Store span information to allow for better error messages

src/attribute.rs

@@ -1,29 +1,50 @@
 use std::collections::HashMap;
 
-use syn::{Ident, Lit, Token};
+use proc_macro2::Span;
+use syn::{Attribute, Ident, Lit, Token};
 use syn::parse::{Parse, ParseBuffer, ParseStream};
 use syn::parse::discouraged::Speculative;
 use syn::token::Token;
 
+#[derive(Clone)]
 pub(crate) enum Value {
     None,
     Lit(Lit),
     Ident(Ident),
 }
 
+#[derive(Clone)]
 pub(crate) struct Dict {
-    pub(crate) inner: HashMap<String, Value>
+    pub(crate) inner: HashMap<String, (Value, Span)>
 }
 
 impl Dict {
-    fn require_keys(&self, keys: &[&str]) -> Result<(), String> {
-        match keys.iter().find(|key| !self.inner.contains_key(**key)) {
+    pub(crate) fn new() -> Self {
+        Dict { inner: HashMap::new() }
+    }
+
+    pub(crate) fn from_attrs(name: &str, attrs: &[Attribute]) -> syn::Result<Self> {
+        let mut dict = Dict::new();
+
+        let sub_dicts = attrs.iter()
+            .filter(|attr| attr.path.is_ident(name))
+            .map(|attr| attr.parse_args::<Dict>());
+
+        for sub_dict in sub_dicts {
+            dict.inner.extend(sub_dict?.inner.into_iter());
+        }
+
+        Ok(dict)
+    }
+
+    pub(crate) fn require_keys(&self, keys: &[&str]) -> Result<(), String> {
+        match keys.iter().find(|&&key| !self.inner.contains_key(key)) {
             Some(absent_key) => Err(absent_key.to_string()),
             None => Ok(())
         }
     }
 
-    fn allow_keys(&self, keys: &[&str]) -> Result<(), String> {
+    pub(crate) fn allow_keys(&self, keys: &[&str]) -> Result<(), String> {
         match self.inner.keys().find(|key| !keys.contains(&key.as_str())) {
             Some(disallowed_key) => Err(disallowed_key.clone()),
             None => Ok(())
@@ -37,7 +58,7 @@ impl Parse for Dict {
             inner: input
                 .parse_terminated::<KeyValPair, Token![,]>(KeyValPair::parse)?
                 .into_iter()
-                .map(|pair| (pair.key, pair.val))
+                .map(|pair| (pair.key, (pair.val, pair.span)))
                 .collect::<HashMap<_, _>>()
         })
     }
@@ -46,13 +67,13 @@ impl Parse for Dict {
 struct KeyValPair {
     key: String,
     val: Value,
+    span: Span,
 }
 
 impl Parse for KeyValPair {
     fn parse(input: ParseStream) -> syn::Result<Self> {
         let key = input
-            .parse::<Ident>()?
-            .to_string();
+            .parse::<Ident>()?;
 
         let val = if input.peek(Token![=]) {
             input.parse::<Token![=]>()?;
@@ -61,13 +82,20 @@ impl Parse for KeyValPair {
             } else if let Ok(ident) = speculative_parse::<Ident>(input) {
                 Value::Ident(ident)
             } else {
-                return Err(input.error(format!("expected either a literal or identifier as the value corresponding to the key \"{}\", but found neither", key)));
+                let err_msg = format!(
+                    "expected either a literal or identifier as the value corresponding to the \
+                     key \"{}\", but found neither", key);
+                return Err(input.error(err_msg));
             }
         } else {
             Value::None
         };
 
-        Ok(KeyValPair { key, val })
+        Ok(KeyValPair {
+            key: key.to_string(),
+            val,
+            span: key.span(),
+        })
     }
 }