fix(duckdb): unbreak the build, make --engine duckdb reachable, and green up clippy

src/engine/duckdb/mod.rs

@@ -39,7 +39,13 @@ pub struct DuckDbEngine;
 /// Default schema used when no `--schema` is provided.
 const DEFAULT_SCHEMA: &str = "main";
 
+// `DatabaseEngine` declares these methods as returning futures, so each impl must be
+// `async` whether or not its body awaits. The DuckDB driver is synchronous, so all three
+// methods here have no `.await` and trip `clippy::unused_async` -- a lint that cannot be
+// satisfied without hand-rolling `impl Future` bodies purely to appease it. The
+// `postgres` and `mysql` impls are unaffected because their drivers genuinely await.
 impl DatabaseEngine for DuckDbEngine {
+    #[allow(clippy::unused_async, clippy::unused_async_trait_impl)]
     async fn validate_connection(config: &ConnectionConfig) -> Result<ConnectionInfo> {
         let file_path = extract_file_path(config)?;
         let conn = open_connection(&file_path)?;
@@ -60,6 +66,7 @@ impl DatabaseEngine for DuckDbEngine {
         })
     }
 
+    #[allow(clippy::unused_async, clippy::unused_async_trait_impl)]
     async fn introspect(
         config: &ConnectionConfig,
         operation: &IntrospectOperation,
@@ -97,6 +104,7 @@ impl DatabaseEngine for DuckDbEngine {
         Ok(result)
     }
 
+    #[allow(clippy::unused_async, clippy::unused_async_trait_impl)]
     async fn execute(
         config: &ConnectionConfig,
         query: &str,
@@ -810,6 +818,11 @@ fn duckdb_value_to_json(value: &Value) -> serde_json::Value {
             Json::Object(obj)
         }
         Value::Union(inner) => duckdb_value_to_json(inner),
+        // `duckdb::types::Value` is #[non_exhaustive] as of duckdb 1.10505.0, so a
+        // wildcard is required. Rendering the Debug form keeps an unknown future
+        // variant VISIBLE in the JSON rather than silently nulling it out; the
+        // compiler's suggested `todo!()` would panic inside a live MCP server.
+        other => Json::String(format!("{other:?}")),
     }
 }
 

src/engine/sqlite/mod.rs

@@ -31,7 +31,13 @@ use crate::error::{PlenumError, Result};
 /// `SQLite` database engine implementation
 pub struct SqliteEngine;
 
+// `DatabaseEngine` declares these methods as returning futures, so each impl must be
+// `async` whether or not its body awaits. The SQLite driver is synchronous, so all three
+// methods here have no `.await` and trip `clippy::unused_async` -- a lint that cannot be
+// satisfied without hand-rolling `impl Future` bodies purely to appease it. The
+// `postgres` and `mysql` impls are unaffected because their drivers genuinely await.
 impl DatabaseEngine for SqliteEngine {
+    #[allow(clippy::unused_async, clippy::unused_async_trait_impl)]
     async fn validate_connection(config: &ConnectionConfig) -> Result<ConnectionInfo> {
         // Validate config is for SQLite
         if config.engine != DatabaseType::SQLite {
@@ -74,6 +80,7 @@ impl DatabaseEngine for SqliteEngine {
         })
     }
 
+    #[allow(clippy::unused_async, clippy::unused_async_trait_impl)]
     async fn introspect(
         config: &ConnectionConfig,
         operation: &IntrospectOperation,
@@ -149,6 +156,7 @@ impl DatabaseEngine for SqliteEngine {
         Ok(result)
     }
 
+    #[allow(clippy::unused_async, clippy::unused_async_trait_impl)]
     async fn execute(
         config: &ConnectionConfig,
         query: &str,

src/main.rs

@@ -69,7 +69,7 @@ enum Commands {
         project_path: Option<String>,
 
         /// Database engine
-        #[arg(long, value_parser = ["postgres", "mysql", "sqlite"])]
+        #[arg(long, value_parser = ["postgres", "mysql", "sqlite", "duckdb"])]
         engine: Option<String>,
 
         /// Host (postgres/mysql)
@@ -108,7 +108,7 @@ enum Commands {
         #[arg(long)]
         database: Option<String>,
 
-        /// `SQLite` file path
+        /// `SQLite` / `DuckDB` file path
         #[arg(long)]
         file: Option<PathBuf>,
 
@@ -153,7 +153,7 @@ enum Commands {
         project_path: Option<String>,
 
         /// Engine override
-        #[arg(long, value_parser = ["postgres", "mysql", "sqlite"])]
+        #[arg(long, value_parser = ["postgres", "mysql", "sqlite", "duckdb"])]
         engine: Option<String>,
 
         /// Host override
@@ -280,7 +280,7 @@ enum Commands {
         project_path: Option<String>,
 
         /// Engine override
-        #[arg(long, value_parser = ["postgres", "mysql", "sqlite"])]
+        #[arg(long, value_parser = ["postgres", "mysql", "sqlite", "duckdb"])]
         engine: Option<String>,
 
         /// Host override