xso-proc: provide FromXml::xml_name_matcher on derived impls

That way, derived implementations can also make use of the performance
benefits.

xso-proc/src/common.rs

@@ -7,8 +7,45 @@
 //! Definitions common to both enums and structs
 
 use proc_macro2::TokenStream;
+use quote::{quote, ToTokens};
 use syn::*;
 
+/// Template which renders to a `xso::fromxml::XmlNameMatcher` value.
+pub(crate) enum XmlNameMatcher {
+    /// Renders as `xso::fromxml::XmlNameMatcher::Any`.
+    #[allow(dead_code)] // We keep it for completeness.
+    Any,
+
+    /// Renders as `xso::fromxml::XmlNameMatcher::InNamespace(#0)`.
+    InNamespace(TokenStream),
+
+    /// Renders as `xso::fromxml::XmlNameMatcher::Specific(#0, #1)`.
+    Specific(TokenStream, TokenStream),
+
+    /// Renders as `#0`.
+    ///
+    /// This is an escape hatch for more complicated constructs, e.g. when
+    /// a superset of multiple matchers is required.
+    Custom(TokenStream),
+}
+
+impl ToTokens for XmlNameMatcher {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        match self {
+            Self::Any => tokens.extend(quote! {
+                ::xso::fromxml::XmlNameMatcher::<'static>::Any
+            }),
+            Self::InNamespace(ref namespace) => tokens.extend(quote! {
+                ::xso::fromxml::XmlNameMatcher::<'static>::InNamespace(#namespace)
+            }),
+            Self::Specific(ref namespace, ref name) => tokens.extend(quote! {
+                ::xso::fromxml::XmlNameMatcher::<'static>::Specific(#namespace, #name)
+            }),
+            Self::Custom(ref stream) => tokens.extend(stream.clone()),
+        }
+    }
+}
+
 /// Parts necessary to construct a `::xso::FromXml` implementation.
 pub(crate) struct FromXmlParts {
     /// Additional items necessary for the implementation.
@@ -19,6 +56,9 @@ pub(crate) struct FromXmlParts {
 
     /// The name of the type which is the `::xso::FromXml::Builder`.
     pub(crate) builder_ty_ident: Ident,
+
+    /// The `XmlNameMatcher` to pre-select elements for this implementation.
+    pub(crate) name_matcher: XmlNameMatcher,
 }
 
 /// Parts necessary to construct a `::xso::AsXml` implementation.