From 00afaa4b1822e9d2c096830e546e96ab4e87f25b Mon Sep 17 00:00:00 2001
From: Andrew Kelley <andrew@ziglang.org>
Date: Fri, 18 Apr 2025 18:23:06 -0700
Subject: [PATCH] std.io.BufferedReader: get rid of the `@constCast`

but it's pushed out to various callsites
---
 lib/std/debug/Dwarf.zig             | 2 +-
 lib/std/debug/Dwarf/expression.zig  | 4 ++--
 lib/std/debug/FixedBufferReader.zig | 2 +-
 lib/std/debug/SelfInfo.zig          | 4 ++--
 lib/std/io/BufferedReader.zig       | 8 +++++---
 5 files changed, 11 insertions(+), 9 deletions(-)

diff --git a/lib/std/debug/Dwarf.zig b/lib/std/debug/Dwarf.zig
index 422d0e5b2e7a9b54e2eac7044be8cc76c5d144b8..4106b7d621bdea03fffb914ecf87ffec9968e76e 100644
--- a/lib/std/debug/Dwarf.zig
+++ b/lib/std/debug/Dwarf.zig
@@ -2236,7 +2236,7 @@ pub const ElfModule = struct {
             const section_bytes = try chopSlice(mapped_mem, shdr.sh_offset, shdr.sh_size);
             sections[section_index.?] = if ((shdr.sh_flags & elf.SHF_COMPRESSED) > 0) blk: {
                 var section_reader: std.io.BufferedReader = undefined;
-                section_reader.initFixed(section_bytes);
+                section_reader.initFixed(@constCast(section_bytes));
                 const chdr = section_reader.takeStruct(elf.Chdr) catch continue;
                 if (chdr.ch_type != .ZLIB) continue;
                 const ch_size = chdr.ch_size;
diff --git a/lib/std/debug/Dwarf/expression.zig b/lib/std/debug/Dwarf/expression.zig
index 4f11830d9c460603106d203d8960e733d5f4b9cb..3bdef6f03bc8b710a906eb8814675c15ded1ab8a 100644
--- a/lib/std/debug/Dwarf/expression.zig
+++ b/lib/std/debug/Dwarf/expression.zig
@@ -262,7 +262,7 @@ pub fn StackMachine(comptime options: Options) type {
         ) Error!?Value {
             if (initial_value) |i| try self.stack.append(allocator, .{ .generic = i });
             var reader: std.io.BufferedReader = undefined;
-            reader.initFixed(expression);
+            reader.initFixed(@constCast(expression));
             while (try self.step(&reader, allocator, context)) {}
             if (self.stack.items.len == 0) return null;
             return self.stack.items[self.stack.items.len - 1];
@@ -721,7 +721,7 @@ pub fn StackMachine(comptime options: Options) type {
                         if (context.thread_context == null) return error.IncompleteExpressionContext;
 
                         var block_reader: std.io.BufferedReader = undefined;
-                        block_reader.initFixed(block);
+                        block_reader.initFixed(@constCast(block));
                         const register = (try readOperand(&block_reader, block[0], context)).?.register;
                         const value = mem.readInt(usize, (try abi.regBytes(context.thread_context.?, register, context.reg_context))[0..@sizeOf(usize)], native_endian);
                         try self.stack.append(allocator, .{ .generic = value });
diff --git a/lib/std/debug/FixedBufferReader.zig b/lib/std/debug/FixedBufferReader.zig
index dc431ad0745a7d1b5c99e5fa3ff1d05e85aa0c29..035495ca5503eaf7334cd0a8e17449831f2e32b2 100644
--- a/lib/std/debug/FixedBufferReader.zig
+++ b/lib/std/debug/FixedBufferReader.zig
@@ -53,7 +53,7 @@ pub fn readIntChecked(
 
 pub fn readLeb128(fbr: *FixedBufferReader, comptime T: type) Error!T {
     var br: std.io.BufferedReader = undefined;
-    br.initFixed(fbr.buf);
+    br.initFixed(@constCast(fbr.buf));
     br.seek = fbr.pos;
     const result = br.takeLeb128(T);
     fbr.pos = br.seek;
diff --git a/lib/std/debug/SelfInfo.zig b/lib/std/debug/SelfInfo.zig
index 3b44ffcd6eae822a8ca26991e859b4adf2540d92..dfc4993297c6a3d77aa704debe7ab8847d618a62 100644
--- a/lib/std/debug/SelfInfo.zig
+++ b/lib/std/debug/SelfInfo.zig
@@ -2026,8 +2026,8 @@ pub const VirtualMachine = struct {
         if (pc < fde.pc_begin or pc >= fde.pc_begin + fde.pc_range) return error.AddressOutOfRange;
 
         var readers: [2]std.io.BufferedReader = undefined;
-        readers[0].initFixed(cie.initial_instructions);
-        readers[1].initFixed(fde.instructions);
+        readers[0].initFixed(@constCast(cie.initial_instructions));
+        readers[1].initFixed(@constCast(fde.instructions));
 
         var prev_row: Row = self.current_row;
         for (&readers, [2]bool{ true, false }) |*reader, is_initial| {
diff --git a/lib/std/io/BufferedReader.zig b/lib/std/io/BufferedReader.zig
index e7c163b316794f49e0ced1749f5fccc72fef47af..18182526860ed864f07cb32c798ab44192e61c29 100644
--- a/lib/std/io/BufferedReader.zig
+++ b/lib/std/io/BufferedReader.zig
@@ -24,12 +24,14 @@ pub fn init(br: *BufferedReader, r: Reader, buffer: []u8) void {
 }
 
 /// Constructs `br` such that it will read from `buffer` and then end.
-/// TODO either remove the const cast here or make methods of this file return a const slice
-pub fn initFixed(br: *BufferedReader, buffer: []const u8) void {
+///
+/// Most methods do not require mutating `buffer`. Those that do are marked,
+/// and if they are avoided then `buffer` can be safely used with `@constCast`.
+pub fn initFixed(br: *BufferedReader, buffer: []u8) void {
     br.* = .{
         .seek = 0,
         .storage = .{
-            .buffer = @constCast(buffer),
+            .buffer = buffer,
             .unbuffered_writer = .failing,
         },
         .unbuffered_reader = .ending,
-- 
2.54.0