feat(core): add TypeInfo, Database::TypeInfo, Database::TypeId, and Column::type_info

sqlx-core/src/column.rs

@@ -1,7 +1,15 @@
+use crate::Database;
+
+/// Represents a column from a query.
 pub trait Column {
-    /// Returns the name or alias of the column.
+    type Database: Database;
+
+    /// Returns the name of the column.
     fn name(&self) -> &str;
 
-    /// Returns the ordinal (also known as the index) of the column.
+    /// Returns the (zero-based) position of the column.
     fn ordinal(&self) -> usize;
+
+    /// Returns type information of the column.
+    fn type_info(&self) -> &<Self::Database as Database>::TypeInfo;
 }

sqlx-core/src/database.rs

@@ -1,6 +1,7 @@
 use std::fmt::Debug;
+use std::hash::Hash;
 
-use crate::{Column, QueryResult, Row};
+use crate::{Column, QueryResult, Row, TypeInfo};
 
 /// A database driver.
 ///
@@ -12,7 +13,7 @@ pub trait Database:
     'static + Sized + Debug + for<'x> HasOutput<'x> + for<'r> HasRawValue<'r>
 {
     /// The concrete [`Column`] implementation for this database.
-    type Column: Column;
+    type Column: Column<Database = Self>;
 
     /// The concrete [`Row`] implementation for this database.
     type Row: Row<Database = Self>;
@@ -21,10 +22,10 @@ pub trait Database:
     type QueryResult: QueryResult;
 
     /// The concrete [`TypeInfo`] implementation for this database.
-    type TypeInfo;
+    type TypeInfo: TypeInfo<Database = Self>;
 
     /// The concrete [`TypeId`] implementation for this database.
-    type TypeId;
+    type TypeId: PartialEq + Hash + Clone + Copy;
 }
 
 /// Associates [`Database`] with an `Output` of a generic lifetime.

sqlx-core/src/type_info.rs

@@ -0,0 +1,38 @@
+use crate::Database;
+
+/// Provides information about a SQL type.
+pub trait TypeInfo {
+    type Database: Database;
+
+    /// Returns the unique identifier for this SQL type.
+    fn id(&self) -> <Self::Database as Database>::TypeId;
+
+    /// Returns `true` if the database could not determine the actual type.
+    ///
+    /// Most commonly this occurs in cases where `NULL` is directly used in
+    /// an expression.
+    ///
+    fn is_unknown(&self) -> bool {
+        false
+    }
+
+    /// Returns `true` if this is a zero-sized type intended to never hold
+    /// a value, such as `void` in C.
+    ///
+    /// PostgreSQL can return this type for simple function expressions
+    /// where the function has no return type.
+    ///
+    fn is_void(&self) -> bool {
+        false
+    }
+
+    /// Returns the name of this SQL type for this database.
+    ///
+    /// Length specifiers will not be included. Only the basename will
+    /// be returned. This should be a rough approximation of how they are
+    /// written in SQL in the given database.
+    ///
+    /// Common type names include `VARCHAR`, `INTEGER`, and `BIGINT`.
+    ///
+    fn name(&self) -> &str;
+}