std: fix a couple compilation errors

lib/std/Progress.zig

@@ -611,7 +611,7 @@ pub fn unlockStdErr() void {
 const stderr_writer: *Writer = &stderr_file_writer.interface;
 /// Protected by `stderr_mutex`.
 var stderr_file_writer: std.fs.File.Writer = .{
-    .interface = std.fs.File.Writer.init_interface(&.{}),
+    .interface = std.fs.File.Writer.initInterface(&.{}),
     .file = if (is_windows) undefined else .stderr(),
     .mode = .streaming,
 };
@@ -628,7 +628,7 @@ pub fn lockStderrWriter(buffer: []u8) *Writer {
     if (is_windows) stderr_file_writer.file = .stderr();
     stderr_writer.flush() catch {};
     stderr_writer.buffer = buffer;
-    return &stderr_writer;
+    return stderr_writer;
 }
 
 pub fn unlockStderrWriter() void {

lib/std/fs/File.zig

@@ -993,7 +993,7 @@ pub const Reader = struct {
                 };
                 var remaining = std.math.cast(u64, offset) orelse return seek_err;
                 while (remaining > 0) {
-                    const n = discard(r, .limited(remaining)) catch |err| {
+                    const n = discard(&r.interface, .limited(remaining)) catch |err| {
                         r.seek_err = err;
                         return err;
                     };
@@ -1252,25 +1252,29 @@ pub const Writer = struct {
     /// vectors through the underlying write calls as possible.
     const max_buffers_len = 16;
 
-    pub fn init(file: File, buffer: []u8) std.io.Writer {
+    pub fn init(file: File, buffer: []u8) Writer {
         return initMode(file, buffer, .positional);
     }
 
-    pub fn initMode(file: File, buffer: []u8, init_mode: Writer.Mode) std.io.Writer {
+    pub fn initMode(file: File, buffer: []u8, init_mode: Writer.Mode) Writer {
         return .{
             .file = file,
-            .interface = .{
-                .context = undefined,
-                .vtable = &.{
-                    .drain = drain,
-                    .sendFile = sendFile,
-                },
-                .buffer = buffer,
-            },
+            .interface = initInterface(buffer),
             .mode = init_mode,
         };
     }
 
+    pub fn initInterface(buffer: []u8) std.io.Writer {
+        return .{
+            .context = undefined,
+            .vtable = &.{
+                .drain = drain,
+                .sendFile = sendFile,
+            },
+            .buffer = buffer,
+        };
+    }
+
     pub fn moveToReader(w: *Writer) Reader {
         defer w.* = undefined;
         return .{
@@ -1335,7 +1339,11 @@ pub const Writer = struct {
         };
     }
 
-    pub fn sendFile(io_writer: *Writer, file_reader: *Reader, limit: std.io.Limit) std.io.Writer.FileError!usize {
+    pub fn sendFile(
+        io_writer: *std.io.Writer,
+        file_reader: *Reader,
+        limit: std.io.Limit,
+    ) std.io.Writer.FileError!usize {
         const w: *Writer = @fieldParentPtr("interface", io_writer);
         const out_fd = w.file.handle;
         const in_fd = file_reader.file.handle;

lib/std/io/Reader.zig

@@ -162,7 +162,7 @@ pub fn defaultDiscard(r: *Reader, limit: Limit) Error!usize {
     };
     if (n > @intFromEnum(limit)) {
         const over_amt = n - @intFromEnum(limit);
-        assert(over_amt <= w.buffer.end); // limit may be exceeded only by an amount within buffer capacity.
+        assert(over_amt <= w.buffer.len); // limit may be exceeded only by an amount within buffer capacity.
         r.seek = w.end - over_amt;
         r.end = w.end;
         return @intFromEnum(limit);