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const std = @import("std");
const expect = std.testing.expect;
const expectError = std.testing.expectError;
fn isFieldOptional(comptime T: type, field_index: usize) !bool {
const fields = @typeInfo(T).@"struct".fields;
return switch (field_index) {
// This prong is analyzed twice with `idx` being a
// comptime-known value each time.
inline 0, 1 => |idx| @typeInfo(fields[idx].type) == .optional,
else => return error.IndexOutOfBounds,
};
}
const Struct1 = struct { a: u32, b: ?u32 };
test "using @typeInfo with runtime values" {
var index: usize = 0;
try expect(!try isFieldOptional(Struct1, index));
index += 1;
try expect(try isFieldOptional(Struct1, index));
index += 1;
try expectError(error.IndexOutOfBounds, isFieldOptional(Struct1, index));
}
// Calls to `isFieldOptional` on `Struct1` get unrolled to an equivalent
// of this function:
fn isFieldOptionalUnrolled(field_index: usize) !bool {
return switch (field_index) {
0 => false,
1 => true,
else => return error.IndexOutOfBounds,
};
}
// test
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