Implement Render even if parsing failed

This makes error messages much more readable.

askama_derive/src/heritage.rs

@@ -36,7 +36,7 @@ impl Heritage<'_> {
 
 type BlockAncestry<'a> = HashMap<&'a str, Vec<(&'a Context<'a>, &'a BlockDef<'a>)>>;
 
-#[derive(Clone)]
+#[derive(Default, Clone)]
 pub(crate) struct Context<'a> {
     pub(crate) nodes: &'a [Node<'a>],
     pub(crate) extends: Option<Rc<Path>>,

askama_derive/src/input.rs

@@ -332,6 +332,14 @@ impl TemplateArgs {
         Ok(args)
     }
 
+    pub(crate) fn fallback() -> Self {
+        Self {
+            source: Some(Source::Source("".to_string())),
+            ext: Some("txt".to_string()),
+            ..Self::default()
+        }
+    }
+
     pub(crate) fn config(&self) -> Result<String, CompileError> {
         read_config_file(self.config.as_deref())
     }

askama_derive/src/lib.rs

@@ -23,10 +23,26 @@ pub fn derive_template(input: TokenStream) -> TokenStream {
     let ast = syn::parse::<syn::DeriveInput>(input).unwrap();
     match build_template(&ast) {
         Ok(source) => source.parse().unwrap(),
-        Err(e) => e.into_compile_error(),
+        Err(e) => {
+            let mut e = e.into_compile_error();
+            if let Ok(source) = build_skeleton(&ast) {
+                let source: TokenStream = source.parse().unwrap();
+                e.extend(source);
+            }
+            e
+        }
     }
 }
 
+fn build_skeleton(ast: &syn::DeriveInput) -> Result<String, CompileError> {
+    let template_args = TemplateArgs::fallback();
+    let config = Config::new("", None)?;
+    let input = TemplateInput::new(ast, &config, &template_args)?;
+    let mut contexts = HashMap::new();
+    contexts.insert(&input.path, Context::default());
+    Generator::new(&input, &contexts, None, MapChain::default()).build(&contexts[&input.path])
+}
+
 /// Takes a `syn::DeriveInput` and generates source code for it
 ///
 /// Reads the metadata from the `template()` attribute to get the template

testing/tests/ui/filter_block_ws.stderr

@@ -6,16 +6,3 @@ error: failed to parse template source at row 1, column 27 near:
   |          ^^^^^^^^
   |
   = note: this error originates in the derive macro `Template` (in Nightly builds, run with -Z macro-backtrace for more info)
-
-error[E0599]: no method named `render` found for struct `A` in the current scope
-  --> tests/ui/filter_block_ws.rs:11:7
-   |
-8  | struct A;
-   | -------- method `render` not found for this struct
-...
-11 |     A.render().unwrap();
-   |       ^^^^^^ method not found in `A`
-   |
-   = help: items from traits can only be used if the trait is implemented and in scope
-   = note: the following trait defines an item `render`, perhaps you need to implement it:
-           candidate #1: `Template`