Convert unused_tuple_struct_fields to warn-by-default

compiler/rustc_lint_defs/src/builtin.rs

@@ -611,7 +611,6 @@ declare_lint! {
     /// ### Example
     ///
     /// ```rust
-    /// #[warn(unused_tuple_struct_fields)]
     /// struct S(i32, i32, i32);
     /// let s = S(1, 2, 3);
     /// let _ = (s.0, s.2);
@@ -626,7 +625,7 @@ declare_lint! {
     /// removing the unused field(s) or, to preserve the numbering of the
     /// remaining fields, change the unused field(s) to have unit type.
     pub UNUSED_TUPLE_STRUCT_FIELDS,
-    Allow,
+    Warn,
     "detects tuple struct fields that are never read"
 }