dev-lang/swift-bootstrap: add 1.1

Introduces the `binary` USE flag (on by default) which allows
bootstrapping from `dev-lang/swift-bin` instead of building from source
using `dev-lang/swift`.

Swift 5.10 is becoming increassingly less desirable as a standalone
Swift version, and more difficult to build over time with newer
toolchain versions. For bootstrapping specifically, using a pre-built
binary is likely an acceptable default.

Signed-off-by: Itai Ferber <itai@itaiferber.net>

dev-lang/swift-bootstrap/metadata.xml

@@ -13,6 +13,9 @@
     install of Swift on a new machine, this package vends a base version of
     Swift that newer versions can BDEPEND on.
   </longdescription>
+  <use>
+    <flag name="binary">Bootstrap from binary (dev-lang/swift-bin:5/10) instead of building from source (dev-lang/swift:5/10).</flag>
+  </use>
   <upstream>
     <remote-id type="github">swiftlang/swift</remote-id>
   </upstream>

dev-lang/swift-bootstrap/swift-bootstrap-1.1.ebuild

@@ -0,0 +1,26 @@
+# Copyright 1999-2024 Gentoo Authors
+# Distributed under the terms of the GNU General Public License v2
+
+EAPI=8
+
+DESCRIPTION="A virtual package providing a base Swift to bootstrap future versions with."
+HOMEPAGE="https://www.swift.org"
+
+S="${WORKDIR}"
+
+LICENSE="GPL-2"
+SLOT="5/10"
+KEYWORDS="~amd64"
+
+IUSE="+binary"
+
+# `dev-lang/swift` BDEPENDS on either this package or `dev-lang/swift` itself. When any version of `dev-lang/swift` is
+# installed, it will be preferred over this package; when no version of `dev-lang/swift` is installed, `emerge` will
+# fall back to installing this.
+#
+# Because `dev-lang/swift` is versioned by SLOT, any updates to newer versions of Swift will record the current version
+# in the @world set, leaving them around; this allows `swift-bootstrap` to eventually get cleaned up.
+RDEPEND="
+	binary? ( dev-lang/swift-bin:5/10 )
+	!binary? ( dev-lang/swift:5/10 )
+"