Show source names in diagnostics

crates/decodal-core/src/eval.rs

@@ -87,6 +87,17 @@ impl<L: SourceLoader> Engine<L> {
         self.materialize_with_path(value, &mut Vec::new())
     }
 
+    pub fn source_name(&self, source: SourceId) -> Option<&str> {
+        self.modules
+            .iter()
+            .find(|module| module.source == source)
+            .map(|module| module.name.as_str())
+    }
+
+    pub fn format_diagnostic(&self, diagnostic: &Diagnostic) -> String {
+        format_diagnostic_with(diagnostic, |source| self.source_name(source))
+    }
+
     fn materialize_with_path(
         &mut self,
         value: &RuntimeValue,
@@ -1163,6 +1174,39 @@ fn primitive_type(name: &str) -> Option<PrimitiveType> {
     }
 }
 
+pub fn format_diagnostic_with<'a>(
+    diagnostic: &Diagnostic,
+    mut source_name: impl FnMut(SourceId) -> Option<&'a str>,
+) -> String {
+    let mut out = format!(
+        "{:?} at {}:{}..{}: {}",
+        diagnostic.kind,
+        format_source(diagnostic.span.source, &mut source_name),
+        diagnostic.span.start,
+        diagnostic.span.end,
+        diagnostic.message,
+    );
+    for label in &diagnostic.labels {
+        out.push_str(&format!(
+            "\nnote at {}:{}..{}: {}",
+            format_source(label.span.source, &mut source_name),
+            label.span.start,
+            label.span.end,
+            label.message,
+        ));
+    }
+    out
+}
+
+fn format_source<'a>(
+    source: SourceId,
+    source_name: &mut impl FnMut(SourceId) -> Option<&'a str>,
+) -> String {
+    source_name(source)
+        .map(String::from)
+        .unwrap_or_else(|| format!("source {}", source.0))
+}
+
 fn constraint_primary_span(constraints: &[ConstraintEntry]) -> Span {
     constraints
         .first()

crates/decodal-core/src/lib.rs

@@ -17,7 +17,7 @@ pub use ast::{Ast, BinaryOp, CompareOp, Expr, ExprId, Field, Literal, Param};
 pub use constraints::normalize_constraints;
 pub use diagnostic::{Diagnostic, DiagnosticKind, Result};
 pub use embedding::{HostField, HostValue};
-pub use eval::Engine;
+pub use eval::{Engine, format_diagnostic_with};
 pub use lexer::{Lexer, Token, TokenKind};
 pub use module::{EmptyLoader, LoadedSource, Module, SourceLoader};
 pub use parser::{ParseOutput, Parser, SourceForm, parse_source, parse_source_with_source_id};