Add support for crate specific .env files

This change will attempt to load an .env file from CARGO_MANIFEST_DIR, if it exists.

For backwards compatibility, if the .env file does not exist, we will fall back to default dotenv behaviour.

Resolves launchbadge#267

sqlx-macros/src/lib.rs

@@ -60,6 +60,17 @@ macro_rules! async_macro (
         let res: Result<proc_macro2::TokenStream> = block_on(async {
             use sqlx::connection::Connect;
 
+            // If a .env file exists at CARGO_MANIFEST_DIR, load environment variables from this,
+            // otherwise fallback to default dotenv behaviour.
+            if let Ok(env_path) = std::env::var("CARGO_MANIFEST_DIR")
+                .map(|dir| std::path::PathBuf::from(dir).join(".env"))
+            {
+                if env_path.exists() {
+                    dotenv::from_path(env_path.as_path())
+                        .map_err(|e| format!("failed to load environment from {:?}, {}", env_path, e))?
+                }
+            }
+
             let db_url = Url::parse(&dotenv::var("DATABASE_URL").map_err(|_| "DATABASE_URL not set")?)?;
 
             match db_url.scheme() {