xso-proc: improve spans for error messages

For codec-based error messages, this reduces the amount of errors per
violation to one. For all others, it improves the placement of the error
slightly, but we still get duplicates.

I couldn't figure out the remaining discrepancies in the spans ...

xso-proc/src/compound.rs

@@ -430,12 +430,14 @@ impl Compound {
                 } => {
                     let feed = feed_fn(builder.clone());
 
+                    let mut substate_data_ident = substate_data.clone();
+                    substate_data_ident.set_span(ty.span());
                     states.push(State::new_with_builder(
                         state_name.clone(),
                         builder_data_ident,
                         &builder_data_ty,
                     ).with_field(
-                        substate_data,
+                        &substate_data_ident,
                         &builder,
                     ).with_mut(substate_data).with_impl(quote! {
                         match #feed(&mut #substate_data, ev, ctx)? {

xso-proc/src/field/attribute.rs

@@ -9,8 +9,8 @@
 //! In particular, it provides the `#[xml(attribute)]` implementation.
 
 use proc_macro2::Span;
-use quote::{quote, ToTokens};
-use syn::*;
+use quote::{quote, quote_spanned, ToTokens};
+use syn::{spanned::Spanned, *};
 
 use std::borrow::Cow;
 
@@ -115,8 +115,9 @@ impl Field for AttributeField {
 
         let finalize = match self.codec {
             Some(ref codec) => {
-                let decode = text_codec_decode_fn(ty.clone());
-                quote! {
+                let span = codec.span();
+                let decode = text_codec_decode_fn(ty.clone(), span);
+                quote_spanned! { span=>
                     |value| #decode(&#codec, value)
                 }
             }
@@ -170,8 +171,15 @@ impl Field for AttributeField {
 
         let generator = match self.codec {
             Some(ref codec) => {
-                let encode = text_codec_encode_fn(ty.clone());
-                quote! { #encode(&#codec, #bound_name)? }
+                let span = codec.span();
+                let encode = text_codec_encode_fn(ty.clone(), span);
+                // NOTE: We need to fudge the span of `bound_name` here,
+                // because its span points outside the macro (the identifier
+                // of the field), which means that quote_spanned will not
+                // override it, which would make the error message ugly.
+                let mut bound_name = bound_name.clone();
+                bound_name.set_span(span);
+                quote_spanned! { span=> #encode(&#codec, #bound_name)? }
             }
             None => {
                 let as_optional_xml_text = as_optional_xml_text_fn(ty.clone());

xso-proc/src/field/child.rs

@@ -68,7 +68,8 @@ impl Field for ChildField {
 
                 let from_events = from_events_fn(element_ty.clone());
 
-                let matcher = quote! { #from_events(name, attrs, ctx) };
+                let span = element_ty.span();
+                let matcher = quote_spanned! { span=> #from_events(name, attrs, ctx) };
                 let builder = from_xml_builder_ty(element_ty.clone());
 
                 (
@@ -178,9 +179,10 @@ impl Field for ChildField {
                 let as_xml_iter = as_xml_iter_fn(item_ty.clone());
                 let item_iter = item_iter_ty(item_ty.clone(), lifetime.clone());
 
+                let span = item_ty.span();
                 (
                     TokenStream::default(),
-                    quote! { #as_xml_iter(#bound_name)? },
+                    quote_spanned! { span=> #as_xml_iter(#bound_name)? },
                     item_iter,
                 )
             }

xso-proc/src/field/text.rs

@@ -9,8 +9,8 @@
 //! In particular, it provides the `#[xml(text)]` implementation.
 
 use proc_macro2::Span;
-use quote::quote;
-use syn::*;
+use quote::{quote, quote_spanned};
+use syn::{spanned::Spanned, *};
 
 use crate::error_message::ParentRef;
 use crate::scope::{AsItemsScope, FromEventsScope};
@@ -38,8 +38,9 @@ impl Field for TextField {
         let field_access = scope.access_field(member);
         let finalize = match self.codec {
             Some(ref codec) => {
-                let decode = text_codec_decode_fn(ty.clone());
-                quote! {
+                let span = codec.span();
+                let decode = text_codec_decode_fn(ty.clone(), span);
+                quote_spanned! { span=>
                     #decode(&#codec, #field_access)?
                 }
             }
@@ -71,8 +72,15 @@ impl Field for TextField {
     ) -> Result<FieldIteratorPart> {
         let generator = match self.codec {
             Some(ref codec) => {
-                let encode = text_codec_encode_fn(ty.clone());
-                quote! { #encode(&#codec, #bound_name)? }
+                let span = codec.span();
+                let encode = text_codec_encode_fn(ty.clone(), span);
+                // NOTE: We need to fudge the span of `bound_name` here,
+                // because its span points outside the macro (the identifier
+                // of the field), which means that quote_spanned will not
+                // override it, which would make the error message ugly.
+                let mut bound_name = bound_name.clone();
+                bound_name.set_span(span);
+                quote_spanned! { span=> #encode(&#codec, #bound_name)? }
             }
             None => {
                 let as_xml_text = as_xml_text_fn(ty.clone());

xso-proc/src/types.rs

@@ -316,43 +316,46 @@ pub(crate) fn as_xml_text_fn(ty: Type) -> Expr {
 
 /// Construct a [`syn::Path`] referring to `::xso::TextCodec::<#for_ty>`,
 /// returning the span of `for_ty` alongside it.
-fn text_codec_of(for_ty: Type) -> (Span, Path) {
-    let span = for_ty.span();
-    (
-        span,
-        Path {
-            leading_colon: Some(syn::token::PathSep {
-                spans: [span, span],
-            }),
-            segments: [
-                PathSegment {
-                    ident: Ident::new("xso", span),
-                    arguments: PathArguments::None,
-                },
-                PathSegment {
-                    ident: Ident::new("TextCodec", span),
-                    arguments: PathArguments::AngleBracketed(AngleBracketedGenericArguments {
-                        colon2_token: Some(syn::token::PathSep {
-                            spans: [span, span],
-                        }),
-                        lt_token: syn::token::Lt { spans: [span] },
-                        args: [GenericArgument::Type(for_ty)].into_iter().collect(),
-                        gt_token: syn::token::Gt { spans: [span] },
+///
+/// The span used is `codec_span`, in order to ensure that error messages
+/// about a missing implementation point at the codec, not at the type.
+fn text_codec_of(for_ty: Type, codec_span: Span) -> Path {
+    let span = codec_span;
+    Path {
+        leading_colon: Some(syn::token::PathSep {
+            spans: [span, span],
+        }),
+        segments: [
+            PathSegment {
+                ident: Ident::new("xso", span),
+                arguments: PathArguments::None,
+            },
+            PathSegment {
+                ident: Ident::new("TextCodec", span),
+                arguments: PathArguments::AngleBracketed(AngleBracketedGenericArguments {
+                    colon2_token: Some(syn::token::PathSep {
+                        spans: [span, span],
                     }),
-                },
-            ]
-            .into_iter()
-            .collect(),
-        },
-    )
+                    lt_token: syn::token::Lt { spans: [span] },
+                    args: [GenericArgument::Type(for_ty)].into_iter().collect(),
+                    gt_token: syn::token::Gt { spans: [span] },
+                }),
+            },
+        ]
+        .into_iter()
+        .collect(),
+    }
 }
 
 /// Construct a [`syn::Expr`] referring to
 /// `::xso::TextCodec::<#for_ty>::encode`.
-pub(crate) fn text_codec_encode_fn(for_ty: Type) -> Expr {
-    let (span, mut path) = text_codec_of(for_ty);
+///
+/// The span used is `codec_span`, in order to ensure that error messages
+/// about a missing implementation point at the codec, not at the type.
+pub(crate) fn text_codec_encode_fn(for_ty: Type, codec_span: Span) -> Expr {
+    let mut path = text_codec_of(for_ty, codec_span);
     path.segments.push(PathSegment {
-        ident: Ident::new("encode", span),
+        ident: Ident::new("encode", codec_span),
         arguments: PathArguments::None,
     });
     Expr::Path(ExprPath {
@@ -364,10 +367,13 @@ pub(crate) fn text_codec_encode_fn(for_ty: Type) -> Expr {
 
 /// Construct a [`syn::Expr`] referring to
 /// `::xso::TextCodec::<#for_ty>::decode`.
-pub(crate) fn text_codec_decode_fn(for_ty: Type) -> Expr {
-    let (span, mut path) = text_codec_of(for_ty);
+///
+/// The span used is `codec_span`, in order to ensure that error messages
+/// about a missing implementation point at the codec, not at the type.
+pub(crate) fn text_codec_decode_fn(for_ty: Type, codec_span: Span) -> Expr {
+    let mut path = text_codec_of(for_ty, codec_span);
     path.segments.push(PathSegment {
-        ident: Ident::new("decode", span),
+        ident: Ident::new("decode", codec_span),
         arguments: PathArguments::None,
     });
     Expr::Path(ExprPath {