| 1 | const std = @import("std"); |
| 2 | const expect = std.testing.expect; |
| 3 | const expectError = std.testing.expectError; |
| 4 | |
| 5 | fn isFieldOptional(comptime T: type, field_index: usize) !bool { |
| 6 | const field_types = @typeInfo(T).@"struct".field_types; |
| 7 | return switch (field_index) { |
| 8 | // This prong is analyzed twice with `idx` being a |
| 9 | // comptime-known value each time. |
| 10 | inline 0, 1 => |idx| @typeInfo(field_types[idx]) == .optional, |
| 11 | else => return error.IndexOutOfBounds, |
| 12 | }; |
| 13 | } |
| 14 | |
| 15 | const Struct1 = struct { a: u32, b: ?u32 }; |
| 16 | |
| 17 | test "using @typeInfo with runtime values" { |
| 18 | var index: usize = 0; |
| 19 | try expect(!try isFieldOptional(Struct1, index)); |
| 20 | index += 1; |
| 21 | try expect(try isFieldOptional(Struct1, index)); |
| 22 | index += 1; |
| 23 | try expectError(error.IndexOutOfBounds, isFieldOptional(Struct1, index)); |
| 24 | } |
| 25 | |
| 26 | // Calls to `isFieldOptional` on `Struct1` get unrolled to an equivalent |
| 27 | // of this function: |
| 28 | fn isFieldOptionalUnrolled(field_index: usize) !bool { |
| 29 | return switch (field_index) { |
| 30 | 0 => false, |
| 31 | 1 => true, |
| 32 | else => return error.IndexOutOfBounds, |
| 33 | }; |
| 34 | } |
| 35 | |
| 36 | // test |