Add annotate_snippets for better parsing errors (#477)

Adds nicer errors for Podlang code, using the `annotate_snippets` crate, the same crate used by the Rust compiler to generate contextual errors. This prints a short snippet of the code containing the error within the error message, highlighting the part that needs to be fixed.

It also includes a change to the `load_module` function, changing a `Vec` function argument to a slice.

src/lang/mod.rs

@@ -26,6 +26,7 @@
 //! Large predicates are automatically split into chains of smaller predicates;
 //! `apply_predicate` handles this transparently.
 //!
+pub mod diagnostics;
 pub mod error;
 pub mod frontend_ast;
 pub mod frontend_ast_lower;
@@ -37,7 +38,8 @@ pub mod pretty_print;
 
 use std::sync::Arc;
 
-pub use error::LangError;
+pub use diagnostics::render_error;
+pub use error::{LangError, LangErrorKind};
 pub use frontend_ast_split::{SplitChainInfo, SplitChainPiece, SplitResult};
 pub use module::{Module, MultiOperationError};
 pub use parser::{parse_podlang, Pairs, ParseError, Rule};
@@ -57,7 +59,17 @@ pub fn load_module(
     source: &str,
     name: &str,
     params: &Params,
-    available_modules: Vec<Arc<Module>>,
+    available_modules: &[Arc<Module>],
+) -> Result<Module, LangError> {
+    load_module_inner(source, name, params, available_modules)
+        .map_err(|e| e.with_source(source.to_string(), None))
+}
+
+fn load_module_inner(
+    source: &str,
+    name: &str,
+    params: &Params,
+    available_modules: &[Arc<Module>],
 ) -> Result<Module, LangError> {
     let pairs = parse_podlang(source)?;
     let document_pair = pairs
@@ -89,6 +101,15 @@ pub fn parse_request(
     source: &str,
     params: &Params,
     available_modules: &[Arc<Module>],
+) -> Result<PodRequest, LangError> {
+    parse_request_inner(source, params, available_modules)
+        .map_err(|e| e.with_source(source.to_string(), None))
+}
+
+fn parse_request_inner(
+    source: &str,
+    params: &Params,
+    available_modules: &[Arc<Module>],
 ) -> Result<PodRequest, LangError> {
     let pairs = parse_podlang(source)?;
     let document_pair = pairs
@@ -160,7 +181,7 @@ mod tests {
         "#;
 
         let params = Params::default();
-        let module = load_module(input, "test_module", &params, vec![])?;
+        let module = load_module(input, "test_module", &params, &[])?;
 
         assert_eq!(module.batch.predicates().len(), 1);
 
@@ -235,7 +256,7 @@ mod tests {
         "#;
 
         let params = Params::default();
-        let module = load_module(input, "test_module", &params, vec![])?;
+        let module = load_module(input, "test_module", &params, &[])?;
 
         assert_eq!(module.batch.predicates().len(), 1);
 
@@ -281,7 +302,7 @@ mod tests {
         "#;
 
         let params = Params::default();
-        let module = Arc::new(load_module(module_input, "my_module", &params, vec![])?);
+        let module = Arc::new(load_module(module_input, "my_module", &params, &[])?);
 
         assert_eq!(module.batch.predicates().len(), 1);
 
@@ -330,7 +351,7 @@ mod tests {
         "#;
 
         let params = Params::default();
-        let module = Arc::new(load_module(module_input, "some_module", &params, vec![])?);
+        let module = Arc::new(load_module(module_input, "some_module", &params, &[])?);
 
         let module_hash = module.id().encode_hex::<String>();
 
@@ -585,7 +606,7 @@ mod tests {
             )
         "#;
 
-        let module = load_module(input, "ethdos", &params, vec![])?;
+        let module = load_module(input, "ethdos", &params, &[])?;
 
         assert_eq!(
             module.batch.predicates().len(),
@@ -855,7 +876,7 @@ mod tests {
         );
 
         // 3. Load as module
-        let module = load_module(&input, "test", &params, vec![extmod])?;
+        let module = load_module(&input, "test", &params, &[extmod])?;
 
         assert_eq!(
             module.batch.predicates().len(),
@@ -1011,8 +1032,8 @@ mod tests {
 
         assert!(result.is_err());
 
-        match result.err().unwrap() {
-            LangError::Validation(e) => match *e {
+        match result.err().unwrap().kind {
+            LangErrorKind::Validation(e) => match *e {
                 frontend_ast_validate::ValidationError::ModuleNotFound { name, .. } => {
                     // The error now carries the hex-formatted hash
                     assert_eq!(name, fake_hash);
@@ -1034,12 +1055,12 @@ mod tests {
             )
         "#;
 
-        let result = load_module(input, "test", &params, vec![]);
+        let result = load_module(input, "test", &params, &[]);
 
         assert!(result.is_err());
 
-        match result.err().unwrap() {
-            LangError::Validation(e) => match *e {
+        match result.err().unwrap().kind {
+            LangErrorKind::Validation(e) => match *e {
                 frontend_ast_validate::ValidationError::UndefinedWildcard {
                     name,
                     pred_name,