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Using mem.writeInt at comptime can change the type of a structs field #1866

Hejsil opened this Issue Jan 3, 2019 · 1 comment


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Hejsil commented Jan 3, 2019

const std = @import("std");
const debug = std.debug;
const mem = std.mem;

const Bytes = struct {
    bytes: [4]u8,

    pub fn init(v: u32) Bytes {
        var res: Bytes = undefined;
        mem.writeIntLittle(u32, &res.bytes, v);

        return res;

test "" {
    const numLittle = comptime Bytes.init(0x12345678);
    debug.assert(mem.eql(u8, []u8{ 0x78, 0x56, 0x34, 0x12 }, numLittle.bytes));
test.zig:18:71: error: expected type '[]const u8', found 'u32'
    debug.assert(mem.eql(u8, []u8{ 0x78, 0x56, 0x34, 0x12 }, numLittle.bytes));

@andrewrk andrewrk added this to the 0.4.0 milestone Jan 3, 2019

@andrewrk andrewrk added the bug label Jan 3, 2019


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andrewrk commented Jan 3, 2019

I'm guessing that some analyze code incorrectly used the type of the ConstExprVal rather than the type from the type system. Recently the codebase has moved toward the type of ConstExprVal being allowed to disagree with the type system type (which represents a comptime pointer cast). So this introduced new bugs. On the plus side, more advanced kinds of pointer casting are now working at comptime.

@Hejsil Hejsil referenced this issue Jan 6, 2019


Fix #1866 #1874

andrewrk added a commit that referenced this issue Jan 6, 2019

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