3 files changed, 94 insertions, 9 deletions

diff --git a/tvix/eval/src/compiler/attrs.rs b/tvix/eval/src/compiler/attrs.rs
index 864cc2cb3da0..91039d6dbfbe 100644
--- a/tvix/eval/src/compiler/attrs.rs
+++ b/tvix/eval/src/compiler/attrs.rs
@@ -33,7 +33,7 @@ impl Compiler<'_, '_> {
             let span = self.span_for(&node);
             self.emit_warning(
                 span,
-                WarningKind::NotImplemented("recursive attribute sets are not yet implemented"),
+                WarningKind::NotImplemented("recursive attribute sets"),
             );
         }
 
diff --git a/tvix/eval/src/eval.rs b/tvix/eval/src/eval.rs
index 585463dde22e..4ed97b329d80 100644
--- a/tvix/eval/src/eval.rs
+++ b/tvix/eval/src/eval.rs
@@ -13,7 +13,7 @@ pub fn interpret(code: &str, location: Option<PathBuf>) -> EvalResult<Value> {
         location
             .as_ref()
             .map(|p| p.to_string_lossy().to_string())
-            .unwrap_or_else(|| "<repl>".into()),
+            .unwrap_or_else(|| "[tvix-repl]".into()),
         code.into(),
     );
     let codemap = Rc::new(codemap);
@@ -47,7 +47,7 @@ pub fn interpret(code: &str, location: Option<PathBuf>) -> EvalResult<Value> {
             location,
             &file,
             global_builtins(),
-            &mut DisassemblingObserver::new(codemap, std::io::stderr()),
+            &mut DisassemblingObserver::new(codemap.clone(), std::io::stderr()),
         )
     } else {
         crate::compiler::compile(
@@ -60,12 +60,7 @@ pub fn interpret(code: &str, location: Option<PathBuf>) -> EvalResult<Value> {
     }?;
 
     for warning in result.warnings {
-        eprintln!(
-            "warning: {:?} at `{}`[line {}]",
-            warning.kind,
-            file.source_slice(warning.span),
-            file.find_line(warning.span.low()) + 1
-        )
+        warning.fancy_format_stderr(&codemap);
     }
 
     for error in &result.errors {
diff --git a/tvix/eval/src/warnings.rs b/tvix/eval/src/warnings.rs
index b37d0fc91863..a09c5746d387 100644
--- a/tvix/eval/src/warnings.rs
+++ b/tvix/eval/src/warnings.rs
@@ -1,6 +1,9 @@
 //! Implements warnings that are emitted in cases where code passed to
 //! Tvix exhibits problems that the user could address.
 
+use codemap::CodeMap;
+use codemap_diagnostic::{ColorConfig, Diagnostic, Emitter, Level, SpanLabel, SpanStyle};
+
 #[derive(Debug)]
 pub enum WarningKind {
     DeprecatedLiteralURL,
@@ -18,3 +21,90 @@ pub struct EvalWarning {
     pub kind: WarningKind,
     pub span: codemap::Span,
 }
+
+impl EvalWarning {
+    /// Render a fancy, human-readable output of this warning and
+    /// return it as a String. Note that this version of the output
+    /// does not include any colours or font styles.
+    pub fn fancy_format_str(&self, codemap: &CodeMap) -> String {
+        let mut out = vec![];
+        Emitter::vec(&mut out, Some(codemap)).emit(&[self.diagnostic(codemap)]);
+        String::from_utf8_lossy(&out).to_string()
+    }
+
+    /// Render a fancy, human-readable output of this warning and
+    /// print it to stderr. If rendered in a terminal that supports
+    /// colours and font styles, the output will include those.
+    pub fn fancy_format_stderr(&self, codemap: &CodeMap) {
+        Emitter::stderr(ColorConfig::Auto, Some(codemap)).emit(&[self.diagnostic(codemap)]);
+    }
+
+    /// Create the optional span label displayed as an annotation on
+    /// the underlined span of the warning.
+    fn span_label(&self) -> Option<String> {
+        match self.kind {
+            WarningKind::UnusedBinding | WarningKind::ShadowedGlobal(_) => {
+                Some("variable declared here".into())
+            }
+            _ => None,
+        }
+    }
+
+    /// Create the primary warning message displayed to users for a
+    /// warning.
+    fn message(&self, codemap: &CodeMap) -> String {
+        match self.kind {
+            WarningKind::DeprecatedLiteralURL => {
+                format!("URL literal syntax is deprecated, use a quoted string instead")
+            }
+
+            WarningKind::UselessInherit => {
+                format!("inherited variable already exists with the same value")
+            }
+
+            WarningKind::UnusedBinding => {
+                let file = codemap.find_file(self.span.low());
+
+                format!(
+                    "variable '{}' is declared, but never used:",
+                    file.source_slice(self.span)
+                )
+            }
+
+            WarningKind::ShadowedGlobal(name) => {
+                format!("declared variable '{}' shadows a built-in global!", name)
+            }
+
+            WarningKind::NotImplemented(what) => {
+                format!("feature not yet implemented in tvix: {}", what)
+            }
+        }
+    }
+
+    /// Return the unique warning code for this variant which can be
+    /// used to refer users to documentation.
+    fn code(&self) -> &'static str {
+        match self.kind {
+            WarningKind::DeprecatedLiteralURL => "W001",
+            WarningKind::UselessInherit => "W002",
+            WarningKind::UnusedBinding => "W003",
+            WarningKind::ShadowedGlobal(_) => "W004",
+            WarningKind::NotImplemented(_) => "W999",
+        }
+    }
+
+    fn diagnostic(&self, codemap: &CodeMap) -> Diagnostic {
+        let span_label = SpanLabel {
+            label: self.span_label(),
+            span: self.span,
+            style: SpanStyle::Primary,
+        };
+
+        Diagnostic {
+            level: Level::Warning,
+            message: self.message(codemap),
+            spans: vec![span_label],
+            code: Some(self.code().into()),
+        }
+    }
+}