ziglang
Regression with "Unable to evaluate constant expression" in len of fixed size arrays
Zig Version
0.11.0-dev.2571+31738de28
Steps to Reproduce and Observed Behavior
Run zig build-exe repro.zig:
// repro.zig
const std = @import("std");
pub fn main() anyerror!void {
var hasher = std.hash.Wyhash.init(0);
var remain: []const u8 = "hello world!";
var buf: [hasher.buf.len]u8 = undefined;
std.debug.print("buf {s} {s}", .{ &buf, remain });
}
❯ zig build-exe repro.zig
/Users/jarred/Desktop/repro.zig:5:21: error: unable to evaluate comptime expression
var buf: [hasher.buf.len]u8 = undefined;
~~~~~~^~~~
referenced by:
comptime_0: /Users/jarred/zig/0.11.0-dev.2571+31738de28/files/lib/std/start.zig:66:50
remaining reference traces hidden; use '-freference-trace' to see all reference traces
hasher.buf.len is comptime known because it is a fixed size array and therefore can be determined by the type definition of the Wyhash struct's buf field.
pub const Wyhash = struct {
state: WyhashStateless,
buf: [32]u8,
// ^
buf_len: usize,
// ...
Similarly, this fails to compile:
pub const Wyhash = struct {
buf: [32]u8 = undefined,
};
const std = @import("std");
pub fn main() anyerror!void {
var remain: []const u8 = "hello world!";
var hasher = Wyhash{};
var buf: [hasher.buf.len]u8 = undefined;
std.debug.print("buf {s} {s}", .{ &buf, remain });
}
Expected Behavior
Compile successfully
This worked until at least 0.11.0-dev.2371+a31450375, which means it is a regression.
Yes caused by that and yes related to that - it happens when you have another layer of field access before len. Here's an example of code that didn't work (at top-level!) prior to the change:
const S = struct {
arr: [5]u8 = undefined,
};
var x: S = .{};
comptime {
_ = x.arr.len;
}
Because function-level comptime now works properly, the same issue manifests itself there.
Workaround:
comptime {
// broken
_ = s.arr.len;
// works
_ = @typeInfo(std.meta.FieldType(@TypeOf(s), .arr)).Array.len;
}
Would love to see this fixed, rooting for #15287.
