feat(cli): add migrate subcommand for generating a build script

suggest embedding migrations on `sqlx migrate add` in a new project

sqlx-cli/src/lib.rs

@@ -36,6 +36,7 @@ pub async fn run(opt: Opt) -> anyhow::Result<()> {
                 ignore_missing,
             } => migrate::revert(&migrate.source, &database_url, dry_run, ignore_missing).await?,
             MigrateCommand::Info => migrate::info(&migrate.source, &database_url).await?,
+            MigrateCommand::BuildScript { force } => migrate::build_script(&migrate.source, force)?,
         },
 
         Command::Database(database) => match database.command {

sqlx-cli/src/migrate.rs

@@ -42,6 +42,11 @@ pub async fn add(
 ) -> anyhow::Result<()> {
     fs::create_dir_all(migration_source).context("Unable to create migrations directory")?;
 
+    // if the migrations directory is empty
+    let has_existing_migrations = fs::read_dir(migration_source)
+        .map(|mut dir| dir.next().is_some())
+        .unwrap_or(false);
+
     let migrator = Migrator::new(Path::new(migration_source)).await?;
     // This checks if all existing migrations are of the same type as the reverisble flag passed
     for migration in migrator.iter() {
@@ -74,6 +79,31 @@ pub async fn add(
         )?;
     }
 
+    if !has_existing_migrations {
+        let quoted_source = if migration_source != "migrations" {
+            format!("{:?}", migration_source)
+        } else {
+            "".to_string()
+        };
+
+        print!(
+            r#"
+Congratulations on creating your first migration!
+
+Did you know you can embed your migrations in your application binary?
+On startup, after creating your database connection or pool, add:
+
+sqlx::migrate!({}).run(<&your_pool OR &mut your_connection>).await?;
+
+Note that the compiler won't pick up new migrations if no Rust source files have changed.
+You can create a Cargo build script to work around this with `sqlx migrate build-script`.
+
+See: https://docs.rs/sqlx/0.5/sqlx/macro.migrate.html
+"#,
+            quoted_source
+        );
+    }
+
     Ok(())
 }
 
@@ -245,3 +275,30 @@ pub async fn revert(
 
     Ok(())
 }
+
+pub fn build_script(migration_source: &str, force: bool) -> anyhow::Result<()> {
+    anyhow::ensure!(
+        Path::new("Cargo.toml").exists(),
+        "must be run in a Cargo project root"
+    );
+
+    anyhow::ensure!(
+        (force || !Path::new("build.rs").exists()),
+        "build.rs already exists; use --force to overwrite"
+    );
+
+    let contents = format!(
+        r#"// generated by `sqlx migrate build-script`
+fn main() {{
+    // trigger recompilation when a new migration is added
+    println!("cargo:rerun-if-changed={}");
+}}"#,
+        migration_source
+    );
+
+    fs::write("build.rs", contents)?;
+
+    println!("Created `build.rs`; be sure to check it into version control!");
+
+    Ok(())
+}

sqlx-cli/src/opt.rs

@@ -130,4 +130,13 @@ pub enum MigrateCommand {
 
     /// List all available migrations.
     Info,
+
+    /// Generate a `build.rs` to trigger recompilation when a new migration is added.
+    ///
+    /// Must be run in a Cargo project root.
+    BuildScript {
+        /// Overwrite the build script if it already exists.
+        #[clap(long)]
+        force: bool,
+    },
 }

src/macros.rs

@@ -745,6 +745,8 @@ macro_rules! query_file_scalar_unchecked (
 /// }
 /// ```
 ///
+/// You can also do this automatically by running `sqlx migrate build-script`.
+///
 /// See: [The Cargo Book: 3.8 Build Scripts; Outputs of the Build Script](https://doc.rust-lang.org/stable/cargo/reference/build-scripts.html#outputs-of-the-build-script)
 ///
 /// #### Nightly Rust: `cfg` Flag