diff --git a/lib/compiler/aro/aro/Compilation.zig b/lib/compiler/aro/aro/Compilation.zig
index d7377d7e52..159296a9a9 100644
--- a/lib/compiler/aro/aro/Compilation.zig
+++ b/lib/compiler/aro/aro/Compilation.zig
@@ -546,7 +546,7 @@ pub fn generateBuiltinMacros(comp: *Compilation, system_defines_mode: SystemDefi
     }
 
     try buf.appendSlice("#define __STDC__ 1\n");
-    try buf.writer().print("#define __STDC_HOSTED__ {d}\n", .{@intFromBool(comp.target.os.tag != .freestanding)});
+    try buf.print("#define __STDC_HOSTED__ {d}\n", .{@intFromBool(comp.target.os.tag != .freestanding)});
 
     // standard macros
     try buf.appendSlice(
diff --git a/src/arch/wasm/Emit.zig b/src/arch/wasm/Emit.zig
index d97aebe553..272d5519a6 100644
--- a/src/arch/wasm/Emit.zig
+++ b/src/arch/wasm/Emit.zig
@@ -3,7 +3,7 @@ const Emit = @This();
 const std = @import("std");
 const assert = std.debug.assert;
 const Allocator = std.mem.Allocator;
-const leb = std.leb;
+const ArrayList = std.ArrayList;
 
 const Wasm = link.File.Wasm;
 const Mir = @import("Mir.zig");
@@ -15,7 +15,7 @@ const codegen = @import("../../codegen.zig");
 mir: Mir,
 wasm: *Wasm,
 /// The binary representation that will be emitted by this module.
-code: *std.ArrayListUnmanaged(u8),
+code: *ArrayList(u8),
 
 pub const Error = error{
     OutOfMemory,
@@ -85,7 +85,7 @@ pub fn lowerToCode(emit: *Emit) Error!void {
             if (is_obj) {
                 @panic("TODO");
             } else {
-                leb.writeUleb128(code.fixedWriter(), 1 + @intFromEnum(indirect_func_idx)) catch unreachable;
+                writeUleb128(code, 1 + @intFromEnum(indirect_func_idx));
             }
             inst += 1;
             continue :loop tags[inst];
@@ -99,7 +99,7 @@ pub fn lowerToCode(emit: *Emit) Error!void {
             code.appendAssumeCapacity(@intFromEnum(std.wasm.Opcode.i32_const));
             // MIR is lowered during flush, so there is indeed only one thread at this time.
             const errors_len = 1 + comp.zcu.?.intern_pool.global_error_set.getNamesFromMainThread().len;
-            leb.writeIleb128(code.fixedWriter(), errors_len) catch unreachable;
+            writeSleb128(code, errors_len);
 
             inst += 1;
             continue :loop tags[inst];
@@ -122,7 +122,7 @@ pub fn lowerToCode(emit: *Emit) Error!void {
                 continue :loop tags[inst];
             } else {
                 const addr: u32 = wasm.errorNameTableAddr();
-                leb.writeIleb128(code.fixedWriter(), addr) catch unreachable;
+                writeSleb128(code, addr);
 
                 inst += 1;
                 continue :loop tags[inst];
@@ -131,7 +131,7 @@ pub fn lowerToCode(emit: *Emit) Error!void {
         .br_if, .br, .memory_grow, .memory_size => {
             try code.ensureUnusedCapacity(gpa, 11);
             code.appendAssumeCapacity(@intFromEnum(tags[inst]));
-            leb.writeUleb128(code.fixedWriter(), datas[inst].label) catch unreachable;
+            writeUleb128(code, datas[inst].label);
 
             inst += 1;
             continue :loop tags[inst];
@@ -140,7 +140,7 @@ pub fn lowerToCode(emit: *Emit) Error!void {
         .local_get, .local_set, .local_tee => {
             try code.ensureUnusedCapacity(gpa, 11);
             code.appendAssumeCapacity(@intFromEnum(tags[inst]));
-            leb.writeUleb128(code.fixedWriter(), datas[inst].local) catch unreachable;
+            writeUleb128(code, datas[inst].local);
 
             inst += 1;
             continue :loop tags[inst];
@@ -153,8 +153,8 @@ pub fn lowerToCode(emit: *Emit) Error!void {
             try code.ensureUnusedCapacity(gpa, 11 + 10 * labels.len);
             code.appendAssumeCapacity(@intFromEnum(std.wasm.Opcode.br_table));
             // -1 because default label is not part of length/depth.
-            leb.writeUleb128(code.fixedWriter(), extra.data.length - 1) catch unreachable;
-            for (labels) |label| leb.writeUleb128(code.fixedWriter(), label) catch unreachable;
+            writeUleb128(code, extra.data.length - 1);
+            for (labels) |label| writeUleb128(code, label);
 
             inst += 1;
             continue :loop tags[inst];
@@ -199,9 +199,9 @@ pub fn lowerToCode(emit: *Emit) Error!void {
                 code.appendNTimesAssumeCapacity(0, 5);
             } else {
                 const index: Wasm.Flush.FuncTypeIndex = .fromTypeIndex(func_ty_index, &wasm.flush_buffer);
-                leb.writeUleb128(code.fixedWriter(), @intFromEnum(index)) catch unreachable;
+                writeUleb128(code, @intFromEnum(index));
             }
-            leb.writeUleb128(code.fixedWriter(), @as(u32, 0)) catch unreachable; // table index
+            writeUleb128(code, @as(u32, 0)); // table index
 
             inst += 1;
             continue :loop tags[inst];
@@ -263,7 +263,7 @@ pub fn lowerToCode(emit: *Emit) Error!void {
                 code.appendNTimesAssumeCapacity(0, 5);
             } else {
                 const sp_global: Wasm.GlobalIndex = .stack_pointer;
-                std.leb.writeUleb128(code.fixedWriter(), @intFromEnum(sp_global)) catch unreachable;
+                writeUleb128(code, @intFromEnum(sp_global));
             }
 
             inst += 1;
@@ -291,7 +291,7 @@ pub fn lowerToCode(emit: *Emit) Error!void {
         .i32_const => {
             try code.ensureUnusedCapacity(gpa, 6);
             code.appendAssumeCapacity(@intFromEnum(std.wasm.Opcode.i32_const));
-            leb.writeIleb128(code.fixedWriter(), datas[inst].imm32) catch unreachable;
+            writeSleb128(code, datas[inst].imm32);
 
             inst += 1;
             continue :loop tags[inst];
@@ -300,7 +300,7 @@ pub fn lowerToCode(emit: *Emit) Error!void {
             try code.ensureUnusedCapacity(gpa, 11);
             code.appendAssumeCapacity(@intFromEnum(std.wasm.Opcode.i64_const));
             const int64: i64 = @bitCast(mir.extraData(Mir.Imm64, datas[inst].payload).data.toInt());
-            leb.writeIleb128(code.fixedWriter(), int64) catch unreachable;
+            writeSleb128(code, int64);
 
             inst += 1;
             continue :loop tags[inst];
@@ -476,33 +476,33 @@ pub fn lowerToCode(emit: *Emit) Error!void {
             const extra_index = datas[inst].payload;
             const opcode = mir.extra[extra_index];
             code.appendAssumeCapacity(@intFromEnum(std.wasm.Opcode.misc_prefix));
-            leb.writeUleb128(code.fixedWriter(), opcode) catch unreachable;
+            writeUleb128(code, opcode);
             switch (@as(std.wasm.MiscOpcode, @enumFromInt(opcode))) {
                 // bulk-memory opcodes
                 .data_drop => {
                     const segment = mir.extra[extra_index + 1];
-                    leb.writeUleb128(code.fixedWriter(), segment) catch unreachable;
+                    writeUleb128(code, segment);
 
                     inst += 1;
                     continue :loop tags[inst];
                 },
                 .memory_init => {
                     const segment = mir.extra[extra_index + 1];
-                    leb.writeUleb128(code.fixedWriter(), segment) catch unreachable;
-                    leb.writeUleb128(code.fixedWriter(), @as(u32, 0)) catch unreachable; // memory index
+                    writeUleb128(code, segment);
+                    writeUleb128(code, @as(u32, 0)); // memory index
 
                     inst += 1;
                     continue :loop tags[inst];
                 },
                 .memory_fill => {
-                    leb.writeUleb128(code.fixedWriter(), @as(u32, 0)) catch unreachable; // memory index
+                    writeUleb128(code, @as(u32, 0)); // memory index
 
                     inst += 1;
                     continue :loop tags[inst];
                 },
                 .memory_copy => {
-                    leb.writeUleb128(code.fixedWriter(), @as(u32, 0)) catch unreachable; // dst memory index
-                    leb.writeUleb128(code.fixedWriter(), @as(u32, 0)) catch unreachable; // src memory index
+                    writeUleb128(code, @as(u32, 0)); // dst memory index
+                    writeUleb128(code, @as(u32, 0)); // src memory index
 
                     inst += 1;
                     continue :loop tags[inst];
@@ -538,7 +538,7 @@ pub fn lowerToCode(emit: *Emit) Error!void {
             const extra_index = datas[inst].payload;
             const opcode = mir.extra[extra_index];
             code.appendAssumeCapacity(@intFromEnum(std.wasm.Opcode.simd_prefix));
-            leb.writeUleb128(code.fixedWriter(), opcode) catch unreachable;
+            writeUleb128(code, opcode);
             switch (@as(std.wasm.SimdOpcode, @enumFromInt(opcode))) {
                 .v128_store,
                 .v128_load,
@@ -824,7 +824,7 @@ pub fn lowerToCode(emit: *Emit) Error!void {
             const extra_index = datas[inst].payload;
             const opcode = mir.extra[extra_index];
             code.appendAssumeCapacity(@intFromEnum(std.wasm.Opcode.atomics_prefix));
-            leb.writeUleb128(code.fixedWriter(), opcode) catch unreachable;
+            writeUleb128(code, opcode);
             switch (@as(std.wasm.AtomicsOpcode, @enumFromInt(opcode))) {
                 .i32_atomic_load,
                 .i64_atomic_load,
@@ -900,7 +900,7 @@ pub fn lowerToCode(emit: *Emit) Error!void {
                     // Hard-codes memory index 0 since multi-memory proposal is
                     // not yet accepted nor implemented.
                     const memory_index: u32 = 0;
-                    leb.writeUleb128(code.fixedWriter(), memory_index) catch unreachable;
+                    writeUleb128(code, memory_index);
                     inst += 1;
                     continue :loop tags[inst];
                 },
@@ -915,15 +915,15 @@ pub fn lowerToCode(emit: *Emit) Error!void {
 }
 
 /// Asserts 20 unused capacity.
-fn encodeMemArg(code: *std.ArrayListUnmanaged(u8), mem_arg: Mir.MemArg) void {
+fn encodeMemArg(code: *ArrayList(u8), mem_arg: Mir.MemArg) void {
     assert(code.unusedCapacitySlice().len >= 20);
     // Wasm encodes alignment as power of 2, rather than natural alignment.
     const encoded_alignment = @ctz(mem_arg.alignment);
-    leb.writeUleb128(code.fixedWriter(), encoded_alignment) catch unreachable;
-    leb.writeUleb128(code.fixedWriter(), mem_arg.offset) catch unreachable;
+    writeUleb128(code, encoded_alignment);
+    writeUleb128(code, mem_arg.offset);
 }
 
-fn uavRefObj(wasm: *Wasm, code: *std.ArrayListUnmanaged(u8), value: InternPool.Index, offset: i32, is_wasm32: bool) !void {
+fn uavRefObj(wasm: *Wasm, code: *ArrayList(u8), value: InternPool.Index, offset: i32, is_wasm32: bool) !void {
     const comp = wasm.base.comp;
     const gpa = comp.gpa;
     const opcode: std.wasm.Opcode = if (is_wasm32) .i32_const else .i64_const;
@@ -940,7 +940,7 @@ fn uavRefObj(wasm: *Wasm, code: *std.ArrayListUnmanaged(u8), value: InternPool.I
     code.appendNTimesAssumeCapacity(0, if (is_wasm32) 5 else 10);
 }
 
-fn uavRefExe(wasm: *Wasm, code: *std.ArrayListUnmanaged(u8), value: InternPool.Index, offset: i32, is_wasm32: bool) !void {
+fn uavRefExe(wasm: *Wasm, code: *ArrayList(u8), value: InternPool.Index, offset: i32, is_wasm32: bool) !void {
     const comp = wasm.base.comp;
     const gpa = comp.gpa;
     const opcode: std.wasm.Opcode = if (is_wasm32) .i32_const else .i64_const;
@@ -949,10 +949,10 @@ fn uavRefExe(wasm: *Wasm, code: *std.ArrayListUnmanaged(u8), value: InternPool.I
     code.appendAssumeCapacity(@intFromEnum(opcode));
 
     const addr = wasm.uavAddr(value);
-    leb.writeUleb128(code.fixedWriter(), @as(u32, @intCast(@as(i64, addr) + offset))) catch unreachable;
+    writeUleb128(code, @as(u32, @intCast(@as(i64, addr) + offset)));
 }
 
-fn navRefOff(wasm: *Wasm, code: *std.ArrayListUnmanaged(u8), data: Mir.NavRefOff, is_wasm32: bool) !void {
+fn navRefOff(wasm: *Wasm, code: *ArrayList(u8), data: Mir.NavRefOff, is_wasm32: bool) !void {
     const comp = wasm.base.comp;
     const zcu = comp.zcu.?;
     const ip = &zcu.intern_pool;
@@ -975,10 +975,22 @@ fn navRefOff(wasm: *Wasm, code: *std.ArrayListUnmanaged(u8), data: Mir.NavRefOff
         code.appendNTimesAssumeCapacity(0, if (is_wasm32) 5 else 10);
     } else {
         const addr = wasm.navAddr(data.nav_index);
-        leb.writeUleb128(code.fixedWriter(), @as(u32, @intCast(@as(i64, addr) + data.offset))) catch unreachable;
+        writeUleb128(code, @as(u32, @intCast(@as(i64, addr) + data.offset)));
     }
 }
 
-fn appendOutputFunctionIndex(code: *std.ArrayListUnmanaged(u8), i: Wasm.OutputFunctionIndex) void {
-    leb.writeUleb128(code.fixedWriter(), @intFromEnum(i)) catch unreachable;
+fn appendOutputFunctionIndex(code: *ArrayList(u8), i: Wasm.OutputFunctionIndex) void {
+    writeUleb128(code, @intFromEnum(i));
+}
+
+fn writeUleb128(code: *ArrayList(u8), arg: anytype) void {
+    var w: std.Io.Writer = .fixed(code.unusedCapacitySlice());
+    w.writeUleb128(arg) catch unreachable;
+    code.items.len += w.end;
+}
+
+fn writeSleb128(code: *ArrayList(u8), arg: anytype) void {
+    var w: std.Io.Writer = .fixed(code.unusedCapacitySlice());
+    w.writeSleb128(arg) catch unreachable;
+    code.items.len += w.end;
 }
diff --git a/src/codegen.zig b/src/codegen.zig
index 91ce034966..56e6e7c99f 100644
--- a/src/codegen.zig
+++ b/src/codegen.zig
@@ -6,6 +6,7 @@ const link = @import("link.zig");
 const log = std.log.scoped(.codegen);
 const mem = std.mem;
 const math = std.math;
+const ArrayList = std.ArrayList;
 const target_util = @import("target.zig");
 const trace = @import("tracy.zig").trace;
 
@@ -179,7 +180,7 @@ pub fn emitFunction(
     src_loc: Zcu.LazySrcLoc,
     func_index: InternPool.Index,
     any_mir: *const AnyMir,
-    code: *std.ArrayListUnmanaged(u8),
+    code: *ArrayList(u8),
     debug_output: link.File.DebugInfoOutput,
 ) CodeGenError!void {
     const zcu = pt.zcu;
@@ -204,7 +205,7 @@ pub fn generateLazyFunction(
     pt: Zcu.PerThread,
     src_loc: Zcu.LazySrcLoc,
     lazy_sym: link.File.LazySymbol,
-    code: *std.ArrayListUnmanaged(u8),
+    code: *ArrayList(u8),
     debug_output: link.File.DebugInfoOutput,
 ) CodeGenError!void {
     const zcu = pt.zcu;
@@ -236,7 +237,7 @@ pub fn generateLazySymbol(
     lazy_sym: link.File.LazySymbol,
     // TODO don't use an "out" parameter like this; put it in the result instead
     alignment: *Alignment,
-    code: *std.ArrayListUnmanaged(u8),
+    code: *ArrayList(u8),
     debug_output: link.File.DebugInfoOutput,
     reloc_parent: link.File.RelocInfo.Parent,
 ) CodeGenError!void {
@@ -311,7 +312,7 @@ pub fn generateSymbol(
     pt: Zcu.PerThread,
     src_loc: Zcu.LazySrcLoc,
     val: Value,
-    code: *std.ArrayListUnmanaged(u8),
+    code: *ArrayList(u8),
     reloc_parent: link.File.RelocInfo.Parent,
 ) GenerateSymbolError!void {
     const tracy = trace(@src());
@@ -379,7 +380,7 @@ pub fn generateSymbol(
         },
         .err => |err| {
             const int = try pt.getErrorValue(err.name);
-            try code.writer(gpa).writeInt(u16, @intCast(int), endian);
+            mem.writeInt(u16, try code.addManyAsArray(gpa, 2), @intCast(int), endian);
         },
         .error_union => |error_union| {
             const payload_ty = ty.errorUnionPayload(zcu);
@@ -389,7 +390,7 @@ pub fn generateSymbol(
             };
 
             if (!payload_ty.hasRuntimeBitsIgnoreComptime(zcu)) {
-                try code.writer(gpa).writeInt(u16, err_val, endian);
+                mem.writeInt(u16, try code.addManyAsArray(gpa, 2), err_val, endian);
                 return;
             }
 
@@ -399,7 +400,7 @@ pub fn generateSymbol(
 
             // error value first when its type is larger than the error union's payload
             if (error_align.order(payload_align) == .gt) {
-                try code.writer(gpa).writeInt(u16, err_val, endian);
+                mem.writeInt(u16, try code.addManyAsArray(gpa, 2), err_val, endian);
             }
 
             // emit payload part of the error union
@@ -421,7 +422,7 @@ pub fn generateSymbol(
             // Payload size is larger than error set, so emit our error set last
             if (error_align.compare(.lte, payload_align)) {
                 const begin = code.items.len;
-                try code.writer(gpa).writeInt(u16, err_val, endian);
+                mem.writeInt(u16, try code.addManyAsArray(gpa, 2), err_val, endian);
                 const unpadded_end = code.items.len - begin;
                 const padded_end = abi_align.forward(unpadded_end);
                 const padding = math.cast(usize, padded_end - unpadded_end) orelse return error.Overflow;
@@ -476,7 +477,7 @@ pub fn generateSymbol(
                     }));
                     try generateSymbol(bin_file, pt, src_loc, value, code, reloc_parent);
                 }
-                try code.writer(gpa).writeByte(@intFromBool(payload_val != null));
+                try code.append(gpa, @intFromBool(payload_val != null));
                 try code.appendNTimes(gpa, 0, padding);
             }
         },
@@ -721,7 +722,7 @@ fn lowerPtr(
     pt: Zcu.PerThread,
     src_loc: Zcu.LazySrcLoc,
     ptr_val: InternPool.Index,
-    code: *std.ArrayListUnmanaged(u8),
+    code: *ArrayList(u8),
     reloc_parent: link.File.RelocInfo.Parent,
     prev_offset: u64,
 ) GenerateSymbolError!void {
@@ -774,7 +775,7 @@ fn lowerUavRef(
     pt: Zcu.PerThread,
     src_loc: Zcu.LazySrcLoc,
     uav: InternPool.Key.Ptr.BaseAddr.Uav,
-    code: *std.ArrayListUnmanaged(u8),
+    code: *ArrayList(u8),
     reloc_parent: link.File.RelocInfo.Parent,
     offset: u64,
 ) GenerateSymbolError!void {
@@ -834,7 +835,7 @@ fn lowerNavRef(
     lf: *link.File,
     pt: Zcu.PerThread,
     nav_index: InternPool.Nav.Index,
-    code: *std.ArrayListUnmanaged(u8),
+    code: *ArrayList(u8),
     reloc_parent: link.File.RelocInfo.Parent,
     offset: u64,
 ) GenerateSymbolError!void {
diff --git a/src/libs/mingw.zig b/src/libs/mingw.zig
index 2a07141c38..9072318691 100644
--- a/src/libs/mingw.zig
+++ b/src/libs/mingw.zig
@@ -304,7 +304,7 @@ pub fn buildImportLib(comp: *Compilation, lib_name: []const u8) !void {
     const include_dir = try comp.dirs.zig_lib.join(arena, &.{ "libc", "mingw", "def-include" });
 
     if (comp.verbose_cc) print: {
-        var stderr = std.debug.lockStderrWriter();
+        var stderr = std.debug.lockStderrWriter(&.{});
         defer std.debug.unlockStderrWriter();
         nosuspend stderr.print("def file: {s}\n", .{def_file_path}) catch break :print;
         nosuspend stderr.print("include dir: {s}\n", .{include_dir}) catch break :print;
diff --git a/src/link/Coff.zig b/src/link/Coff.zig
index 62430f6c08..41c968534a 100644
--- a/src/link/Coff.zig
+++ b/src/link/Coff.zig
@@ -2179,13 +2179,13 @@ fn writeDataDirectoriesHeaders(coff: *Coff) !void {
 fn writeHeader(coff: *Coff) !void {
     const target = &coff.base.comp.root_mod.resolved_target.result;
     const gpa = coff.base.comp.gpa;
-    var buffer = std.array_list.Managed(u8).init(gpa);
+    var buffer: std.Io.Writer.Allocating = .init(gpa);
     defer buffer.deinit();
-    const writer = buffer.writer();
+    const writer = &buffer.writer;
 
     try buffer.ensureTotalCapacity(coff.getSizeOfHeaders());
     writer.writeAll(&msdos_stub) catch unreachable;
-    mem.writeInt(u32, buffer.items[0x3c..][0..4], msdos_stub.len, .little);
+    mem.writeInt(u32, buffer.writer.buffer[0x3c..][0..4], msdos_stub.len, .little);
 
     writer.writeAll("PE\x00\x00") catch unreachable;
     var flags = coff_util.CoffHeaderFlags{
@@ -2313,7 +2313,7 @@ fn writeHeader(coff: *Coff) !void {
         },
     }
 
-    try coff.pwriteAll(buffer.items, 0);
+    try coff.pwriteAll(buffer.written(), 0);
 }
 
 pub fn padToIdeal(actual_size: anytype) @TypeOf(actual_size) {