std.builtin.Type: improve ergonomics of *const anyopaque fields

For representing struct field default values and array/pointer type
sentinel values, we use `*const anyopaque`, since there is no way for
`std.builtin.Type.StructField` etc to refer back to its `type` field.
However, when introspecting a type, this is quite awkward due to the
pointer casts necessary.

As such, this commit renames the `sentinel` fields to `sentinel_ptr`,
and the `default_value` field to `default_value_ptr`, and introduces
helper methods `sentinel()` and `defaultValue()` to load the values.
These methods are marked as `inline` because their return value, which
is always comptime-known, is very often required at comptime by use
sites, so this avoids having to annotate such calls with `comptime`.

This is a breaking change, although note that 0.14.0 is already a
breaking release for all users of `std.builtin.Type` due to the union
fields being renamed.

lib/std/builtin.zig

@@ -607,8 +607,16 @@ pub const Type = union(enum) {
 
         /// The type of the sentinel is the element type of the pointer, which is
         /// the value of the `child` field in this struct. However there is no way
-        /// to refer to that type here, so we use pointer to `anyopaque`.
-        sentinel: ?*const anyopaque,
+        /// to refer to that type here, so we use `*const anyopaque`.
+        /// See also: `sentinel`
+        sentinel_ptr: ?*const anyopaque,
+
+        /// Loads the pointer type's sentinel value from `sentinel_ptr`.
+        /// Returns `null` if the pointer type has no sentinel.
+        pub inline fn sentinel(comptime ptr: Pointer) ?ptr.child {
+            const sp: *const ptr.child = @ptrCast(@alignCast(ptr.sentinel_ptr orelse return null));
+            return sp.*;
+        }
 
         /// This data structure is used by the Zig language code generation and
         /// therefore must be kept in sync with the compiler implementation.
@@ -628,8 +636,16 @@ pub const Type = union(enum) {
 
         /// The type of the sentinel is the element type of the array, which is
         /// the value of the `child` field in this struct. However there is no way
-        /// to refer to that type here, so we use pointer to `anyopaque`.
-        sentinel: ?*const anyopaque,
+        /// to refer to that type here, so we use `*const anyopaque`.
+        /// See also: `sentinel`.
+        sentinel_ptr: ?*const anyopaque,
+
+        /// Loads the array type's sentinel value from `sentinel_ptr`.
+        /// Returns `null` if the array type has no sentinel.
+        pub inline fn sentinel(comptime arr: Array) ?arr.child {
+            const sp: *const arr.child = @ptrCast(@alignCast(arr.sentinel_ptr orelse return null));
+            return sp.*;
+        }
     };
 
     /// This data structure is used by the Zig language code generation and
@@ -645,9 +661,20 @@ pub const Type = union(enum) {
     pub const StructField = struct {
         name: [:0]const u8,
         type: type,
-        default_value: ?*const anyopaque,
+        /// The type of the default value is the type of this struct field, which
+        /// is the value of the `type` field in this struct. However there is no
+        /// way to refer to that type here, so we use `*const anyopaque`.
+        /// See also: `defaultValue`.
+        default_value_ptr: ?*const anyopaque,
         is_comptime: bool,
         alignment: comptime_int,
+
+        /// Loads the field's default value from `default_value_ptr`.
+        /// Returns `null` if the field has no default value.
+        pub inline fn defaultValue(comptime sf: StructField) ?sf.type {
+            const dp: *const sf.type = @ptrCast(@alignCast(sf.default_value_ptr orelse return null));
+            return dp.*;
+        }
     };
 
     /// This data structure is used by the Zig language code generation and