diff --git a/lib/std/Io/Reader.zig b/lib/std/Io/Reader.zig
index 3384374dd6..c4923127ee 100644
--- a/lib/std/Io/Reader.zig
+++ b/lib/std/Io/Reader.zig
@@ -8,7 +8,7 @@ const Writer = std.io.Writer;
 const assert = std.debug.assert;
 const testing = std.testing;
 const Allocator = std.mem.Allocator;
-const ArrayList = std.ArrayListUnmanaged;
+const ArrayList = std.ArrayList;
 const Limit = std.io.Limit;
 
 pub const Limited = @import("Reader/Limited.zig");
@@ -290,103 +290,63 @@ pub const LimitedAllocError = Allocator.Error || ShortError || error{StreamTooLo
 pub fn allocRemaining(r: *Reader, gpa: Allocator, limit: Limit) LimitedAllocError![]u8 {
     var buffer: ArrayList(u8) = .empty;
     defer buffer.deinit(gpa);
-    try appendRemaining(r, gpa, null, &buffer, limit);
+    try appendRemaining(r, gpa, &buffer, limit);
     return buffer.toOwnedSlice(gpa);
 }
 
 /// Transfers all bytes from the current position to the end of the stream, up
 /// to `limit`, appending them to `list`.
 ///
-/// If `limit` would be exceeded, `error.StreamTooLong` is returned instead. In
-/// such case, the next byte that would be read will be the first one to exceed
-/// `limit`, and all preceeding bytes have been appended to `list`.
-///
-/// If `limit` is not `Limit.unlimited`, asserts `buffer` has nonzero capacity.
+/// If `limit` is reached or exceeded, `error.StreamTooLong` is returned
+/// instead. In such case, the next byte that would be read will be the first
+/// one to exceed `limit`, and all preceeding bytes have been appended to
+/// `list`.
 ///
 /// See also:
 /// * `allocRemaining`
 pub fn appendRemaining(
     r: *Reader,
     gpa: Allocator,
-    comptime alignment: ?std.mem.Alignment,
-    list: *std.ArrayListAlignedUnmanaged(u8, alignment),
+    list: *ArrayList(u8),
     limit: Limit,
 ) LimitedAllocError!void {
-    if (limit == .unlimited) return appendRemainingUnlimited(r, gpa, alignment, list, 1);
-    assert(r.buffer.len != 0); // Needed to detect limit exceeded without losing data.
-    const buffer_contents = r.buffer[r.seek..r.end];
-    const copy_len = limit.minInt(buffer_contents.len);
-    try list.appendSlice(gpa, r.buffer[0..copy_len]);
-    r.seek += copy_len;
-    if (buffer_contents.len - copy_len != 0) return error.StreamTooLong;
-    r.seek = 0;
-    r.end = 0;
-    var remaining = @intFromEnum(limit) - copy_len;
-    // From here, we leave `buffer` empty, appending directly to `list`.
-    var writer: Writer = .{
-        .buffer = undefined,
-        .end = undefined,
-        .vtable = &.{ .drain = Writer.fixedDrain },
-    };
-    while (true) {
-        try list.ensureUnusedCapacity(gpa, 2);
-        const cap = list.unusedCapacitySlice();
-        const dest = cap[0..@min(cap.len, remaining + 1)];
-        writer.buffer = list.allocatedSlice();
-        writer.end = list.items.len;
-        const n = r.vtable.stream(r, &writer, .limited(dest.len)) catch |err| switch (err) {
-            error.WriteFailed => unreachable, // Prevented by the limit.
+    var a: std.Io.Writer.Allocating = .initOwnedSlice(gpa, list.items);
+    a.writer.end = list.items.len;
+    list.* = .empty;
+    defer {
+        list.* = .{
+            .items = a.writer.buffer[0..a.writer.end],
+            .capacity = a.writer.buffer.len,
+        };
+    }
+    var remaining = limit;
+    while (remaining.nonzero()) {
+        const n = stream(r, &a.writer, remaining) catch |err| switch (err) {
             error.EndOfStream => return,
+            error.WriteFailed => return error.OutOfMemory,
             error.ReadFailed => return error.ReadFailed,
         };
-        list.items.len += n;
-        if (n > remaining) {
-            // Move the byte to `Reader.buffer` so it is not lost.
-            assert(n - remaining == 1);
-            assert(r.end == 0);
-            r.buffer[0] = list.items[list.items.len - 1];
-            list.items.len -= 1;
-            r.end = 1;
-            return;
-        }
-        remaining -= n;
+        remaining = remaining.subtract(n).?;
     }
+    return error.StreamTooLong;
 }
 
 pub const UnlimitedAllocError = Allocator.Error || ShortError;
 
-pub fn appendRemainingUnlimited(
-    r: *Reader,
-    gpa: Allocator,
-    comptime alignment: ?std.mem.Alignment,
-    list: *std.ArrayListAlignedUnmanaged(u8, alignment),
-    bump: usize,
-) UnlimitedAllocError!void {
-    const buffer_contents = r.buffer[r.seek..r.end];
-    try list.ensureUnusedCapacity(gpa, buffer_contents.len + bump);
-    list.appendSliceAssumeCapacity(buffer_contents);
-    // If statement protects `ending`.
-    if (r.end != 0) {
-        r.seek = 0;
-        r.end = 0;
-    }
-    // From here, we leave `buffer` empty, appending directly to `list`.
-    var writer: Writer = .{
-        .buffer = undefined,
-        .end = undefined,
-        .vtable = &.{ .drain = Writer.fixedDrain },
-    };
-    while (true) {
-        try list.ensureUnusedCapacity(gpa, bump);
-        writer.buffer = list.allocatedSlice();
-        writer.end = list.items.len;
-        const n = r.vtable.stream(r, &writer, .limited(list.unusedCapacitySlice().len)) catch |err| switch (err) {
-            error.WriteFailed => unreachable, // Prevented by the limit.
-            error.EndOfStream => return,
-            error.ReadFailed => return error.ReadFailed,
+pub fn appendRemainingUnlimited(r: *Reader, gpa: Allocator, list: *ArrayList(u8)) UnlimitedAllocError!void {
+    var a: std.Io.Writer.Allocating = .initOwnedSlice(gpa, list.items);
+    a.writer.end = list.items.len;
+    list.* = .empty;
+    defer {
+        list.* = .{
+            .items = a.writer.buffer[0..a.writer.end],
+            .capacity = a.writer.buffer.len,
         };
-        list.items.len += n;
     }
+    _ = streamRemaining(r, &a.writer) catch |err| switch (err) {
+        error.WriteFailed => return error.OutOfMemory,
+        error.ReadFailed => return error.ReadFailed,
+    };
 }
 
 /// Writes bytes from the internally tracked stream position to `data`.
@@ -1295,7 +1255,10 @@ fn takeMultipleOf7Leb128(r: *Reader, comptime Result: type) TakeLeb128Error!Resu
 
 /// Ensures `capacity` more data can be buffered without rebasing.
 pub fn rebase(r: *Reader, capacity: usize) RebaseError!void {
-    if (r.end + capacity <= r.buffer.len) return;
+    if (r.end + capacity <= r.buffer.len) {
+        @branchHint(.likely);
+        return;
+    }
     return r.vtable.rebase(r, capacity);
 }
 
diff --git a/lib/std/Io/Writer.zig b/lib/std/Io/Writer.zig
index ecf8cdba62..6c5798f7f8 100644
--- a/lib/std/Io/Writer.zig
+++ b/lib/std/Io/Writer.zig
@@ -329,7 +329,7 @@ pub fn rebase(w: *Writer, preserve: usize, unused_capacity_len: usize) Error!voi
         @branchHint(.likely);
         return;
     }
-    try w.vtable.rebase(w, preserve, unused_capacity_len);
+    return w.vtable.rebase(w, preserve, unused_capacity_len);
 }
 
 pub fn defaultRebase(w: *Writer, preserve: usize, minimum_len: usize) Error!void {
@@ -2349,6 +2349,13 @@ pub fn unreachableDrain(w: *Writer, data: []const []const u8, splat: usize) Erro
     unreachable;
 }
 
+pub fn unreachableRebase(w: *Writer, preserve: usize, capacity: usize) Error!void {
+    _ = w;
+    _ = preserve;
+    _ = capacity;
+    unreachable;
+}
+
 /// Provides a `Writer` implementation based on calling `Hasher.update`, sending
 /// all data also to an underlying `Writer`.
 ///
@@ -2489,10 +2496,6 @@ pub fn Hashing(comptime Hasher: type) type {
 pub const Allocating = struct {
     allocator: Allocator,
     writer: Writer,
-    /// Every call to `drain` ensures at least this amount of unused capacity
-    /// before it returns. This prevents an infinite loop in interface logic
-    /// that calls `drain`.
-    minimum_unused_capacity: usize = 1,
 
     pub fn init(allocator: Allocator) Allocating {
         return .{
@@ -2604,13 +2607,12 @@ pub const Allocating = struct {
         const gpa = a.allocator;
         const pattern = data[data.len - 1];
         const splat_len = pattern.len * splat;
-        const bump = a.minimum_unused_capacity;
         var list = a.toArrayList();
         defer setArrayList(a, list);
         const start_len = list.items.len;
         assert(data.len != 0);
         for (data) |bytes| {
-            list.ensureUnusedCapacity(gpa, bytes.len + splat_len + bump) catch return error.WriteFailed;
+            list.ensureUnusedCapacity(gpa, bytes.len + splat_len + 1) catch return error.WriteFailed;
             list.appendSliceAssumeCapacity(bytes);
         }
         if (splat == 0) {
@@ -2641,11 +2643,12 @@ pub const Allocating = struct {
     }
 
     fn growingRebase(w: *Writer, preserve: usize, minimum_len: usize) Error!void {
-        _ = preserve; // This implementation always preserves the entire buffer.
         const a: *Allocating = @fieldParentPtr("writer", w);
         const gpa = a.allocator;
         var list = a.toArrayList();
         defer setArrayList(a, list);
+        const total = std.math.add(usize, preserve, minimum_len) catch return error.WriteFailed;
+        list.ensureTotalCapacity(gpa, total) catch return error.WriteFailed;
         list.ensureUnusedCapacity(gpa, minimum_len) catch return error.WriteFailed;
     }
 
diff --git a/lib/std/array_list.zig b/lib/std/array_list.zig
index 2272490996..f15e59a424 100644
--- a/lib/std/array_list.zig
+++ b/lib/std/array_list.zig
@@ -1033,7 +1033,7 @@ pub fn Aligned(comptime T: type, comptime alignment: ?mem.Alignment) type {
         pub fn print(self: *Self, gpa: Allocator, comptime fmt: []const u8, args: anytype) error{OutOfMemory}!void {
             comptime assert(T == u8);
             try self.ensureUnusedCapacity(gpa, fmt.len);
-            var aw: std.io.Writer.Allocating = .fromArrayList(gpa, self);
+            var aw: std.Io.Writer.Allocating = .fromArrayList(gpa, self);
             defer self.* = aw.toArrayList();
             return aw.writer.print(fmt, args) catch |err| switch (err) {
                 error.WriteFailed => return error.OutOfMemory,
diff --git a/lib/std/compress/flate/Decompress.zig b/lib/std/compress/flate/Decompress.zig
index a970360a85..47570beab2 100644
--- a/lib/std/compress/flate/Decompress.zig
+++ b/lib/std/compress/flate/Decompress.zig
@@ -62,7 +62,7 @@ pub const Error = Container.Error || error{
 const direct_vtable: Reader.VTable = .{
     .stream = streamDirect,
     .rebase = rebaseFallible,
-    .discard = discard,
+    .discard = discardDirect,
     .readVec = readVec,
 };
 
@@ -105,17 +105,16 @@ fn rebaseFallible(r: *Reader, capacity: usize) Reader.RebaseError!void {
 fn rebase(r: *Reader, capacity: usize) void {
     assert(capacity <= r.buffer.len - flate.history_len);
     assert(r.end + capacity > r.buffer.len);
-    const discard_n = r.end - flate.history_len;
+    const discard_n = @min(r.seek, r.end - flate.history_len);
     const keep = r.buffer[discard_n..r.end];
     @memmove(r.buffer[0..keep.len], keep);
-    assert(keep.len != 0);
     r.end = keep.len;
     r.seek -= discard_n;
 }
 
 /// This could be improved so that when an amount is discarded that includes an
 /// entire frame, skip decoding that frame.
-fn discard(r: *Reader, limit: std.Io.Limit) Reader.Error!usize {
+fn discardDirect(r: *Reader, limit: std.Io.Limit) Reader.Error!usize {
     if (r.end + flate.history_len > r.buffer.len) rebase(r, flate.history_len);
     var writer: Writer = .{
         .vtable = &.{
@@ -167,11 +166,14 @@ fn readVec(r: *Reader, data: [][]u8) Reader.Error!usize {
 
 fn streamIndirectInner(d: *Decompress) Reader.Error!usize {
     const r = &d.reader;
-    if (r.end + flate.history_len > r.buffer.len) rebase(r, flate.history_len);
+    if (r.buffer.len - r.end < flate.history_len) rebase(r, flate.history_len);
     var writer: Writer = .{
         .buffer = r.buffer,
         .end = r.end,
-        .vtable = &.{ .drain = Writer.unreachableDrain },
+        .vtable = &.{
+            .drain = Writer.unreachableDrain,
+            .rebase = Writer.unreachableRebase,
+        },
     };
     defer r.end = writer.end;
     _ = streamFallible(d, &writer, .limited(writer.buffer.len - writer.end)) catch |err| switch (err) {
@@ -1251,8 +1253,6 @@ test "zlib should not overshoot" {
 fn testFailure(container: Container, in: []const u8, expected_err: anyerror) !void {
     var reader: Reader = .fixed(in);
     var aw: Writer.Allocating = .init(testing.allocator);
-    aw.minimum_unused_capacity = flate.history_len;
-    try aw.ensureUnusedCapacity(flate.max_window_len);
     defer aw.deinit();
 
     var decompress: Decompress = .init(&reader, container, &.{});
@@ -1263,8 +1263,6 @@ fn testFailure(container: Container, in: []const u8, expected_err: anyerror) !vo
 fn testDecompress(container: Container, compressed: []const u8, expected_plain: []const u8) !void {
     var in: std.Io.Reader = .fixed(compressed);
     var aw: std.Io.Writer.Allocating = .init(testing.allocator);
-    aw.minimum_unused_capacity = flate.history_len;
-    try aw.ensureUnusedCapacity(flate.max_window_len);
     defer aw.deinit();
 
     var decompress: Decompress = .init(&in, container, &.{});
diff --git a/lib/std/compress/zstd.zig b/lib/std/compress/zstd.zig
index a59df317bb..39073b51c5 100644
--- a/lib/std/compress/zstd.zig
+++ b/lib/std/compress/zstd.zig
@@ -78,15 +78,14 @@ pub const table_size_max = struct {
 };
 
 fn testDecompress(gpa: std.mem.Allocator, compressed: []const u8) ![]u8 {
-    var out: std.ArrayListUnmanaged(u8) = .empty;
-    defer out.deinit(gpa);
-    try out.ensureUnusedCapacity(gpa, default_window_len);
+    var out: std.Io.Writer.Allocating = .init(gpa);
+    defer out.deinit();
 
-    var in: std.io.Reader = .fixed(compressed);
+    var in: std.Io.Reader = .fixed(compressed);
     var zstd_stream: Decompress = .init(&in, &.{}, .{});
-    try zstd_stream.reader.appendRemaining(gpa, null, &out, .unlimited);
+    _ = try zstd_stream.reader.streamRemaining(&out.writer);
 
-    return out.toOwnedSlice(gpa);
+    return out.toOwnedSlice();
 }
 
 fn testExpectDecompress(uncompressed: []const u8, compressed: []const u8) !void {
@@ -99,15 +98,14 @@ fn testExpectDecompress(uncompressed: []const u8, compressed: []const u8) !void
 fn testExpectDecompressError(err: anyerror, compressed: []const u8) !void {
     const gpa = std.testing.allocator;
 
-    var out: std.ArrayListUnmanaged(u8) = .empty;
-    defer out.deinit(gpa);
-    try out.ensureUnusedCapacity(gpa, default_window_len);
+    var out: std.Io.Writer.Allocating = .init(gpa);
+    defer out.deinit();
 
-    var in: std.io.Reader = .fixed(compressed);
+    var in: std.Io.Reader = .fixed(compressed);
     var zstd_stream: Decompress = .init(&in, &.{}, .{});
     try std.testing.expectError(
         error.ReadFailed,
-        zstd_stream.reader.appendRemaining(gpa, null, &out, .unlimited),
+        zstd_stream.reader.streamRemaining(&out.writer),
     );
     try std.testing.expectError(err, zstd_stream.err orelse {});
 }
diff --git a/lib/std/compress/zstd/Decompress.zig b/lib/std/compress/zstd/Decompress.zig
index aacfc70171..08fda244f9 100644
--- a/lib/std/compress/zstd/Decompress.zig
+++ b/lib/std/compress/zstd/Decompress.zig
@@ -73,6 +73,20 @@ pub const Error = error{
     WindowSizeUnknown,
 };
 
+const direct_vtable: Reader.VTable = .{
+    .stream = streamDirect,
+    .rebase = rebaseFallible,
+    .discard = discardDirect,
+    .readVec = readVec,
+};
+
+const indirect_vtable: Reader.VTable = .{
+    .stream = streamIndirect,
+    .rebase = rebaseFallible,
+    .discard = discardIndirect,
+    .readVec = readVec,
+};
+
 /// When connecting `reader` to a `Writer`, `buffer` should be empty, and
 /// `Writer.buffer` capacity has requirements based on `Options.window_len`.
 ///
@@ -84,12 +98,7 @@ pub fn init(input: *Reader, buffer: []u8, options: Options) Decompress {
         .verify_checksum = options.verify_checksum,
         .window_len = options.window_len,
         .reader = .{
-            .vtable = &.{
-                .stream = stream,
-                .rebase = rebase,
-                .discard = discard,
-                .readVec = readVec,
-            },
+            .vtable = if (buffer.len == 0) &direct_vtable else &indirect_vtable,
             .buffer = buffer,
             .seek = 0,
             .end = 0,
@@ -97,11 +106,27 @@ pub fn init(input: *Reader, buffer: []u8, options: Options) Decompress {
     };
 }
 
-fn rebase(r: *Reader, capacity: usize) Reader.RebaseError!void {
+fn streamDirect(r: *Reader, w: *Writer, limit: std.Io.Limit) Reader.StreamError!usize {
+    const d: *Decompress = @alignCast(@fieldParentPtr("reader", r));
+    return stream(d, w, limit);
+}
+
+fn streamIndirect(r: *Reader, w: *Writer, limit: std.Io.Limit) Reader.StreamError!usize {
+    const d: *Decompress = @alignCast(@fieldParentPtr("reader", r));
+    _ = limit;
+    _ = w;
+    return streamIndirectInner(d);
+}
+
+fn rebaseFallible(r: *Reader, capacity: usize) Reader.RebaseError!void {
+    rebase(r, capacity);
+}
+
+fn rebase(r: *Reader, capacity: usize) void {
     const d: *Decompress = @alignCast(@fieldParentPtr("reader", r));
     assert(capacity <= r.buffer.len - d.window_len);
     assert(r.end + capacity > r.buffer.len);
-    const discard_n = r.end - d.window_len;
+    const discard_n = @min(r.seek, r.end - d.window_len);
     const keep = r.buffer[discard_n..r.end];
     @memmove(r.buffer[0..keep.len], keep);
     r.end = keep.len;
@@ -110,9 +135,9 @@ fn rebase(r: *Reader, capacity: usize) Reader.RebaseError!void {
 
 /// This could be improved so that when an amount is discarded that includes an
 /// entire frame, skip decoding that frame.
-fn discard(r: *Reader, limit: std.Io.Limit) Reader.Error!usize {
+fn discardDirect(r: *Reader, limit: std.Io.Limit) Reader.Error!usize {
     const d: *Decompress = @alignCast(@fieldParentPtr("reader", r));
-    r.rebase(d.window_len) catch unreachable;
+    rebase(r, d.window_len);
     var writer: Writer = .{
         .vtable = &.{
             .drain = std.Io.Writer.Discarding.drain,
@@ -134,25 +159,53 @@ fn discard(r: *Reader, limit: std.Io.Limit) Reader.Error!usize {
     return n;
 }
 
-fn readVec(r: *Reader, data: [][]u8) Reader.Error!usize {
-    _ = data;
+fn discardIndirect(r: *Reader, limit: std.Io.Limit) Reader.Error!usize {
     const d: *Decompress = @alignCast(@fieldParentPtr("reader", r));
-    assert(r.seek == r.end);
-    r.rebase(d.window_len) catch unreachable;
+    rebase(r, d.window_len);
     var writer: Writer = .{
         .buffer = r.buffer,
         .end = r.end,
-        .vtable = &.{ .drain = Writer.fixedDrain },
+        .vtable = &.{ .drain = Writer.unreachableDrain },
     };
-    r.end += r.vtable.stream(r, &writer, .limited(writer.buffer.len - writer.end)) catch |err| switch (err) {
+    {
+        defer r.end = writer.end;
+        _ = stream(d, &writer, .limited(writer.buffer.len - writer.end)) catch |err| switch (err) {
+            error.WriteFailed => unreachable,
+            else => |e| return e,
+        };
+    }
+    const n = limit.minInt(r.end - r.seek);
+    r.seek += n;
+    return n;
+}
+
+fn readVec(r: *Reader, data: [][]u8) Reader.Error!usize {
+    _ = data;
+    const d: *Decompress = @alignCast(@fieldParentPtr("reader", r));
+    return streamIndirectInner(d);
+}
+
+fn streamIndirectInner(d: *Decompress) Reader.Error!usize {
+    const r = &d.reader;
+    if (r.buffer.len - r.end < zstd.block_size_max) rebase(r, zstd.block_size_max);
+    assert(r.buffer.len - r.end >= zstd.block_size_max);
+    var writer: Writer = .{
+        .buffer = r.buffer,
+        .end = r.end,
+        .vtable = &.{
+            .drain = Writer.unreachableDrain,
+            .rebase = Writer.unreachableRebase,
+        },
+    };
+    defer r.end = writer.end;
+    _ = stream(d, &writer, .limited(writer.buffer.len - writer.end)) catch |err| switch (err) {
         error.WriteFailed => unreachable,
         else => |e| return e,
     };
     return 0;
 }
 
-fn stream(r: *Reader, w: *Writer, limit: Limit) Reader.StreamError!usize {
-    const d: *Decompress = @alignCast(@fieldParentPtr("reader", r));
+fn stream(d: *Decompress, w: *Writer, limit: Limit) Reader.StreamError!usize {
     const in = d.input;
 
     state: switch (d.state) {
@@ -170,7 +223,7 @@ fn stream(r: *Reader, w: *Writer, limit: Limit) Reader.StreamError!usize {
                 else => |e| return e,
             };
             const magic = try in.takeEnumNonexhaustive(Frame.Magic, .little);
-            initFrame(d, w.buffer.len, magic) catch |err| {
+            initFrame(d, magic) catch |err| {
                 d.err = err;
                 return error.ReadFailed;
             };
@@ -198,13 +251,13 @@ fn stream(r: *Reader, w: *Writer, limit: Limit) Reader.StreamError!usize {
     }
 }
 
-fn initFrame(d: *Decompress, window_size_max: usize, magic: Frame.Magic) !void {
+fn initFrame(d: *Decompress, magic: Frame.Magic) !void {
     const in = d.input;
     switch (magic.kind() orelse return error.BadMagic) {
         .zstandard => {
             const header = try Frame.Zstandard.Header.decode(in);
             d.state = .{ .in_frame = .{
-                .frame = try Frame.init(header, window_size_max, d.verify_checksum),
+                .frame = try Frame.init(header, d.window_len, d.verify_checksum),
                 .checksum = null,
                 .decompressed_size = 0,
                 .decode = .init,
@@ -258,7 +311,6 @@ fn readInFrame(d: *Decompress, w: *Writer, limit: Limit, state: *State.InFrame)
                     try decode.readInitialFseState(&bit_stream);
 
                     // Ensures the following calls to `decodeSequence` will not flush.
-                    if (window_len + frame_block_size_max > w.buffer.len) return error.OutputBufferUndersize;
                     const dest = (try w.writableSliceGreedyPreserve(window_len, frame_block_size_max))[0..frame_block_size_max];
                     const write_pos = dest.ptr - w.buffer.ptr;
                     for (0..sequences_header.sequence_count - 1) |_| {
@@ -775,7 +827,6 @@ pub const Frame = struct {
                         try w.splatByteAll(d.literal_streams.one[0], len);
                     },
                     .compressed, .treeless => {
-                        if (len > w.buffer.len) return error.OutputBufferUndersize;
                         const buf = try w.writableSlice(len);
                         const huffman_tree = d.huffman_tree.?;
                         const max_bit_count = huffman_tree.max_bit_count;
diff --git a/lib/std/debug/Dwarf.zig b/lib/std/debug/Dwarf.zig
index 2cdeede7b1..01c672b3a0 100644
--- a/lib/std/debug/Dwarf.zig
+++ b/lib/std/debug/Dwarf.zig
@@ -2247,7 +2247,7 @@ pub const ElfModule = struct {
                 var decompress: std.compress.flate.Decompress = .init(&section_reader, .zlib, &.{});
                 var decompressed_section: ArrayList(u8) = .empty;
                 defer decompressed_section.deinit(gpa);
-                decompress.reader.appendRemainingUnlimited(gpa, null, &decompressed_section, std.compress.flate.history_len) catch {
+                decompress.reader.appendRemainingUnlimited(gpa, &decompressed_section) catch {
                     invalidDebugInfoDetected();
                     continue;
                 };
diff --git a/lib/std/http/test.zig b/lib/std/http/test.zig
index 20c7383650..91a57dbbe2 100644
--- a/lib/std/http/test.zig
+++ b/lib/std/http/test.zig
@@ -149,9 +149,8 @@ test "HTTP server handles a chunked transfer coding request" {
         "content-type: text/plain\r\n" ++
         "\r\n" ++
         "message from server!\n";
-    var tiny_buffer: [1]u8 = undefined; // allows allocRemaining to detect limit exceeded
-    var stream_reader = stream.reader(&tiny_buffer);
-    const response = try stream_reader.interface().allocRemaining(gpa, .limited(expected_response.len));
+    var stream_reader = stream.reader(&.{});
+    const response = try stream_reader.interface().allocRemaining(gpa, .limited(expected_response.len + 1));
     defer gpa.free(response);
     try expectEqualStrings(expected_response, response);
 }
@@ -293,8 +292,7 @@ test "Server.Request.respondStreaming non-chunked, unknown content-length" {
     var stream_writer = stream.writer(&.{});
     try stream_writer.interface.writeAll(request_bytes);
 
-    var tiny_buffer: [1]u8 = undefined; // allows allocRemaining to detect limit exceeded
-    var stream_reader = stream.reader(&tiny_buffer);
+    var stream_reader = stream.reader(&.{});
     const response = try stream_reader.interface().allocRemaining(gpa, .unlimited);
     defer gpa.free(response);
 
@@ -364,8 +362,7 @@ test "receiving arbitrary http headers from the client" {
     var stream_writer = stream.writer(&.{});
     try stream_writer.interface.writeAll(request_bytes);
 
-    var tiny_buffer: [1]u8 = undefined; // allows allocRemaining to detect limit exceeded
-    var stream_reader = stream.reader(&tiny_buffer);
+    var stream_reader = stream.reader(&.{});
     const response = try stream_reader.interface().allocRemaining(gpa, .unlimited);
     defer gpa.free(response);
 
diff --git a/lib/std/unicode.zig b/lib/std/unicode.zig
index 0f849fb8b1..fb0ce509df 100644
--- a/lib/std/unicode.zig
+++ b/lib/std/unicode.zig
@@ -4,6 +4,7 @@ const assert = std.debug.assert;
 const testing = std.testing;
 const mem = std.mem;
 const native_endian = builtin.cpu.arch.endian();
+const Allocator = std.mem.Allocator;
 
 /// Use this to replace an unknown, unrecognized, or unrepresentable character.
 ///
@@ -921,7 +922,7 @@ fn utf16LeToUtf8ArrayListImpl(
     comptime surrogates: Surrogates,
 ) (switch (surrogates) {
     .cannot_encode_surrogate_half => Utf16LeToUtf8AllocError,
-    .can_encode_surrogate_half => mem.Allocator.Error,
+    .can_encode_surrogate_half => Allocator.Error,
 })!void {
     assert(result.unusedCapacitySlice().len >= utf16le.len);
 
@@ -965,15 +966,15 @@ fn utf16LeToUtf8ArrayListImpl(
     }
 }
 
-pub const Utf16LeToUtf8AllocError = mem.Allocator.Error || Utf16LeToUtf8Error;
+pub const Utf16LeToUtf8AllocError = Allocator.Error || Utf16LeToUtf8Error;
 
 pub fn utf16LeToUtf8ArrayList(result: *std.array_list.Managed(u8), utf16le: []const u16) Utf16LeToUtf8AllocError!void {
     try result.ensureUnusedCapacity(utf16le.len);
     return utf16LeToUtf8ArrayListImpl(result, utf16le, .cannot_encode_surrogate_half);
 }
 
-/// Caller must free returned memory.
-pub fn utf16LeToUtf8Alloc(allocator: mem.Allocator, utf16le: []const u16) Utf16LeToUtf8AllocError![]u8 {
+/// Caller owns returned memory.
+pub fn utf16LeToUtf8Alloc(allocator: Allocator, utf16le: []const u16) Utf16LeToUtf8AllocError![]u8 {
     // optimistically guess that it will all be ascii.
     var result = try std.array_list.Managed(u8).initCapacity(allocator, utf16le.len);
     errdefer result.deinit();
@@ -982,8 +983,8 @@ pub fn utf16LeToUtf8Alloc(allocator: mem.Allocator, utf16le: []const u16) Utf16L
     return result.toOwnedSlice();
 }
 
-/// Caller must free returned memory.
-pub fn utf16LeToUtf8AllocZ(allocator: mem.Allocator, utf16le: []const u16) Utf16LeToUtf8AllocError![:0]u8 {
+/// Caller owns returned memory.
+pub fn utf16LeToUtf8AllocZ(allocator: Allocator, utf16le: []const u16) Utf16LeToUtf8AllocError![:0]u8 {
     // optimistically guess that it will all be ascii (and allocate space for the null terminator)
     var result = try std.array_list.Managed(u8).initCapacity(allocator, utf16le.len + 1);
     errdefer result.deinit();
@@ -1160,7 +1161,7 @@ pub fn utf8ToUtf16LeArrayList(result: *std.array_list.Managed(u16), utf8: []cons
     return utf8ToUtf16LeArrayListImpl(result, utf8, .cannot_encode_surrogate_half);
 }
 
-pub fn utf8ToUtf16LeAlloc(allocator: mem.Allocator, utf8: []const u8) error{ InvalidUtf8, OutOfMemory }![]u16 {
+pub fn utf8ToUtf16LeAlloc(allocator: Allocator, utf8: []const u8) error{ InvalidUtf8, OutOfMemory }![]u16 {
     // optimistically guess that it will not require surrogate pairs
     var result = try std.array_list.Managed(u16).initCapacity(allocator, utf8.len);
     errdefer result.deinit();
@@ -1169,7 +1170,7 @@ pub fn utf8ToUtf16LeAlloc(allocator: mem.Allocator, utf8: []const u8) error{ Inv
     return result.toOwnedSlice();
 }
 
-pub fn utf8ToUtf16LeAllocZ(allocator: mem.Allocator, utf8: []const u8) error{ InvalidUtf8, OutOfMemory }![:0]u16 {
+pub fn utf8ToUtf16LeAllocZ(allocator: Allocator, utf8: []const u8) error{ InvalidUtf8, OutOfMemory }![:0]u16 {
     // optimistically guess that it will not require surrogate pairs
     var result = try std.array_list.Managed(u16).initCapacity(allocator, utf8.len + 1);
     errdefer result.deinit();
@@ -1750,13 +1751,13 @@ pub const Wtf8Iterator = struct {
     }
 };
 
-pub fn wtf16LeToWtf8ArrayList(result: *std.array_list.Managed(u8), utf16le: []const u16) mem.Allocator.Error!void {
+pub fn wtf16LeToWtf8ArrayList(result: *std.array_list.Managed(u8), utf16le: []const u16) Allocator.Error!void {
     try result.ensureUnusedCapacity(utf16le.len);
     return utf16LeToUtf8ArrayListImpl(result, utf16le, .can_encode_surrogate_half);
 }
 
 /// Caller must free returned memory.
-pub fn wtf16LeToWtf8Alloc(allocator: mem.Allocator, wtf16le: []const u16) mem.Allocator.Error![]u8 {
+pub fn wtf16LeToWtf8Alloc(allocator: Allocator, wtf16le: []const u16) Allocator.Error![]u8 {
     // optimistically guess that it will all be ascii.
     var result = try std.array_list.Managed(u8).initCapacity(allocator, wtf16le.len);
     errdefer result.deinit();
@@ -1766,7 +1767,7 @@ pub fn wtf16LeToWtf8Alloc(allocator: mem.Allocator, wtf16le: []const u16) mem.Al
 }
 
 /// Caller must free returned memory.
-pub fn wtf16LeToWtf8AllocZ(allocator: mem.Allocator, wtf16le: []const u16) mem.Allocator.Error![:0]u8 {
+pub fn wtf16LeToWtf8AllocZ(allocator: Allocator, wtf16le: []const u16) Allocator.Error![:0]u8 {
     // optimistically guess that it will all be ascii (and allocate space for the null terminator)
     var result = try std.array_list.Managed(u8).initCapacity(allocator, wtf16le.len + 1);
     errdefer result.deinit();
@@ -1784,7 +1785,7 @@ pub fn wtf8ToWtf16LeArrayList(result: *std.array_list.Managed(u16), wtf8: []cons
     return utf8ToUtf16LeArrayListImpl(result, wtf8, .can_encode_surrogate_half);
 }
 
-pub fn wtf8ToWtf16LeAlloc(allocator: mem.Allocator, wtf8: []const u8) error{ InvalidWtf8, OutOfMemory }![]u16 {
+pub fn wtf8ToWtf16LeAlloc(allocator: Allocator, wtf8: []const u8) error{ InvalidWtf8, OutOfMemory }![]u16 {
     // optimistically guess that it will not require surrogate pairs
     var result = try std.array_list.Managed(u16).initCapacity(allocator, wtf8.len);
     errdefer result.deinit();
@@ -1793,7 +1794,7 @@ pub fn wtf8ToWtf16LeAlloc(allocator: mem.Allocator, wtf8: []const u8) error{ Inv
     return result.toOwnedSlice();
 }
 
-pub fn wtf8ToWtf16LeAllocZ(allocator: mem.Allocator, wtf8: []const u8) error{ InvalidWtf8, OutOfMemory }![:0]u16 {
+pub fn wtf8ToWtf16LeAllocZ(allocator: Allocator, wtf8: []const u8) error{ InvalidWtf8, OutOfMemory }![:0]u16 {
     // optimistically guess that it will not require surrogate pairs
     var result = try std.array_list.Managed(u16).initCapacity(allocator, wtf8.len + 1);
     errdefer result.deinit();
@@ -1870,7 +1871,7 @@ pub fn wtf8ToUtf8Lossy(utf8: []u8, wtf8: []const u8) error{InvalidWtf8}!void {
     }
 }
 
-pub fn wtf8ToUtf8LossyAlloc(allocator: mem.Allocator, wtf8: []const u8) error{ InvalidWtf8, OutOfMemory }![]u8 {
+pub fn wtf8ToUtf8LossyAlloc(allocator: Allocator, wtf8: []const u8) error{ InvalidWtf8, OutOfMemory }![]u8 {
     const utf8 = try allocator.alloc(u8, wtf8.len);
     errdefer allocator.free(utf8);
 
@@ -1879,7 +1880,7 @@ pub fn wtf8ToUtf8LossyAlloc(allocator: mem.Allocator, wtf8: []const u8) error{ I
     return utf8;
 }
 
-pub fn wtf8ToUtf8LossyAllocZ(allocator: mem.Allocator, wtf8: []const u8) error{ InvalidWtf8, OutOfMemory }![:0]u8 {
+pub fn wtf8ToUtf8LossyAllocZ(allocator: Allocator, wtf8: []const u8) error{ InvalidWtf8, OutOfMemory }![:0]u8 {
     const utf8 = try allocator.allocSentinel(u8, wtf8.len, 0);
     errdefer allocator.free(utf8);
 
diff --git a/lib/std/zig.zig b/lib/std/zig.zig
index cb4be0de64..cbb01b2b38 100644
--- a/lib/std/zig.zig
+++ b/lib/std/zig.zig
@@ -554,8 +554,11 @@ test isUnderscore {
     try std.testing.expect(!isUnderscore("\\x5f"));
 }
 
+/// If the source can be UTF-16LE encoded, this function asserts that `gpa`
+/// will align a byte-sized allocation to at least 2. Allocators that don't do
+/// this are rare.
 pub fn readSourceFileToEndAlloc(gpa: Allocator, file_reader: *std.fs.File.Reader) ![:0]u8 {
-    var buffer: std.ArrayListAlignedUnmanaged(u8, .@"2") = .empty;
+    var buffer: std.ArrayList(u8) = .empty;
     defer buffer.deinit(gpa);
 
     if (file_reader.getSize()) |size| {
@@ -564,7 +567,7 @@ pub fn readSourceFileToEndAlloc(gpa: Allocator, file_reader: *std.fs.File.Reader
         try buffer.ensureTotalCapacityPrecise(gpa, casted_size + 1);
     } else |_| {}
 
-    try file_reader.interface.appendRemaining(gpa, .@"2", &buffer, .limited(max_src_size));
+    try file_reader.interface.appendRemaining(gpa, &buffer, .limited(max_src_size));
 
     // Detect unsupported file types with their Byte Order Mark
     const unsupported_boms = [_][]const u8{
@@ -581,7 +584,7 @@ pub fn readSourceFileToEndAlloc(gpa: Allocator, file_reader: *std.fs.File.Reader
     // If the file starts with a UTF-16 little endian BOM, translate it to UTF-8
     if (std.mem.startsWith(u8, buffer.items, "\xff\xfe")) {
         if (buffer.items.len % 2 != 0) return error.InvalidEncoding;
-        return std.unicode.utf16LeToUtf8AllocZ(gpa, @ptrCast(buffer.items)) catch |err| switch (err) {
+        return std.unicode.utf16LeToUtf8AllocZ(gpa, @ptrCast(@alignCast(buffer.items))) catch |err| switch (err) {
             error.DanglingSurrogateHalf => error.UnsupportedEncoding,
             error.ExpectedSecondSurrogateHalf => error.UnsupportedEncoding,
             error.UnexpectedSecondSurrogateHalf => error.UnsupportedEncoding,