1 files changed, 168 insertions, 0 deletions
diff --git a/dhall/src/error/builder.rs b/dhall/src/error/builder.rs
new file mode 100644
index 0000000..39f8dfb
--- /dev/null
+++ b/dhall/src/error/builder.rs
@@ -0,0 +1,168 @@
+use annotate_snippets::{
+    display_list::DisplayList,
+    formatter::DisplayListFormatter,
+    snippet::{Annotation, AnnotationType, Slice, Snippet, SourceAnnotation},
+};
+
+use crate::syntax::{ParsedSpan, Span};
+
+#[derive(Debug, Clone, Default)]
+pub struct ErrorBuilder {
+    title: FreeAnnotation,
+    annotations: Vec<SpannedAnnotation>,
+    footer: Vec<FreeAnnotation>,
+    /// Inducate that the current builder has already been consumed and consuming it again should
+    /// panic.
+    consumed: bool,
+}
+
+#[derive(Debug, Clone)]
+struct SpannedAnnotation {
+    span: ParsedSpan,
+    message: String,
+    annotation_type: AnnotationType,
+}
+
+#[derive(Debug, Clone)]
+struct FreeAnnotation {
+    message: String,
+    annotation_type: AnnotationType,
+}
+
+impl SpannedAnnotation {
+    fn into_annotation(self) -> SourceAnnotation {
+        SourceAnnotation {
+            label: self.message,
+            annotation_type: self.annotation_type,
+            range: self.span.as_char_range(),
+        }
+    }
+}
+
+impl FreeAnnotation {
+    fn into_annotation(self) -> Annotation {
+        Annotation {
+            label: Some(self.message),
+            id: None,
+            annotation_type: self.annotation_type,
+        }
+    }
+}
+
+/// A builder that uses the annotate_snippets library to display nice error messages about source
+/// code locations.
+impl ErrorBuilder {
+    pub fn new(message: impl ToString) -> Self {
+        ErrorBuilder {
+            title: FreeAnnotation {
+                message: message.to_string(),
+                annotation_type: AnnotationType::Error,
+            },
+            annotations: Vec::new(),
+            footer: Vec::new(),
+            consumed: false,
+        }
+    }
+
+    pub fn span_annot(
+        &mut self,
+        span: Span,
+        message: impl ToString,
+        annotation_type: AnnotationType,
+    ) -> &mut Self {
+        // Ignore spans not coming from a source file
+        let span = match span {
+            Span::Parsed(span) => span,
+            _ => return self,
+        };
+        self.annotations.push(SpannedAnnotation {
+            span,
+            message: message.to_string(),
+            annotation_type,
+        });
+        self
+    }
+    pub fn footer_annot(
+        &mut self,
+        message: impl ToString,
+        annotation_type: AnnotationType,
+    ) -> &mut Self {
+        self.footer.push(FreeAnnotation {
+            message: message.to_string(),
+            annotation_type,
+        });
+        self
+    }
+
+    pub fn span_err(
+        &mut self,
+        span: Span,
+        message: impl ToString,
+    ) -> &mut Self {
+        self.span_annot(span, message, AnnotationType::Error)
+    }
+    pub fn span_help(
+        &mut self,
+        span: Span,
+        message: impl ToString,
+    ) -> &mut Self {
+        self.span_annot(span, message, AnnotationType::Help)
+    }
+    pub fn help(&mut self, message: impl ToString) -> &mut Self {
+        self.footer_annot(message, AnnotationType::Help)
+    }
+    pub fn note(&mut self, message: impl ToString) -> &mut Self {
+        self.footer_annot(message, AnnotationType::Note)
+    }
+
+    // TODO: handle multiple files
+    pub fn format(&mut self) -> String {
+        if self.consumed {
+            panic!("tried to format the same ErrorBuilder twice")
+        }
+        let this = std::mem::replace(self, ErrorBuilder::default());
+        self.consumed = true;
+        drop(self); // Get rid of the self reference so we don't use it by mistake.
+
+        let slices = if this.annotations.is_empty() {
+            Vec::new()
+        } else {
+            let input = this.annotations[0].span.to_input();
+            let annotations = this
+                .annotations
+                .into_iter()
+                .map(|annot| annot.into_annotation())
+                .collect();
+            vec![Slice {
+                source: input,
+                line_start: 1, // TODO
+                origin: Some("<current file>".to_string()),
+                fold: true,
+                annotations,
+            }]
+        };
+        let footer = this
+            .footer
+            .into_iter()
+            .map(|annot| annot.into_annotation())
+            .collect();
+
+        let snippet = Snippet {
+            title: Some(this.title.into_annotation()),
+            slices,
+            footer,
+        };
+        let dl = DisplayList::from(snippet);
+        let dlf = DisplayListFormatter::new(true, false);
+        format!("{}", dlf.format(&dl))
+    }
+}
+
+impl Default for FreeAnnotation {
+    fn default() -> Self {
+        FreeAnnotation {
+            message: String::new(),
+            annotation_type: AnnotationType::Error,
+        }
+    }
+}