add BoundedArrayAligned (#14580)

This is useful for creating byte buffers of actually-different-things.
Copied the argument order from `Allocator.alignedAlloc`

I noted that `ArrayListAligned` is going out of it's way to not set the
alignment at comptime when it is not specified. However, I was not able
to do that the same way here, and good people on IRC, @ifreund in
particular (thanks!) assured me that

    [N]T align(@alignOf(T))

is equivalent to

    [N]T

lib/std/bounded_array.zig

@@ -16,9 +16,30 @@ const testing = std.testing;
 /// var a_clone = a; // creates a copy - the structure doesn't use any internal pointers
 /// ```
 pub fn BoundedArray(comptime T: type, comptime buffer_capacity: usize) type {
+    return BoundedArrayAligned(T, @alignOf(T), buffer_capacity);
+}
+
+/// A structure with an array, length and alignment, that can be used as a
+/// slice.
+///
+/// Useful to pass around small explicitly-aligned arrays whose exact size is
+/// only known at runtime, but whose maximum size is known at comptime, without
+/// requiring an `Allocator`.
+/// ```zig
+//  var a = try BoundedArrayAligned(u8, 16, 2).init(0);
+//  try a.append(255);
+//  try a.append(255);
+//  const b = @ptrCast(*const [1]u16, a.constSlice().ptr);
+//  try testing.expectEqual(@as(u16, 65535), b[0]);
+/// ```
+pub fn BoundedArrayAligned(
+    comptime T: type,
+    comptime alignment: u29,
+    comptime buffer_capacity: usize,
+) type {
     return struct {
         const Self = @This();
-        buffer: [buffer_capacity]T = undefined,
+        buffer: [buffer_capacity]T align(alignment) = undefined,
         len: usize = 0,
 
         /// Set the actual length of the slice.
@@ -30,15 +51,15 @@ pub fn BoundedArray(comptime T: type, comptime buffer_capacity: usize) type {
 
         /// View the internal array as a slice whose size was previously set.
         pub fn slice(self: anytype) switch (@TypeOf(&self.buffer)) {
-            *[buffer_capacity]T => []T,
-            *const [buffer_capacity]T => []const T,
+            *align(alignment) [buffer_capacity]T => []align(alignment) T,
+            *align(alignment) const [buffer_capacity]T => []align(alignment) const T,
             else => unreachable,
         } {
             return self.buffer[0..self.len];
         }
 
         /// View the internal array as a constant slice whose size was previously set.
-        pub fn constSlice(self: *const Self) []const T {
+        pub fn constSlice(self: *const Self) []align(alignment) const T {
             return self.slice();
         }
 
@@ -94,7 +115,7 @@ pub fn BoundedArray(comptime T: type, comptime buffer_capacity: usize) type {
 
         /// Resize the slice, adding `n` new elements, which have `undefined` values.
         /// The return value is a slice pointing to the uninitialized elements.
-        pub fn addManyAsArray(self: *Self, comptime n: usize) error{Overflow}!*[n]T {
+        pub fn addManyAsArray(self: *Self, comptime n: usize) error{Overflow}!*align(alignment) [n]T {
             const prev_len = self.len;
             try self.resize(self.len + n);
             return self.slice()[prev_len..][0..n];
@@ -118,7 +139,7 @@ pub fn BoundedArray(comptime T: type, comptime buffer_capacity: usize) type {
         /// This can be useful for writing directly into it.
         /// Note that such an operation must be followed up with a
         /// call to `resize()`
-        pub fn unusedCapacitySlice(self: *Self) []T {
+        pub fn unusedCapacitySlice(self: *Self) []align(alignment) T {
             return self.buffer[self.len..];
         }
 
@@ -365,3 +386,15 @@ test "BoundedArray" {
     try w.writeAll(s);
     try testing.expectEqualStrings(s, a.constSlice());
 }
+
+test "BoundedArrayAligned" {
+    var a = try BoundedArrayAligned(u8, 16, 4).init(0);
+    try a.append(0);
+    try a.append(0);
+    try a.append(255);
+    try a.append(255);
+
+    const b = @ptrCast(*const [2]u16, a.constSlice().ptr);
+    try testing.expectEqual(@as(u16, 0), b[0]);
+    try testing.expectEqual(@as(u16, 65535), b[1]);
+}

lib/std/std.zig

@@ -9,6 +9,7 @@ pub const AutoArrayHashMapUnmanaged = array_hash_map.AutoArrayHashMapUnmanaged;
 pub const AutoHashMap = hash_map.AutoHashMap;
 pub const AutoHashMapUnmanaged = hash_map.AutoHashMapUnmanaged;
 pub const BoundedArray = @import("bounded_array.zig").BoundedArray;
+pub const BoundedArrayAligned = @import("bounded_array.zig").BoundedArrayAligned;
 pub const Build = @import("Build.zig");
 pub const BufMap = @import("buf_map.zig").BufMap;
 pub const BufSet = @import("buf_set.zig").BufSet;