std: add type-erased reader; base GenericReader on it

The idea here is to avoid code bloat by having only one actual io.Reader
implementation, which is type erased, and then implement a GenericReader
that preserves type information on top of that as thin glue code.

The strategy here is for that glue code to `@errSetCast` the result of
the type-erased reader functions, however, while trying to do that I
ran into #17343.

lib/std/io.zig

@@ -10,6 +10,7 @@ const fs = std.fs;
 const mem = std.mem;
 const meta = std.meta;
 const File = std.fs.File;
+const Allocator = std.mem.Allocator;
 
 pub const Mode = enum {
     /// I/O operates normally, waiting for the operating system syscalls to complete.
@@ -105,7 +106,243 @@ pub fn getStdIn() File {
     };
 }
 
-pub const Reader = @import("io/reader.zig").Reader;
+pub fn GenericReader(
+    comptime Context: type,
+    comptime ReadError: type,
+    /// Returns the number of bytes read. It may be less than buffer.len.
+    /// If the number of bytes read is 0, it means end of stream.
+    /// End of stream is not an error condition.
+    comptime readFn: fn (context: Context, buffer: []u8) ReadError!usize,
+) type {
+    return struct {
+        context: Context,
+
+        pub const Error = ReadError;
+        pub const NoEofError = ReadError || error{
+            EndOfStream,
+        };
+
+        pub inline fn read(self: Self, buffer: []u8) Error!usize {
+            return readFn(self.context, buffer);
+        }
+
+        pub inline fn readAll(self: Self, buffer: []u8) Error!usize {
+            return @errorCast(self.typeErased().readAll(buffer));
+        }
+
+        pub inline fn readAtLeast(self: Self, buffer: []u8, len: usize) Error!usize {
+            return @errorCast(self.typeErased().readAtLeast(buffer, len));
+        }
+
+        pub inline fn readNoEof(self: Self, buf: []u8) NoEofError!void {
+            return @errorCast(self.typeErased().readNoEof(buf));
+        }
+
+        pub inline fn readAllArrayList(
+            self: Self,
+            array_list: *std.ArrayList(u8),
+            max_append_size: usize,
+        ) (error{StreamTooLong} || Error)!void {
+            return @errorCast(self.typeErased().readAllArrayList(array_list, max_append_size));
+        }
+
+        pub inline fn readAllArrayListAligned(
+            self: Self,
+            comptime alignment: ?u29,
+            array_list: *std.ArrayListAligned(u8, alignment),
+            max_append_size: usize,
+        ) (error{StreamTooLong} || Error)!void {
+            return @errorCast(self.typeErased().readAllArrayListAligned(
+                alignment,
+                array_list,
+                max_append_size,
+            ));
+        }
+
+        pub inline fn readAllAlloc(self: Self, allocator: Allocator, max_size: usize) Error![]u8 {
+            return @errorCast(self.typeErased().readAllAlloc(allocator, max_size));
+        }
+
+        pub inline fn readUntilDelimiterArrayList(
+            self: Self,
+            array_list: *std.ArrayList(u8),
+            delimiter: u8,
+            max_size: usize,
+        ) Error!void {
+            return @errorCast(self.typeErased().readUntilDelimiterArrayList(
+                array_list,
+                delimiter,
+                max_size,
+            ));
+        }
+
+        pub inline fn readUntilDelimiterAlloc(
+            self: Self,
+            allocator: Allocator,
+            delimiter: u8,
+            max_size: usize,
+        ) Error![]u8 {
+            return @errorCast(self.typeErased().readUntilDelimiterAlloc(
+                allocator,
+                delimiter,
+                max_size,
+            ));
+        }
+
+        pub inline fn readUntilDelimiter(self: Self, buf: []u8, delimiter: u8) Error![]u8 {
+            return @errorCast(self.typeErased().readUntilDelimiter(buf, delimiter));
+        }
+
+        pub inline fn readUntilDelimiterOrEofAlloc(
+            self: Self,
+            allocator: Allocator,
+            delimiter: u8,
+            max_size: usize,
+        ) Error!?[]u8 {
+            return @errorCast(self.typeErased().readUntilDelimiterOrEofAlloc(
+                allocator,
+                delimiter,
+                max_size,
+            ));
+        }
+
+        pub inline fn readUntilDelimiterOrEof(self: Self, buf: []u8, delimiter: u8) Error!?[]u8 {
+            return @errorCast(self.typeErased().readUntilDelimiterOrEof(buf, delimiter));
+        }
+
+        pub inline fn streamUntilDelimiter(
+            self: Self,
+            writer: anytype,
+            delimiter: u8,
+            optional_max_size: ?usize,
+        ) (NoEofError || error{StreamTooLong} || @TypeOf(writer).Error)!void {
+            return @errorCast(self.typeErased().streamUntilDelimiter(
+                writer.typeErased(),
+                delimiter,
+                optional_max_size,
+            ));
+        }
+
+        pub inline fn skipUntilDelimiterOrEof(self: Self, delimiter: u8) Error!void {
+            return @errorCast(self.typeErased().skipUntilDelimiterOrEof(delimiter));
+        }
+
+        pub inline fn readByte(self: Self) NoEofError!u8 {
+            return @errorCast(self.typeErased().readByte());
+        }
+
+        pub inline fn readByteSigned(self: Self) NoEofError!i8 {
+            return @errorCast(self.typeErased().readByteSigned());
+        }
+
+        pub inline fn readBytesNoEof(
+            self: Self,
+            comptime num_bytes: usize,
+        ) NoEofError![num_bytes]u8 {
+            return @errorCast(self.typeErased().readBytesNoEof(num_bytes));
+        }
+
+        pub inline fn readIntoBoundedBytes(
+            self: Self,
+            comptime num_bytes: usize,
+            bounded: *std.BoundedArray(u8, num_bytes),
+        ) Error!void {
+            return @errorCast(self.typeErased().readIntoBoundedBytes(num_bytes, bounded));
+        }
+
+        pub inline fn readBoundedBytes(
+            self: Self,
+            comptime num_bytes: usize,
+        ) Error!std.BoundedArray(u8, num_bytes) {
+            return @errorCast(self.typeErased().readBoundedBytes(num_bytes));
+        }
+
+        pub inline fn readIntNative(self: Self, comptime T: type) NoEofError!T {
+            return @errorCast(self.typeErased().readIntNative(T));
+        }
+
+        pub inline fn readIntForeign(self: Self, comptime T: type) NoEofError!T {
+            return @errorCast(self.typeErased().readIntForeign(T));
+        }
+
+        pub inline fn readIntLittle(self: Self, comptime T: type) NoEofError!T {
+            return @errorCast(self.typeErased().readIntLittle(T));
+        }
+
+        pub inline fn readIntBig(self: Self, comptime T: type) NoEofError!T {
+            return @errorCast(self.typeErased().readIntBig(T));
+        }
+
+        pub inline fn readInt(self: Self, comptime T: type, endian: std.builtin.Endian) NoEofError!T {
+            return @errorCast(self.typeErased().readInt(T, endian));
+        }
+
+        pub inline fn readVarInt(
+            self: Self,
+            comptime ReturnType: type,
+            endian: std.builtin.Endian,
+            size: usize,
+        ) NoEofError!ReturnType {
+            return @errorCast(self.typeErased().readVarInt(ReturnType, endian, size));
+        }
+
+        pub const SkipBytesOptions = TypeErasedReader.SkipBytesOptions;
+
+        pub inline fn skipBytes(
+            self: Self,
+            num_bytes: u64,
+            comptime options: SkipBytesOptions,
+        ) Error!void {
+            return @errorCast(self.typeErased().skipBytes(num_bytes, options));
+        }
+
+        pub inline fn isBytes(self: Self, slice: []const u8) Error!bool {
+            return @errorCast(self.typeErased().isBytes(slice));
+        }
+
+        pub fn readStruct(self: Self, comptime T: type) Error!T {
+            return @errorCast(self.typeErased().readStruct(T));
+        }
+
+        pub inline fn readStructBig(self: Self, comptime T: type) Error!T {
+            return @errorCast(self.typeErased().readStructBig(T));
+        }
+
+        pub const ReadEnumError = Error || error{
+            /// An integer was read, but it did not match any of the tags in the supplied enum.
+            InvalidValue,
+        };
+
+        pub inline fn readEnum(
+            self: Self,
+            comptime Enum: type,
+            endian: std.builtin.Endian,
+        ) ReadEnumError!Enum {
+            return @errorCast(self.typeErased().readEnum(Enum, endian));
+        }
+
+        pub inline fn typeErased(self: *const Self) TypeErasedReader {
+            return .{
+                .context = @ptrCast(&self.context),
+                .readFn = typeErasedReadFn,
+            };
+        }
+
+        const Self = @This();
+
+        fn typeErasedReadFn(context: *const anyopaque, buffer: []u8) anyerror!usize {
+            const ptr: *const Context = @alignCast(@ptrCast(context));
+            return readFn(ptr.*, buffer);
+        }
+    };
+}
+
+/// Deprecated; consider switching to `TypeErasedReader` or use `GenericReader`
+/// to use previous API.
+pub const Reader = GenericReader;
+
+pub const TypeErasedReader = @import("io/Reader.zig");
+
 pub const Writer = @import("io/writer.zig").Writer;
 pub const SeekableStream = @import("io/seekable_stream.zig").SeekableStream;
 
@@ -168,7 +405,7 @@ test "null_writer" {
 }
 
 pub fn poll(
-    allocator: std.mem.Allocator,
+    allocator: Allocator,
     comptime StreamEnum: type,
     files: PollFiles(StreamEnum),
 ) Poller(StreamEnum) {
@@ -418,6 +655,7 @@ pub fn PollFiles(comptime StreamEnum: type) type {
 }
 
 test {
+    _ = TypeErasedReader;
     _ = @import("io/bit_reader.zig");
     _ = @import("io/bit_writer.zig");
     _ = @import("io/buffered_atomic_file.zig");
@@ -427,7 +665,6 @@ test {
     _ = @import("io/counting_writer.zig");
     _ = @import("io/counting_reader.zig");
     _ = @import("io/fixed_buffer_stream.zig");
-    _ = @import("io/reader.zig");
     _ = @import("io/writer.zig");
     _ = @import("io/peek_stream.zig");
     _ = @import("io/seekable_stream.zig");