From f1fa66771c5dc5e6779729db66e674ba9ec063e2 Mon Sep 17 00:00:00 2001
From: torque <torque@users.noreply.github.com>
Date: Sun, 5 Nov 2023 12:10:02 -0800
Subject: [PATCH] std.io: add AnyWriter

This is a non-generic writer that holds a type-erased pointer to a
generic writer and is suitable for use in scenarios, such as runtime
callbacks, where generic functions cannot be used.

The approach taken here is a bit different from AnyReader in that the
type-erased version has not replaced the generic version. Instead, an
interface mixin is used to share the same interface between the two
implementations.
---
 lib/std/io.zig        |  3 +++
 lib/std/io/test.zig   |  9 +++++++++
 lib/std/io/writer.zig | 38 ++++++++++++++++++++++++++++++++++++--
 3 files changed, 48 insertions(+), 2 deletions(-)

diff --git a/lib/std/io.zig b/lib/std/io.zig
index e19b2e0e163c..3c0aee4b3060 100644
--- a/lib/std/io.zig
+++ b/lib/std/io.zig
@@ -340,6 +340,9 @@ pub const Reader = GenericReader;
 pub const AnyReader = @import("io/Reader.zig");
 
 pub const Writer = @import("io/writer.zig").Writer;
+pub const AnyWriter = @import("io/writer.zig").AnyWriter;
+pub const WriterInterface = @import("io/writer.zig").WriterInterface;
+
 pub const SeekableStream = @import("io/seekable_stream.zig").SeekableStream;
 
 pub const BufferedWriter = @import("io/buffered_writer.zig").BufferedWriter;
diff --git a/lib/std/io/test.zig b/lib/std/io/test.zig
index 6de0fe1e5387..0263d105e2f2 100644
--- a/lib/std/io/test.zig
+++ b/lib/std/io/test.zig
@@ -190,3 +190,12 @@ test "GenericReader methods can return error.EndOfStream" {
         fbs.reader().isBytes("foo"),
     );
 }
+
+test "AnyWriter write to buffer" {
+    var buffer = std.ArrayList(u8).init(std.testing.allocator);
+    defer buffer.deinit();
+    const writer = buffer.writer().any();
+
+    try writer.print("{s}", .{"greetings"});
+    try std.testing.expectEqualStrings("greetings", buffer.items);
+}
diff --git a/lib/std/io/writer.zig b/lib/std/io/writer.zig
index f1c0efda9037..64d4892671cd 100644
--- a/lib/std/io/writer.zig
+++ b/lib/std/io/writer.zig
@@ -11,12 +11,46 @@ pub fn Writer(
         context: Context,
 
         const Self = @This();
-        pub const Error = WriteError;
 
-        pub fn write(self: Self, bytes: []const u8) Error!usize {
+        pub fn write(self: Self, bytes: []const u8) Self.Error!usize {
             return writeFn(self.context, bytes);
         }
 
+        pub inline fn any(self: *const Self) AnyWriter {
+            return .{
+                .context = @ptrCast(self),
+                .writeFn = typeErasedWriteFn,
+            };
+        }
+
+        fn typeErasedWriteFn(context: *const anyopaque, bytes: []const u8) anyerror!usize {
+            const self: *const Self = @ptrCast(@alignCast(context));
+            return self.write(bytes);
+        }
+
+        pub usingnamespace WriterInterface(Self, WriteError);
+    };
+}
+
+pub const AnyWriter = struct {
+    context: *const anyopaque,
+    writeFn: *const fn (context: *const anyopaque, bytes: []const u8) AnyWriter.Error!usize,
+
+    pub fn write(self: AnyWriter, bytes: []const u8) AnyWriter.Error!usize {
+        return self.writeFn(self.context, bytes);
+    }
+
+    pub inline fn any(self: AnyWriter) AnyWriter {
+        return self;
+    }
+
+    pub usingnamespace WriterInterface(AnyWriter, anyerror);
+};
+
+pub fn WriterInterface(comptime Self: type, comptime ErrSet: type) type {
+    return struct {
+        pub const Error = ErrSet;
+
         pub fn writeAll(self: Self, bytes: []const u8) Error!void {
             var index: usize = 0;
             while (index != bytes.len) {