::stdout().write_all(acc.as_ref())?;
+        io::stdout().flush()?;
+    }
+    Ok(())
+}
+
+fn rustfmt(code: &str) -> anyhow::Result<String> {
+    let tempdir = tempfile::Builder::new()
+        .prefix("ac-library-rs-xtask")
+        .tempdir()?;
+
+    let path = tempdir.path().join("expanded.rs");
+
+    std::fs::write(&path, code)?;
+
+    let rustfmt_exe = Path::new(&env::var_os("CARGO").with_context(|| "missing `$CARGO`")?)
+        .with_file_name("rustfmt")
+        .with_extension(env::consts::EXE_EXTENSION);
+
+    if !rustfmt_exe.exists() {
+        bail!(
+            "`{}` does not exist. Run `rustup component add rustfmt` first",
+            rustfmt_exe.display(),
+        );
+    }
+
+    cmd!(rustfmt_exe, "--edition", "2018", &path)
+        .run()
+        .with_context(|| "could not format the output")?;
+
+    let output = std::fs::read_to_string(path)?;
+    tempdir.close()?;
+    Ok(output)
+}
diff --git a/xtask/src/lib.rs b/xtask/src/lib.rs
new file mode 100644
index 0000000..d3e3fdd
--- /dev/null
+++ b/xtask/src/lib.rs
@@ -0,0 +1,37 @@
+mod commands;
+pub mod shell;
+
+use crate::commands::export::OptExport;
+use crate::shell::Shell;
+use std::io::Write as _;
+use structopt::StructOpt;
+
+#[derive(StructOpt, Debug)]
+#[structopt(bin_name("cargo xtask"))]
+pub enum Opt {
+    /// Export the library
+    Export(OptExport),
+}
+
+pub fn run(opt: Opt, shell: &mut Shell) -> anyhow::Result<()> {
+    match opt {
+        Opt::Export(opt) => commands::export::run(opt, shell),
+    }
+}
+
+pub fn exit_with_error(err: anyhow::Error, shell: &mut Shell) -> ! {
+    let _ = shell.error(&err);
+
+    for cause in err.chain().skip(1) {
+        let _ = writeln!(shell.err(), "\nCaused by:");
+
+        for line in cause.to_string().lines() {
+            let _ = match line {
+                "" => writeln!(shell.err()),
+                line => writeln!(shell.err(), "  {}", line),
+            };
+        }
+    }
+
+    std::process::exit(1);
+}
diff --git a/xtask/src/main.rs b/xtask/src/main.rs
new file mode 100644
index 0000000..620399d
--- /dev/null
+++ b/xtask/src/main.rs
@@ -0,0 +1,10 @@
+use structopt::StructOpt as _;
+use xtask::{shell::Shell, Opt};
+
+fn main() {
+    let opt = Opt::from_args();
+    let mut shell = Shell::new();
+    if let Err(err) = xtask::run(opt, &mut shell) {
+        xtask::exit_with_error(err, &mut shell);
+    }
+}
diff --git a/xtask/src/shell.rs b/xtask/src/shell.rs
new file mode 100644
index 0000000..b1ff808
--- /dev/null
+++ b/xtask/src/shell.rs
@@ -0,0 +1,63 @@
+use std::{
+    fmt,
+    io::{self, Write as _},
+};
+use termcolor::{Color, ColorChoice, ColorSpec, StandardStream, WriteColor as _};
+
+pub struct Shell {
+    stderr: StandardStream,
+}
+
+impl Shell {
+    pub fn new() -> Self {
+        Self {
+            stderr: StandardStream::stderr(if atty::is(atty::Stream::Stderr) {
+                ColorChoice::Auto
+            } else {
+                ColorChoice::Never
+            }),
+        }
+    }
+
+    pub(crate) fn err(&mut self) -> &mut StandardStream {
+        &mut self.stderr
+    }
+
+    pub(crate) fn status(
+        &mut self,
+        status: impl fmt::Display,
+        message: impl fmt::Display,
+    ) -> io::Result<()> {
+        self.print(status, message, Color::Green, true)
+    }
+
+    pub fn error(&mut self, message: impl fmt::Display) -> io::Result<()> {
+        self.print("error", message, Color::Red, false)
+    }
+
+    fn print(
+        &mut self,
+        status: impl fmt::Display,
+        message: impl fmt::Display,
+        color: Color,
+        justified: bool,
+    ) -> io::Result<()> {
+        self.stderr
+            .set_color(ColorSpec::new().set_bold(true).set_fg(Some(color)))?;
+        if justified {
+            write!(self.stderr, "{:>12}", status)?;
+        } else {
+            write!(self.stderr, "{}", status)?;
+            self.stderr.set_color(ColorSpec::new().set_bold(true))?;
+            write!(self.stderr, ":")?;
+        }
+        self.stderr.reset()?;
+        writeln!(self.stderr, " {}", message)
+    }
+}
+
+impl Default for Shell {
+    fn default() -> Self {
+        Self::new()
+    }
+}