postgres: Use $USER as default for the username

Author: Nilix007

sqlx-core/src/postgres/connection.rs

@@ -110,8 +110,12 @@ pub struct PgConnection {
 
 // https://www.postgresql.org/docs/12/protocol-flow.html#id-1.10.5.7.3
 async fn startup(stream: &mut PgStream, url: &Url) -> crate::Result<BackendKeyData> {
-    // Defaults to postgres@.../postgres
-    let username = url.username().unwrap_or(Cow::Borrowed("postgres"));
+    // Defaults to $USER@.../$USER
+    // and falls back to postgres@.../postgres
+    let username = url
+        .username()
+        .or_else(|| std::env::var("USER").map(Cow::Owned).ok())
+        .unwrap_or(Cow::Borrowed("postgres"));
     let database = url.database().unwrap_or(&username);
 
     // See this doc for more runtime parameters