make all garage_db::Engine variants un-conditional

Having all Engine enum variants conditional causes compilation errors
when *none* of the DB engine features is enabled. This is not an issue
for full garage build, but affects crates that use garage_db as
dependency.

Change all variants to be present at all times. It solves compilation
errors and also allows us to better differentiate between invalid DB
engine name and engine with support not compiled in current binary.

src/db/lib.rs

@@ -172,30 +172,31 @@ impl Db {
 }
 
 /// List of supported database engine types
+///
+/// The `enum` holds list of *all* database engines that are are be supported by crate, no matter
+/// if relevant feature is enabled or not. It allows us to distinguish between invalid engine
+/// and valid engine, whose support is not enabled via feature flag.
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum Engine {
-	#[cfg(feature = "lmdb")]
 	Lmdb,
-
-	#[cfg(feature = "sqlite")]
 	Sqlite,
-
-	#[cfg(feature = "sled")]
 	Sled,
 }
 
+impl Engine {
+	/// Return variant name as static `&str`
+	pub fn as_str(&self) -> &'static str {
+		match self {
+			Self::Lmdb => "lmdb",
+			Self::Sqlite => "sqlite",
+			Self::Sled => "sled",
+		}
+	}
+}
+
 impl std::fmt::Display for Engine {
 	fn fmt(&self, fmt: &mut std::fmt::Formatter) -> std::fmt::Result {
-		match self {
-			#[cfg(feature = "lmdb")]
-			Self::Lmdb => fmt.write_str("lmdb"),
-
-			#[cfg(feature = "sqlite")]
-			Self::Sqlite => fmt.write_str("sqlite"),
-
-			#[cfg(feature = "sled")]
-			Self::Sled => fmt.write_str("sled"),
-		}
+		self.as_str().fmt(fmt)
 	}
 }
 
@@ -204,15 +205,9 @@ impl std::str::FromStr for Engine {
 
 	fn from_str(text: &str) -> Result<Engine> {
 		match text {
-			#[cfg(feature = "lmdb")]
 			"lmdb" | "heed" => Ok(Self::Lmdb),
-
-			#[cfg(feature = "sqlite")]
 			"sqlite" | "sqlite3" | "rusqlite" => Ok(Self::Sqlite),
-
-			#[cfg(feature = "sled")]
 			"sled" => Ok(Self::Sled),
-
 			kind => Err(Error(format!("Invalid DB engine: {}", kind).into())),
 		}
 	}

src/garage/cli/convert_db.rs

@@ -91,5 +91,13 @@ fn open_db(path: PathBuf, engine: Engine, open: &OpenDbOpt) -> Result<Db> {
 			let db = env_builder.open(&path)?;
 			Ok(lmdb_adapter::LmdbDb::init(db))
 		}
+
+		// Pattern is unreachable when all supported DB engines are compiled into binary. The allow
+		// attribute is added so that we won't have to change this match in case stop building
+		// support for one or more engines by default.
+		#[allow(unreachable_patterns)]
+		engine => Err(Error(
+			format!("Engine support not available in this build: {}", engine).into(),
+		)),
 	}
 }