Zig 0.16.0

zigbreaking.md

Zig 0.15.x

Language / type system

• usingnamespace keyword removed entirely; refactor to explicit imports / conditional expressions (the mixin pattern now uses zero-bit fields plus @fieldParentPtr("name", m)).
• async and await keywords removed; @frameSize builtin removed. To be replaced by stdlib I/O-based implementations.
• Packed union fields no longer accept align attributes (matches packed structs).
• New rules for undefined: only operators that cannot trigger illegal behavior accept undefined operands; arithmetic on undefined is now a compile error / illegal behavior.
• Integer-to-float coercion is a compile error at comptime when precision would be lost; integer literals must be written as float literals (e.g. 123_456_789 → 123_456_789.0).
• Inline assembly clobber lists changed from string syntax to typed struct syntax (zig fmt auto-upgrades).
• @ptrCast extended to allow single-item-pointer → slice (slice byte count matches operand).
• Boolean vector operators (!, &, |, ^, and/or semantics) now defined on bool vectors.

Standard library

• "Writergate" — std.io reader/writer interfaces overhauled into std.Io:
  • std.io.GenericReader / std.io.AnyReader → std.Io.Reader
  • std.io.GenericWriter / std.io.AnyWriter → std.Io.Writer
  • std.io.SeekableStream removed; use concrete std.fs.File.Reader / std.fs.File.Writer.
  • std.io.BitReader, std.io.BitWriter removed.
  • std.Io.LimitedReader, std.Io.BufferedReader removed.
  • std.io.CountingWriter removed; use std.Io.Writer.Discarding or std.Io.Writer.Allocating.
  • std.io.BufferedWriter removed; pass an explicit buffer to a file writer.
  • std.fifo module removed entirely (std.fifo.LinearFifo gone).
  • std.RingBuffer removed (replaced by I/O stream ring buffers).
  • std.fs.File.reader() / .writer() → .deprecatedReader() / .deprecatedWriter(); new .reader(buffer) / .writer(buffer) require a caller-provided buffer and explicit flush().
  • std.fs.File.WriteFileOptions, std.fs.File.writeFileAll, std.fs.File.writeFileAllUnseekable removed.
  • std.fs.Dir.atomicFile now requires write_buffer in options; fs.AtomicFile.File field replaced with File.Writer.
  • std.posix.sendfile removed in favor of std.fs.File.Reader.sendFile.
  • std.fs.Dir.copyFile no longer returns error.OutOfMemory.
• Formatting overhaul:
  • format method signature changed from fn format(self, comptime fmt: []const u8, options: std.fmt.FormatOptions, writer: anytype) !void to fn format(self, writer: *std.Io.Writer) std.Io.Writer.Error!void.
  • {} no longer calls a type's format method; you must use {f}.
  • std.fmt.FormatOptions removed; alignment over Unicode codepoints removed (ASCII/bytes only).
  • std.fmt.format → std.Io.Writer.print.
  • std.fmt.Formatter → std.fmt.Alt (requires explicit context type).
  • std.fmt.fmtSliceHexLower / fmtSliceHexUpper removed → use {x} / {X}.
  • std.fmt.fmtSliceEscapeLower / fmtSliceEscapeUpper → std.ascii.hexEscape.
  • std.fmt.fmtIntSizeDec / fmtIntSizeBin removed → use {B} / {Bi}.
  • std.fmt.fmtDuration / fmtDurationSigned removed → use {D}.
  • std.zig.fmtEscapes → std.zig.fmtString.
  • New specifiers: {t} (tag name), {b64} (base64).
• Collections — ArrayList and friends migrated to unmanaged-by-default:
  • std.ArrayList(T) (managed) → std.array_list.Managed(T); the unmanaged variant is now the default std.ArrayList.
  • std.ArrayListAligned → std.array_list.AlignedManaged.
  • std.BoundedArray removed; use ArrayListUnmanaged with a fixed buffer.
  • std.DoublyLinkedList(T) de-genericified: nodes are now std.DoublyLinkedList.Node embedded in user structs and traversed with @fieldParentPtr.
  • std.compress.flate.CircularBuffer removed.
• Compression: std.compress.flate.Decompress now takes a container parameter; higher-level compression and checksum helpers removed (do checksums out-of-band, use third-party packages for compression).

Build system / toolchain

• Deprecated implicit root-module fields on std.Build.ExecutableOptions (and siblings) removed, including root_source_file; must use the root_module field introduced in 0.14.0.
• std.Build.Step.Compile.sanitize_c changed type from ?bool to ?std.zig.SanitizeC; replace true/false with .full/.off.
• -fsanitize-c split into -fsanitize-c=full and -fsanitize-c=trap.
• x86_64 self-hosted backend is now the default for Debug mode (except NetBSD, OpenBSD, Windows, which still default to LLVM).
• New CLI: zig test-obj (compile tests to an object file instead of an executable).
• LLVM bumped to 20.1.8.
• zig cc: -static and -dynamic now respected.
• zig objcopy regressed; some subcommands now return "unimplemented".

Sources:

• 0.15.1 Release Notes

Zig 0.16.0

Language / type system

• @Type builtin removed. Replaced by @Int, @Tuple, @Pointer, @Fn, @Struct, @Union, @Enum, @EnumLiteral.
• @cImport deprecated. Use the build system: b.addTranslateC(...).
• @intFromFloat deprecated. @floor, @ceil, @round, @trunc now convert directly to integer types.
• Unary float builtins (@sqrt, @sin, @floor, …) forward result types into earlier operations.
• Integer → float coercion is implicit when all values fit losslessly.
• Runtime indexing of vectors with non-comptime values is forbidden — coerce to array first.
• Array ↔ vector in-memory coercion removed.
• Returning the address of a local variable is now a compile error (returning address of expired local variable).
• *u8 and *align(1) u8 are now distinct types (they coerce, but are not identical).
• Pointers to comptime-only types (e.g. *comptime_int) are no longer themselves comptime-only.
• Packed structs/unions:
  • Switch on packed types compares solely on the backing integer.
  • All packed-union fields must have identical @bitSizeOf matching a backing integer.
  • Pointers are forbidden as fields of packed structs/unions.
  • Packed unions accept explicit backing-integer syntax: packed union(u16) { … }.
  • Packed structs/unions with inferred backing integer, and enums with inferred tag types, are no longer valid extern types.
• Lazy field analysis: structs/unions/enums/opaques only resolve when size or field types are required — affects code that probed types via @typeInfo.
• Zero-bit tuple fields no longer auto-promote to comptime fields.
• New dependency-loop cases; error messages explain the loop.

std.Io — the big one

Anything that "potentially blocks control flow or introduces nondeterminism" now lives behind a std.Io instance.

• File system fully migrated std.fs → std.Io.Dir / std.Io.File:
  • fs.Dir → std.Io.Dir
  • fs.File → std.Io.File
  • fs.File.Mode → std.Io.File.Permissions
  • fs.cwd → std.Io.Dir.cwd
  • fs.realpath → std.Io.Dir.realPathFileAbsolute
  • fs.Dir.makeDir → std.Io.Dir.createDir
  • fs.Dir.makePath → std.Io.Dir.createDirPath
  • fs.Dir.makeOpenDir → std.Io.Dir.createDirPathOpen
  • fs.Dir.rename now takes two Dir parameters; returns error.DirNotEmpty instead of error.PathAlreadyExists.
  • fs.Dir.chmod → std.Io.Dir.setPermissions
  • fs.Dir.chown → std.Io.Dir.setOwner
  • fs.File.setEndPos → std.Io.File.setLength
  • fs.File.getEndPos → std.Io.File.length
  • fs.File.read → std.Io.File.readStreaming
  • fs.File.write → std.Io.File.writeStreaming
  • fs.File.updateTimes → std.Io.File.setTimestamps
  • All *Z and *W path variants removed (realpathZ, realpathW, makeDirAbsoluteZ, deleteDirAbsoluteZ, deleteTreeAbsolute, etc. — ~300 functions deleted).
• Time:
  • std.time.Instant → std.Io.Timestamp
  • std.time.Timer → std.Io.Timestamp
  • std.time.timestamp() → std.Io.Timestamp.now(io)
• Randomness:
  • std.crypto.random.bytes → io.random
  • std.crypto.random → std.Random.IoSource via io.interface()
  • posix.getrandom → io.random
• Synchronisation primitives all relocated under std.Io:
  • std.Thread.Mutex → std.Io.Mutex
  • std.Thread.Condition → std.Io.Condition
  • std.Thread.RwLock → std.Io.RwLock
  • std.Thread.Semaphore → std.Io.Semaphore
  • std.Thread.WaitGroup → std.Io.Group
  • std.Thread.ResetEvent → std.Io.Event
  • std.Thread.Futex → std.Io.Futex
  • std.Thread.Pool removed.
  • std.Thread.Mutex.Recursive removed.
  • std.once removed.
  • heap.ThreadSafeAllocator removed.
• Process:
  • std.process.Child.init → std.process.spawn(...) with options struct.
  • std.process.execv → std.process.replace.

Standard library — non-Io changes

• fmt.Formatter → fmt.Alt.
• fmt.format → std.Io.Writer.print.
• fmt.bufPrintZ → fmt.bufPrintSentinel.
• Removed: SegmentedList, meta.declList, Io.GenericWriter, Io.AnyWriter, Io.null_writer, Io.CountingReader.
• Error renames:
  • error.RenameAcrossMountPoints → error.CrossDevice
  • error.NotSameFileSystem → error.CrossDevice
  • error.SharingViolation → error.FileBusy
  • error.EnvironmentVariableNotFound → error.EnvironmentVariableMissing
• Debug:
  • captureStackTrace → captureCurrentStackTrace
  • dumpStackTraceFromBase → dumpCurrentStackTrace
  • writeStackTraceWindows → writeCurrentStackTrace
  • std.debug.StackIterator no longer pub.
• BitSet / EnumSet initEmpty/initFull replaced with declaration literals.
• Container migration to "unmanaged" forms continues across the std lib.
• DynLib: Windows support removed (LoadLibraryExW + GetProcAddress directly).
• std.posix almost entirely removed; OS-specific APIs gone.

Build system / toolchain

• Local package overrides in the build system.
• Project-local package fetching (instead of global cache).
• Unit-test timeouts.
• New --error-style and --multiline-errors flags.
• LLVM 21; loop vectorisation disabled to dodge a regression.
• Solaris / AIX / z/OS removed; illumos still supported.