core: address review comments

- break formatting values out into a function so that we can
  catch errors and never fail
- eliminate the use of toOwnedSentinelSlice since we are using
  an arena to clean up memory

src/cli/args.zig

@@ -132,12 +132,55 @@ pub fn parse(
                 // track more error messages.
                 error.OutOfMemory => return err,
                 error.InvalidField => "unknown field",
-                error.ValueRequired => "value required",
+                error.ValueRequired => formatValueRequired(T, arena_alloc, key) catch "value required",
-                error.InvalidValue => msg: {
+                error.InvalidValue => formatInvalidValue(T, arena_alloc, key, value) catch "invalid value",
+                else => try std.fmt.allocPrintZ(
+                    arena_alloc,
+                    "unknown error {}",
+                    .{err},
+                ),
+            };
+
+            // Add our diagnostic
+            try dst._diagnostics.append(arena_alloc, .{
+                .key = try arena_alloc.dupeZ(u8, key),
+                .message = message,
+                .location = diags.Location.fromIter(iter),
+            });
+        };
+    }
+}
+
+fn formatValueRequired(
+    comptime T: type,
+    arena_alloc: std.mem.Allocator,
+    key: []const u8,
+) std.mem.Allocator.Error![:0]const u8 {
+    var buf = std.ArrayList(u8).init(arena_alloc);
+    errdefer buf.deinit();
+    const writer = buf.writer();
+    try writer.print("value required", .{});
+    try formatValues(T, key, writer);
+    try writer.writeByte(0);
+    return buf.items[0 .. buf.items.len - 1 :0];
+}
+
+fn formatInvalidValue(
+    comptime T: type,
+    arena_alloc: std.mem.Allocator,
+    key: []const u8,
+    value: ?[]const u8,
+) std.mem.Allocator.Error![:0]const u8 {
     var buf = std.ArrayList(u8).init(arena_alloc);
     errdefer buf.deinit();
     const writer = buf.writer();
     try writer.print("invalid value \"{?s}\"", .{value});
+    try formatValues(T, key, writer);
+    try writer.writeByte(0);
+    return buf.items[0 .. buf.items.len - 1 :0];
+}
+
+fn formatValues(comptime T: type, key: []const u8, writer: anytype) std.mem.Allocator.Error!void {
     const typeinfo = @typeInfo(T);
     inline for (typeinfo.Struct.fields) |f| {
         if (std.mem.eql(u8, key, f.name)) {
@@ -154,23 +197,6 @@ pub fn parse(
             break;
         }
     }
-                    break :msg try buf.toOwnedSliceSentinel(0);
-                },
-                else => try std.fmt.allocPrintZ(
-                    arena_alloc,
-                    "unknown error {}",
-                    .{err},
-                ),
-            };
-
-            // Add our diagnostic
-            try dst._diagnostics.append(arena_alloc, .{
-                .key = try arena_alloc.dupeZ(u8, key),
-                .message = message,
-                .location = diags.Location.fromIter(iter),
-            });
-        };
-    }
 }
 
 /// Returns true if this type can track diagnostics.