InternPool: eliminate var_args_param_type

This was a "fake" type used to handle C varargs parameters, much like
generic poison. In fact, it is treated identically to generic poison in
all cases other than one (the final coercion of a call argument), which
is trivially special-cased. Thus, it makes sense to remove this special
tag and instead use `generic_poison_type` in its place. This fixes
several bugs in Sema related to missing handling of this tag.

Resolves: ziglang#19781

src/Air.zig

@@ -891,8 +891,7 @@ pub const Inst = struct {
  /// The most-significant bit of the value is a tag bit. This bit is 1 if the value represents an
  /// instruction index and 0 if it represents an InternPool index.
  ///
- /// The hardcoded refs `none` and `var_args_param_type` are exceptions to this rule: they have
- /// their tag bit set but refer to the InternPool.
+ /// The ref `none` is an exception: it has the tag bit set but refers to the InternPool.
  pub const Ref = enum(u32) {
  u0_type = @intFromEnum(InternPool.Index.u0_type),
  i0_type = @intFromEnum(InternPool.Index.i0_type),
@@ -979,9 +978,6 @@ pub const Inst = struct {
  empty_struct = @intFromEnum(InternPool.Index.empty_struct),
  generic_poison = @intFromEnum(InternPool.Index.generic_poison),
 
- /// This Ref does not correspond to any AIR instruction or constant
- /// value. It is used to handle argument types of var args functions.
- var_args_param_type = @intFromEnum(InternPool.Index.var_args_param_type),
  /// This Ref does not correspond to any AIR instruction or constant
  /// value and may instead be used as a sentinel to indicate null.
  none = @intFromEnum(InternPool.Index.none),
@@ -994,7 +990,6 @@ pub const Inst = struct {
 
  pub fn toInternedAllowNone(ref: Ref) ?InternPool.Index {
  return switch (ref) {
- .var_args_param_type => .var_args_param_type,
  .none => .none,
  else => if (@intFromEnum(ref) >> 31 == 0)
  @enumFromInt(@as(u31, @truncate(@intFromEnum(ref))))
@@ -1010,7 +1005,6 @@ pub const Inst = struct {
 
  pub fn toIndexAllowNone(ref: Ref) ?Index {
  return switch (ref) {
- .var_args_param_type, .none => null,
  else => if (@intFromEnum(ref) >> 31 != 0)
  @enumFromInt(@as(u31, @truncate(@intFromEnum(ref))))
  else
@@ -1557,7 +1551,6 @@ pub fn deinit(air: *Air, gpa: std.mem.Allocator) void {
 
 pub fn internedToRef(ip_index: InternPool.Index) Inst.Ref {
  return switch (ip_index) {
- .var_args_param_type => .var_args_param_type,
  .none => .none,
  else => {
  assert(@intFromEnum(ip_index) >> 31 == 0);

src/InternPool.zig

@@ -2818,7 +2818,6 @@ pub const Index = enum(u32) {
  generic_poison,
 
  /// Used by Air/Sema only.
- var_args_param_type = std.math.maxInt(u32) - 1,
  none = std.math.maxInt(u32),
 
  _,
@@ -8938,7 +8937,6 @@ pub fn typeOf(ip: *const InternPool, index: Index) Index {
  .memoized_call => unreachable,
  },
 
- .var_args_param_type => unreachable,
  .none => unreachable,
  };
 }
@@ -9153,8 +9151,6 @@ pub fn zigTypeTagOrPoison(ip: *const InternPool, index: Index) error{GenericPois
  .empty_struct => unreachable,
  .generic_poison => unreachable,
 
- .var_args_param_type => unreachable, // special tag
-
  _ => switch (ip.items.items(.tag)[@intFromEnum(index)]) {
  .removed => unreachable,
 

src/Sema.zig

@@ -1901,7 +1901,6 @@ pub fn resolveConstStringIntern(
 
 pub fn resolveType(sema: *Sema, block: *Block, src: LazySrcLoc, zir_ref: Zir.Inst.Ref) !Type {
  const air_inst = try sema.resolveInst(zir_ref);
- assert(air_inst != .var_args_param_type);
  const ty = try sema.analyzeAsType(block, src, air_inst);
  if (ty.isGenericPoison()) return error.GenericPoison;
  return ty;
@@ -4572,12 +4571,10 @@ fn zirValidateRefTy(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileErr
  const src = un_tok.src();
  // In case of GenericPoison, we don't actually have a type, so this will be
  // treated as an untyped address-of operator.
- if (un_tok.operand == .var_args_param_type) return;
  const operand_air_inst = sema.resolveInst(un_tok.operand) catch |err| switch (err) {
  error.GenericPoison => return,
  else => |e| return e,
  };
- if (operand_air_inst == .var_args_param_type) return;
  const ty_operand = sema.analyzeAsType(block, src, operand_air_inst) catch |err| switch (err) {
  error.GenericPoison => return,
  else => |e| return e,
@@ -7371,16 +7368,17 @@ const CallArgsInfo = union(enum) {
  sema: *Sema,
  block: *Block,
  arg_index: usize,
- param_ty: Type,
+ /// `null` means this is a varargs param.
+ maybe_param_ty: ?Type,
  func_ty_info: InternPool.Key.FuncType,
  func_inst: Air.Inst.Ref,
  ) CompileError!Air.Inst.Ref {
  const mod = sema.mod;
  const param_count = func_ty_info.param_types.len;
- switch (param_ty.toIntern()) {
- .generic_poison_type, .var_args_param_type => {},
+ if (maybe_param_ty) |param_ty| switch (param_ty.toIntern()) {
+ .generic_poison_type => {},
  else => try sema.queueFullTypeResolution(param_ty),
- }
+ };
  const uncoerced_arg: Air.Inst.Ref = switch (cai) {
  inline .resolved, .call_builtin => |resolved| resolved.args[arg_index],
  .zir_call => |zir_call| arg_val: {
@@ -7409,7 +7407,8 @@ const CallArgsInfo = union(enum) {
  // TODO set comptime_reason
  }
  // Give the arg its result type
- sema.inst_map.putAssumeCapacity(zir_call.call_inst, Air.internedToRef(param_ty.toIntern()));
+ const provide_param_ty = if (maybe_param_ty) |t| t else Type.generic_poison;
+ sema.inst_map.putAssumeCapacity(zir_call.call_inst, Air.internedToRef(provide_param_ty.toIntern()));
  // Resolve the arg!
  const uncoerced_arg = try sema.resolveInlineBody(block, arg_body, zir_call.call_inst);
 
@@ -7426,9 +7425,11 @@ const CallArgsInfo = union(enum) {
  break :arg_val uncoerced_arg;
  },
  };
+ const param_ty = maybe_param_ty orelse {
+ return sema.coerceVarArgParam(block, uncoerced_arg, cai.argSrc(block, arg_index));
+ };
  switch (param_ty.toIntern()) {
  .generic_poison_type => return uncoerced_arg,
- .var_args_param_type => return sema.coerceVarArgParam(block, uncoerced_arg, cai.argSrc(block, arg_index)),
  else => return sema.coerceExtra(
  block,
  param_ty,
@@ -7970,9 +7971,9 @@ fn analyzeCall(
  const args = try sema.arena.alloc(Air.Inst.Ref, args_info.count());
  for (args, 0..) |*arg_out, arg_idx| {
  // Non-generic, so param types are already resolved
- const param_ty = if (arg_idx < func_ty_info.param_types.len) ty: {
+ const param_ty: ?Type = if (arg_idx < func_ty_info.param_types.len) ty: {
  break :ty Type.fromInterned(func_ty_info.param_types.get(ip)[arg_idx]);
- } else Type.fromInterned(InternPool.Index.var_args_param_type);
+ } else null;
  assert(!param_ty.isGenericPoison());
  arg_out.* = try args_info.analyzeArg(sema, block, arg_idx, param_ty, func_ty_info, func);
  try sema.validateRuntimeValue(block, args_info.argSrc(block, arg_idx), arg_out.*);
@@ -10226,12 +10227,10 @@ fn analyzeAs(
 ) CompileError!Air.Inst.Ref {
  const mod = sema.mod;
  const operand = try sema.resolveInst(zir_operand);
- if (zir_dest_type == .var_args_param_type) return operand;
  const operand_air_inst = sema.resolveInst(zir_dest_type) catch |err| switch (err) {
  error.GenericPoison => return operand,
  else => |e| return e,
  };
- if (operand_air_inst == .var_args_param_type) return operand;
  const dest_ty = sema.analyzeAsType(block, src, operand_air_inst) catch |err| switch (err) {
  error.GenericPoison => return operand,
  else => |e| return e,
@@ -35656,8 +35655,6 @@ pub fn resolveTypeFields(sema: *Sema, ty: Type) CompileError!void {
  const ty_ip = ty.toIntern();
 
  switch (ty_ip) {
- .var_args_param_type => unreachable,
-
  .none => unreachable,
 
  .u0_type,
@@ -37176,7 +37173,6 @@ pub fn typeHasOnePossibleValue(sema: *Sema, ty: Type) CompileError!?Value {
  .empty_struct,
  .generic_poison,
  // invalid
- .var_args_param_type,
  .none,
  => unreachable,
 

src/codegen/c/Type.zig

@@ -1468,7 +1468,6 @@ pub const Pool = struct {
  .bool_false,
  .empty_struct,
  .generic_poison,
- .var_args_param_type,
  .none,
  => unreachable,
 

src/codegen/llvm.zig

@@ -3235,7 +3235,6 @@ pub const Object = struct {
  .bool_false,
  .empty_struct,
  .generic_poison,
- .var_args_param_type,
  .none,
  => unreachable,
  else => switch (ip.indexToKey(t.toIntern())) {