diff --git a/lib/docs/wasm/Walk.zig b/lib/docs/wasm/Walk.zig
index 90df301f22..2c41e9c940 100644
--- a/lib/docs/wasm/Walk.zig
+++ b/lib/docs/wasm/Walk.zig
@@ -433,14 +433,18 @@ fn parse(file_name: []const u8, source: []u8) Oom!Ast {
         defer ast.deinit(gpa);
 
         const token_offsets = ast.tokens.items(.start);
-        var rendered_err: std.ArrayListUnmanaged(u8) = .{};
-        defer rendered_err.deinit(gpa);
+        var rendered_err: std.Io.Writer.Allocating = .init(gpa);
+        defer rendered_err.deinit();
         for (ast.errors) |err| {
             const err_offset = token_offsets[err.token] + ast.errorOffset(err);
             const err_loc = std.zig.findLineColumn(ast.source, err_offset);
             rendered_err.clearRetainingCapacity();
-            try ast.renderError(err, rendered_err.writer(gpa));
-            log.err("{s}:{d}:{d}: {s}", .{ file_name, err_loc.line + 1, err_loc.column + 1, rendered_err.items });
+            ast.renderError(err, &rendered_err.writer) catch |e| switch (e) {
+                error.WriteFailed => return error.OutOfMemory,
+            };
+            log.err("{s}:{d}:{d}: {s}", .{
+                file_name, err_loc.line + 1, err_loc.column + 1, rendered_err.getWritten(),
+            });
         }
         return Ast.parse(gpa, "", .zig);
     }
diff --git a/lib/std/Io/Writer.zig b/lib/std/Io/Writer.zig
index 8543f0bce5..dd72607b3f 100644
--- a/lib/std/Io/Writer.zig
+++ b/lib/std/Io/Writer.zig
@@ -2481,6 +2481,12 @@ pub const Allocating = struct {
         return list.ensureUnusedCapacity(a.allocator, additional_count);
     }
 
+    pub fn ensureTotalCapacity(a: *Allocating, new_capacity: usize) Allocator.Error!void {
+        var list = a.toArrayList();
+        defer a.setArrayList(list);
+        return list.ensureTotalCapacity(a.allocator, new_capacity);
+    }
+
     pub fn toOwnedSlice(a: *Allocating) error{OutOfMemory}![]u8 {
         var list = a.toArrayList();
         defer a.setArrayList(list);
diff --git a/lib/std/zig.zig b/lib/std/zig.zig
index 6a64adc547..fa45af129c 100644
--- a/lib/std/zig.zig
+++ b/lib/std/zig.zig
@@ -530,18 +530,11 @@ test isUnderscore {
     try std.testing.expect(!isUnderscore("\\x5f"));
 }
 
-pub fn readSourceFileToEndAlloc(gpa: Allocator, input: std.fs.File, size_hint: usize) ![:0]u8 {
+pub fn readSourceFileToEndAlloc(gpa: Allocator, file_reader: *std.fs.File.Reader) ![:0]u8 {
     var buffer: std.ArrayListAlignedUnmanaged(u8, .@"2") = .empty;
     defer buffer.deinit(gpa);
 
-    try buffer.ensureUnusedCapacity(gpa, size_hint);
-
-    input.readIntoArrayList(gpa, .limited(max_src_size), .@"2", &buffer) catch |err| switch (err) {
-        error.ConnectionResetByPeer => unreachable,
-        error.ConnectionTimedOut => unreachable,
-        error.NotOpenForReading => unreachable,
-        else => |e| return e,
-    };
+    try file_reader.interface.appendRemaining(gpa, .@"2", &buffer, .limited(max_src_size));
 
     // Detect unsupported file types with their Byte Order Mark
     const unsupported_boms = [_][]const u8{
diff --git a/lib/std/zig/AstGen.zig b/lib/std/zig/AstGen.zig
index 175403b295..52fc41a2c9 100644
--- a/lib/std/zig/AstGen.zig
+++ b/lib/std/zig/AstGen.zig
@@ -11278,10 +11278,14 @@ fn parseStrLit(
     offset: u32,
 ) InnerError!void {
     const raw_string = bytes[offset..];
-    var buf_managed = buf.toManaged(astgen.gpa);
-    const result = std.zig.string_literal.parseWrite(buf_managed.writer(), raw_string);
-    buf.* = buf_managed.moveToUnmanaged();
-    switch (try result) {
+    const result = r: {
+        var aw: std.io.Writer.Allocating = .fromArrayList(astgen.gpa, buf);
+        defer buf.* = aw.toArrayList();
+        break :r std.zig.string_literal.parseWrite(&aw.writer, raw_string) catch |err| switch (err) {
+            error.WriteFailed => return error.OutOfMemory,
+        };
+    };
+    switch (result) {
         .success => return,
         .failure => |err| return astgen.failWithStrLitError(err, token, bytes, offset),
     }
@@ -11324,17 +11328,18 @@ fn appendErrorNodeNotes(
     notes: []const u32,
 ) Allocator.Error!void {
     @branchHint(.cold);
+    const gpa = astgen.gpa;
     const string_bytes = &astgen.string_bytes;
     const msg: Zir.NullTerminatedString = @enumFromInt(string_bytes.items.len);
-    try string_bytes.writer(astgen.gpa).print(format ++ "\x00", args);
+    try string_bytes.print(gpa, format ++ "\x00", args);
     const notes_index: u32 = if (notes.len != 0) blk: {
         const notes_start = astgen.extra.items.len;
-        try astgen.extra.ensureTotalCapacity(astgen.gpa, notes_start + 1 + notes.len);
+        try astgen.extra.ensureTotalCapacity(gpa, notes_start + 1 + notes.len);
         astgen.extra.appendAssumeCapacity(@intCast(notes.len));
         astgen.extra.appendSliceAssumeCapacity(notes);
         break :blk @intCast(notes_start);
     } else 0;
-    try astgen.compile_errors.append(astgen.gpa, .{
+    try astgen.compile_errors.append(gpa, .{
         .msg = msg,
         .node = node.toOptional(),
         .token = .none,
@@ -11418,7 +11423,7 @@ fn appendErrorTokNotesOff(
     const gpa = astgen.gpa;
     const string_bytes = &astgen.string_bytes;
     const msg: Zir.NullTerminatedString = @enumFromInt(string_bytes.items.len);
-    try string_bytes.writer(gpa).print(format ++ "\x00", args);
+    try string_bytes.print(gpa, format ++ "\x00", args);
     const notes_index: u32 = if (notes.len != 0) blk: {
         const notes_start = astgen.extra.items.len;
         try astgen.extra.ensureTotalCapacity(gpa, notes_start + 1 + notes.len);
@@ -11454,7 +11459,7 @@ fn errNoteTokOff(
     @branchHint(.cold);
     const string_bytes = &astgen.string_bytes;
     const msg: Zir.NullTerminatedString = @enumFromInt(string_bytes.items.len);
-    try string_bytes.writer(astgen.gpa).print(format ++ "\x00", args);
+    try string_bytes.print(astgen.gpa, format ++ "\x00", args);
     return astgen.addExtra(Zir.Inst.CompileErrors.Item{
         .msg = msg,
         .node = .none,
@@ -11473,7 +11478,7 @@ fn errNoteNode(
     @branchHint(.cold);
     const string_bytes = &astgen.string_bytes;
     const msg: Zir.NullTerminatedString = @enumFromInt(string_bytes.items.len);
-    try string_bytes.writer(astgen.gpa).print(format ++ "\x00", args);
+    try string_bytes.print(astgen.gpa, format ++ "\x00", args);
     return astgen.addExtra(Zir.Inst.CompileErrors.Item{
         .msg = msg,
         .node = node.toOptional(),
@@ -13715,13 +13720,14 @@ fn emitDbgStmtForceCurrentIndex(gz: *GenZir, lc: LineColumn) !void {
     } });
 }
 
-fn lowerAstErrors(astgen: *AstGen) !void {
+fn lowerAstErrors(astgen: *AstGen) error{OutOfMemory}!void {
     const gpa = astgen.gpa;
     const tree = astgen.tree;
     assert(tree.errors.len > 0);
 
-    var msg: std.ArrayListUnmanaged(u8) = .empty;
-    defer msg.deinit(gpa);
+    var msg: std.io.Writer.Allocating = .init(gpa);
+    defer msg.deinit();
+    const msg_w = &msg.writer;
 
     var notes: std.ArrayListUnmanaged(u32) = .empty;
     defer notes.deinit(gpa);
@@ -13749,26 +13755,26 @@ fn lowerAstErrors(astgen: *AstGen) !void {
             break :blk idx - tok_start;
         };
 
-        const err: Ast.Error = .{
+        const ast_err: Ast.Error = .{
             .tag = Ast.Error.Tag.invalid_byte,
             .token = tok,
             .extra = .{ .offset = bad_off },
         };
         msg.clearRetainingCapacity();
-        try tree.renderError(err, msg.writer(gpa));
-        return try astgen.appendErrorTokNotesOff(tok, bad_off, "{s}", .{msg.items}, notes.items);
+        tree.renderError(ast_err, msg_w) catch return error.OutOfMemory;
+        return try astgen.appendErrorTokNotesOff(tok, bad_off, "{s}", .{msg.getWritten()}, notes.items);
     }
 
     var cur_err = tree.errors[0];
     for (tree.errors[1..]) |err| {
         if (err.is_note) {
-            try tree.renderError(err, msg.writer(gpa));
-            try notes.append(gpa, try astgen.errNoteTok(err.token, "{s}", .{msg.items}));
+            tree.renderError(err, msg_w) catch return error.OutOfMemory;
+            try notes.append(gpa, try astgen.errNoteTok(err.token, "{s}", .{msg.getWritten()}));
         } else {
             // Flush error
             const extra_offset = tree.errorOffset(cur_err);
-            try tree.renderError(cur_err, msg.writer(gpa));
-            try astgen.appendErrorTokNotesOff(cur_err.token, extra_offset, "{s}", .{msg.items}, notes.items);
+            tree.renderError(cur_err, msg_w) catch return error.OutOfMemory;
+            try astgen.appendErrorTokNotesOff(cur_err.token, extra_offset, "{s}", .{msg.getWritten()}, notes.items);
             notes.clearRetainingCapacity();
             cur_err = err;
 
@@ -13781,8 +13787,8 @@ fn lowerAstErrors(astgen: *AstGen) !void {
 
     // Flush error
     const extra_offset = tree.errorOffset(cur_err);
-    try tree.renderError(cur_err, msg.writer(gpa));
-    try astgen.appendErrorTokNotesOff(cur_err.token, extra_offset, "{s}", .{msg.items}, notes.items);
+    tree.renderError(cur_err, msg_w) catch return error.OutOfMemory;
+    try astgen.appendErrorTokNotesOff(cur_err.token, extra_offset, "{s}", .{msg.getWritten()}, notes.items);
 }
 
 const DeclarationName = union(enum) {
diff --git a/src/Compilation.zig b/src/Compilation.zig
index 8406134352..461528c939 100644
--- a/src/Compilation.zig
+++ b/src/Compilation.zig
@@ -5328,7 +5328,7 @@ pub fn cImport(comp: *Compilation, c_src: []const u8, owner_mod: *Package.Module
         var out_zig_file = try o_dir.createFile(cimport_zig_basename, .{});
         defer out_zig_file.close();
 
-        const formatted = try tree.render(comp.gpa);
+        const formatted = try tree.renderAlloc(comp.gpa);
         defer comp.gpa.free(formatted);
 
         try out_zig_file.writeAll(formatted);
diff --git a/src/Package/Manifest.zig b/src/Package/Manifest.zig
index 1d71b60fa3..f5a4a8c314 100644
--- a/src/Package/Manifest.zig
+++ b/src/Package/Manifest.zig
@@ -471,10 +471,14 @@ const Parse = struct {
         offset: u32,
     ) InnerError!void {
         const raw_string = bytes[offset..];
-        var buf_managed = buf.toManaged(p.gpa);
-        const result = std.zig.string_literal.parseWrite(buf_managed.writer(), raw_string);
-        buf.* = buf_managed.moveToUnmanaged();
-        switch (try result) {
+        const result = r: {
+            var aw: std.io.Writer.Allocating = .fromArrayList(p.gpa, buf);
+            defer buf.* = aw.toArrayList();
+            break :r std.zig.string_literal.parseWrite(&aw.writer, raw_string) catch |err| switch (err) {
+                error.WriteFailed => return error.OutOfMemory,
+            };
+        };
+        switch (result) {
             .success => {},
             .failure => |err| try p.appendStrLitError(err, token, bytes, offset),
         }
diff --git a/src/fmt.zig b/src/fmt.zig
index bbb02e0c34..88c0e3a06f 100644
--- a/src/fmt.zig
+++ b/src/fmt.zig
@@ -34,7 +34,7 @@ const Fmt = struct {
     color: Color,
     gpa: Allocator,
     arena: Allocator,
-    out_buffer: std.ArrayList(u8),
+    out_buffer: std.Io.Writer.Allocating,
 
     const SeenMap = std.AutoHashMap(fs.File.INode, void);
 };
@@ -102,7 +102,9 @@ pub fn run(
         }
 
         const stdin: fs.File = .stdin();
-        const source_code = std.zig.readSourceFileToEndAlloc(gpa, stdin, null) catch |err| {
+        var stdio_buffer: [1024]u8 = undefined;
+        var file_reader: fs.File.Reader = stdin.reader(&stdio_buffer);
+        const source_code = std.zig.readSourceFileToEndAlloc(gpa, &file_reader) catch |err| {
             fatal("unable to read stdin: {}", .{err});
         };
         defer gpa.free(source_code);
@@ -146,7 +148,7 @@ pub fn run(
             try std.zig.printAstErrorsToStderr(gpa, tree, "<stdin>", color);
             process.exit(2);
         }
-        const formatted = try tree.render(gpa);
+        const formatted = try tree.renderAlloc(gpa);
         defer gpa.free(formatted);
 
         if (check_flag) {
@@ -169,7 +171,7 @@ pub fn run(
         .check_ast = check_ast_flag,
         .force_zon = force_zon,
         .color = color,
-        .out_buffer = std.ArrayList(u8).init(gpa),
+        .out_buffer = .init(gpa),
     };
     defer fmt.seen.deinit();
     defer fmt.out_buffer.deinit();
@@ -230,6 +232,9 @@ const FmtError = error{
     NetNameDeleted,
     InvalidArgument,
     ProcessNotFound,
+    ConnectionTimedOut,
+    NotOpenForReading,
+    StreamTooLong,
 } || fs.File.OpenError;
 
 fn fmtPath(fmt: *Fmt, file_path: []const u8, check_mode: bool, dir: fs.Dir, sub_path: []const u8) FmtError!void {
@@ -295,12 +300,15 @@ fn fmtPathFile(
     if (stat.kind == .directory)
         return error.IsDir;
 
+    var read_buffer: [1024]u8 = undefined;
+    var file_reader: fs.File.Reader = source_file.reader(&read_buffer);
+    file_reader.size = stat.size;
+
     const gpa = fmt.gpa;
-    const source_code = try std.zig.readSourceFileToEndAlloc(
-        gpa,
-        source_file,
-        std.math.cast(usize, stat.size) orelse return error.FileTooBig,
-    );
+    const source_code = std.zig.readSourceFileToEndAlloc(gpa, &file_reader) catch |err| switch (err) {
+        error.ReadFailed => return file_reader.err.?,
+        else => |e| return e,
+    };
     defer gpa.free(source_code);
 
     source_file.close();
@@ -363,11 +371,13 @@ fn fmtPathFile(
     }
 
     // As a heuristic, we make enough capacity for the same as the input source.
-    fmt.out_buffer.shrinkRetainingCapacity(0);
+    fmt.out_buffer.clearRetainingCapacity();
     try fmt.out_buffer.ensureTotalCapacity(source_code.len);
 
-    try tree.renderToArrayList(&fmt.out_buffer, .{});
-    if (mem.eql(u8, fmt.out_buffer.items, source_code))
+    tree.render(gpa, &fmt.out_buffer.writer, .{}) catch |err| switch (err) {
+        error.WriteFailed, error.OutOfMemory => return error.OutOfMemory,
+    };
+    if (mem.eql(u8, fmt.out_buffer.getWritten(), source_code))
         return;
 
     if (check_mode) {
@@ -378,7 +388,7 @@ fn fmtPathFile(
         var af = try dir.atomicFile(sub_path, .{ .mode = stat.mode });
         defer af.deinit();
 
-        try af.file.writeAll(fmt.out_buffer.items);
+        try af.file.writeAll(fmt.out_buffer.getWritten());
         try af.finish();
         const stdout = std.fs.File.stdout().deprecatedWriter();
         try stdout.print("{s}\n", .{file_path});
diff --git a/src/main.zig b/src/main.zig
index 37b137ea94..019ae8bc7e 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -4550,7 +4550,7 @@ fn cmdTranslateC(
                     error.SemanticAnalyzeFail => break :f .{ .error_bundle = errors },
                 };
                 defer tree.deinit(comp.gpa);
-                break :f .{ .success = try tree.render(arena) };
+                break :f .{ .success = try tree.renderAlloc(arena) };
             },
         };
 
@@ -6058,7 +6058,8 @@ fn cmdAstCheck(
             };
         } else fs.File.stdin();
         defer if (zig_source_path != null) f.close();
-        break :s std.zig.readSourceFileToEndAlloc(arena, f, null) catch |err| {
+        var file_reader: fs.File.Reader = f.reader(&stdio_buffer);
+        break :s std.zig.readSourceFileToEndAlloc(arena, &file_reader) catch |err| {
             fatal("unable to load file '{s}' for ast-check: {s}", .{ display_path, @errorName(err) });
         };
     };
@@ -6416,14 +6417,16 @@ fn cmdChangelist(
         var f = fs.cwd().openFile(old_source_path, .{}) catch |err|
             fatal("unable to open old source file '{s}': {s}", .{ old_source_path, @errorName(err) });
         defer f.close();
-        break :source std.zig.readSourceFileToEndAlloc(arena, f, std.zig.max_src_size) catch |err|
+        var file_reader: fs.File.Reader = f.reader(&stdio_buffer);
+        break :source std.zig.readSourceFileToEndAlloc(arena, &file_reader) catch |err|
             fatal("unable to read old source file '{s}': {s}", .{ old_source_path, @errorName(err) });
     };
     const new_source = source: {
         var f = fs.cwd().openFile(new_source_path, .{}) catch |err|
             fatal("unable to open new source file '{s}': {s}", .{ new_source_path, @errorName(err) });
         defer f.close();
-        break :source std.zig.readSourceFileToEndAlloc(arena, f, std.zig.max_src_size) catch |err|
+        var file_reader: fs.File.Reader = f.reader(&stdio_buffer);
+        break :source std.zig.readSourceFileToEndAlloc(arena, &file_reader) catch |err|
             fatal("unable to read new source file '{s}': {s}", .{ new_source_path, @errorName(err) });
     };
 
@@ -6946,7 +6949,7 @@ fn cmdFetch(
         ast.deinit(gpa);
     }
 
-    var fixups: Ast.Fixups = .{};
+    var fixups: Ast.Render.Fixups = .{};
     defer fixups.deinit(gpa);
 
     var saved_path_or_url = path_or_url;
@@ -7047,12 +7050,13 @@ fn cmdFetch(
         try fixups.append_string_after_node.put(gpa, manifest.version_node, dependencies_text);
     }
 
-    var rendered = std.ArrayList(u8).init(gpa);
-    defer rendered.deinit();
-    try ast.renderToArrayList(&rendered, fixups);
+    var aw: std.Io.Writer.Allocating = .init(gpa);
+    defer aw.deinit();
+    try ast.render(gpa, &aw.writer, fixups);
+    const rendered = aw.getWritten();
 
-    build_root.directory.handle.writeFile(.{ .sub_path = Package.Manifest.basename, .data = rendered.items }) catch |err| {
-        fatal("unable to write {s} file: {s}", .{ Package.Manifest.basename, @errorName(err) });
+    build_root.directory.handle.writeFile(.{ .sub_path = Package.Manifest.basename, .data = rendered }) catch |err| {
+        fatal("unable to write {s} file: {t}", .{ Package.Manifest.basename, err });
     };
 
     return cleanExit();
diff --git a/tools/gen_spirv_spec.zig b/tools/gen_spirv_spec.zig
index 8840a476c4..c8a3cf5ced 100644
--- a/tools/gen_spirv_spec.zig
+++ b/tools/gen_spirv_spec.zig
@@ -120,7 +120,7 @@ pub fn main() !void {
         error_bundle.renderToStdErr(color.renderOptions());
     }
 
-    const formatted_output = try tree.render(allocator);
+    const formatted_output = try tree.renderAlloc(allocator);
     _ = try std.fs.File.stdout().write(formatted_output);
 }