macros + sqlite: fix error for null-typed columns

sqlx-core/src/mysql/types/mod.rs

@@ -106,6 +106,10 @@ impl TypeInfo for MySqlTypeInfo {
             _ => self.id.0 == other.id.0 && self.is_unsigned == other.is_unsigned,
         }
     }
+
+    fn is_null_type(&self) -> bool {
+        self.id == TypeId::NULL
+    }
 }
 
 impl<'de, T> Decode<'de, MySql> for Option<T>

sqlx-core/src/postgres/types/mod.rs

@@ -81,6 +81,11 @@ impl TypeInfo for PgTypeInfo {
         // TODO: 99% of postgres types are direct equality for [compatible]; when we add something that isn't (e.g, JSON/JSONB), fix this here
         self.id.0 == other.id.0
     }
+
+    fn is_null_type(&self) -> bool {
+        // Postgres doesn't have a "null" type
+        false
+    }
 }
 
 impl<'de, T> Decode<'de, Postgres> for Option<T>

sqlx-core/src/sqlite/types/mod.rs

@@ -66,6 +66,10 @@ impl TypeInfo for SqliteTypeInfo {
     fn compatible(&self, other: &Self) -> bool {
         self.affinity == other.affinity
     }
+
+    fn is_null_type(&self) -> bool {
+        self.r#type == SqliteType::Null
+    }
 }
 
 impl<'de, T> Decode<'de, Sqlite> for Option<T>

sqlx-core/src/types.rs

@@ -18,6 +18,14 @@ pub trait TypeInfo: Debug + Display + Clone {
     /// Compares type information to determine if `other` is compatible at the Rust level
     /// with `self`.
     fn compatible(&self, other: &Self) -> bool;
+
+    /// Return `true` if this is the database flavor's "null" sentinel type.
+    ///
+    /// For type info coming from the description of a prepared statement, this means
+    /// that the server could not infer the expected type of a bind parameter or result column;
+    /// the latter is most often the case with columns that are the result of an expression
+    /// in a weakly-typed database like MySQL or SQLite.
+    fn is_null_type(&self) -> bool;
 }
 
 /// Indicates that a SQL type is supported for a database.