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disallow readStruct for packed structs #21562

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disallow readStruct for packed structs #21562

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4 changes: 3 additions & 1 deletion lib/std/io/Reader.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -326,7 +326,9 @@ pub fn isBytes(self: Self, slice: []const u8) anyerror!bool {

pub fn readStruct(self: Self, comptime T: type) anyerror!T {
// Only extern and packed structs have defined in-memory layout.
comptime assert(@typeInfo(T).@"struct".layout != .auto);
// Packed structs may have padding due to alignment of the backing integer.
// Therefore, only extern structs are allowed.
comptime assert(@typeInfo(T).@"struct".layout == .@"extern");
Comment on lines +329 to +331
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Suggested change
// Packed structs may have padding due to alignment of the backing integer.
// Therefore, only extern structs are allowed.
comptime assert(@typeInfo(T).@"struct".layout == .@"extern");
comptime assert(@typeInfo(T) == .@"struct");
comptime assert(std.meta.hasUniqueRepresentation(T));

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Regular structs do not have a defined field order and the order may be changed by the compiler. This is not a sound suggestion as it will break when such happens.

var res: [1]T = undefined;
try self.readNoEof(mem.sliceAsBytes(res[0..]));
return res[0];
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