xso: fix no_std support

This reverts the Revert in 6ec275d3 which had to be done in order to
un-break usage of xso in std crates.

xso/ChangeLog

@@ -31,6 +31,7 @@ Version NEXT:
         the base64 crate (if the `base64` feature is enabled).
       - New `codec` field on `attribute` meta, to support decoding and
         encoding using any `TextCodec`.
+      - Support for `no_std` usage (the alloc crate is required, though).
 
 Version 0.1.2:
 2024-07-26 Jonas Schäfer <jonas@zombofant.net>

xso/src/lib.rs

@@ -45,6 +45,22 @@ pub mod exports {
     pub use minidom;
     pub use rxml;
 
+    // These re-exports are necessary to support both std and no_std in code
+    // generated by the macros.
+    //
+    // If we attempted to use ::alloc directly from macros, std builds would
+    // not work because alloc is not generally present in builds using std.
+    // If we used ::std, no_std builds would obviously not work. By exporting
+    // std as alloc in std builds, we can safely use the alloc types from
+    // there.
+    //
+    // Obviously, we have to be careful in xso-proc to not refer to types
+    // which are not in alloc.
+    #[cfg(not(feature = "std"))]
+    pub extern crate alloc;
+    #[cfg(feature = "std")]
+    pub extern crate std as alloc;
+
     /// The built-in `bool` type.
     ///
     /// This is re-exported for use by macros in cases where we cannot rely on