fix(zero length, no context): Add Option to distinguish between "curr…

…ent line" and "just the span"

Fixes #355

Change the number of context lines from usize to Option<usize> to allow
choosing between "just the span" (as implemented previously when using
0 context line) using `None`, and displaying the error line without
extra context (not possible before) using `Some(0)`.

src/eyreish/wrapper.rs

@@ -283,7 +283,7 @@ mod tests {
             report
                 .source_code()
                 .unwrap()
-                .read_span(&(0..5).into(), 0, 0)
+                .read_span(&(0..5).into(), None, None)
                 .unwrap()
                 .data()
                 .to_vec(),

src/handlers/graphical.rs

@@ -28,7 +28,7 @@ pub struct GraphicalReportHandler {
     pub(crate) termwidth: usize,
     pub(crate) theme: GraphicalTheme,
     pub(crate) footer: Option<String>,
-    pub(crate) context_lines: usize,
+    pub(crate) context_lines: Option<usize>,
     pub(crate) tab_width: usize,
     pub(crate) with_cause_chain: bool,
     pub(crate) wrap_lines: bool,
@@ -55,7 +55,7 @@ impl GraphicalReportHandler {
             termwidth: 200,
             theme: GraphicalTheme::default(),
             footer: None,
-            context_lines: 1,
+            context_lines: Some(1),
             tab_width: 4,
             with_cause_chain: true,
             wrap_lines: true,
@@ -74,7 +74,7 @@ impl GraphicalReportHandler {
             termwidth: 200,
             theme,
             footer: None,
-            context_lines: 1,
+            context_lines: Some(1),
             tab_width: 4,
             wrap_lines: true,
             with_cause_chain: true,
@@ -159,7 +159,7 @@ impl GraphicalReportHandler {
         self
     }
 
-    /// Sets the word splitter to usewhen wrapping.
+    /// Sets the word splitter to use when wrapping.
     pub fn with_word_splitter(mut self, word_splitter: textwrap::WordSplitter) -> Self {
         self.word_splitter = Some(word_splitter);
         self
@@ -172,7 +172,22 @@ impl GraphicalReportHandler {
     }
 
     /// Sets the number of lines of context to show around each error.
-    pub fn with_context_lines(mut self, lines: usize) -> Self {
+    ///
+    /// If `0`, then only the span content will be shown (equivalent to
+    /// `with_opt_context_lines(None)`).\
+    /// Use `with_opt_context_lines(Some(0))` if you want the whole line
+    /// containing the error without extra context.
+    pub fn with_context_lines(self, lines: usize) -> Self {
+        self.with_opt_context_lines((lines != 0).then_some(lines))
+    }
+
+    /// Sets the number of lines of context to show around each error.
+    ///
+    /// `None` means only the span content (and possibly the content in between
+    /// multiple adjacent labels) will be shown.\
+    /// `Some(0)` will show the whole line containing the label.\
+    /// `Some(n)` will show the whole line plus n line before and after the label.
+    pub fn with_opt_context_lines(mut self, lines: Option<usize>) -> Self {
         self.context_lines = lines;
         self
     }
@@ -559,7 +574,7 @@ impl GraphicalReportHandler {
         // as the reference point for line/column information.
         let primary_contents = match primary_label {
             Some(label) => source
-                .read_span(label.inner(), 0, 0)
+                .read_span(label.inner(), None, None)
                 .map_err(|_| fmt::Error)?,
             None => contents,
         };

src/handlers/json.rs

@@ -159,7 +159,7 @@ impl JSONReportHandler {
     ) -> fmt::Result {
         if let Some(mut labels) = diagnostic.labels() {
             if let Some(label) = labels.next() {
-                if let Ok(span_content) = source.read_span(label.inner(), 0, 0) {
+                if let Ok(span_content) = source.read_span(label.inner(), None, None) {
                     let filename = span_content.name().unwrap_or_default();
                     return write!(f, r#""filename": "{}","#, escape(filename));
                 }

src/handlers/narratable.rs

@@ -13,7 +13,7 @@ non-graphical environments, such as non-TTY output.
 */
 #[derive(Debug, Clone)]
 pub struct NarratableReportHandler {
-    context_lines: usize,
+    context_lines: Option<usize>,
     with_cause_chain: bool,
     footer: Option<String>,
 }
@@ -24,7 +24,7 @@ impl NarratableReportHandler {
     pub const fn new() -> Self {
         Self {
             footer: None,
-            context_lines: 1,
+            context_lines: Some(1),
             with_cause_chain: true,
         }
     }
@@ -49,7 +49,22 @@ impl NarratableReportHandler {
     }
 
     /// Sets the number of lines of context to show around each error.
-    pub const fn with_context_lines(mut self, lines: usize) -> Self {
+    ///
+    /// If `0`, then only the span content will be shown (equivalent to
+    /// `with_opt_context_lines(None)`).\
+    /// Use `with_opt_context_lines(Some(0))` if you want the whole line
+    /// containing the error without extra context.
+    pub fn with_context_lines(self, lines: usize) -> Self {
+        self.with_opt_context_lines((lines != 0).then_some(lines))
+    }
+
+    /// Sets the number of lines of context to show around each error.
+    ///
+    /// `None` means only the span content (and possibly the content in between
+    /// multiple adjacent labels) will be shown.\
+    /// `Some(0)` will show the whole line containing the label.\
+    /// `Some(n)` will show the whole line plus n line before and after the label.
+    pub fn with_opt_context_lines(mut self, lines: Option<usize>) -> Self {
         self.context_lines = lines;
         self
     }

src/named_source.rs

@@ -56,8 +56,8 @@ impl<S: SourceCode + 'static> SourceCode for NamedSource<S> {
     fn read_span<'a>(
         &'a self,
         span: &crate::SourceSpan,
-        context_lines_before: usize,
-        context_lines_after: usize,
+        context_lines_before: Option<usize>,
+        context_lines_after: Option<usize>,
     ) -> Result<Box<dyn SpanContents<'a> + 'a>, MietteError> {
         let inner_contents =
             self.inner()

src/protocol.rs

@@ -247,11 +247,16 @@ gigabytes or larger in size.
 pub trait SourceCode: Send + Sync {
     /// Read the bytes for a specific span from this `SourceCode`, keeping a
     /// certain number of lines before and after the span as context.
+    /// When the `context_lines_before` (resp. `context_lines_after`) is  set to
+    /// `None`, the span content must start (resp. stop) at the **span** boundary.
+    /// When set to `Some(0)`, the content should start (resp. stop) at the
+    /// **line** boundary. When set to `Some(n)`, the content should include the
+    /// _n_ lines before (resp. after) the span.
     fn read_span<'a>(
         &'a self,
         span: &SourceSpan,
-        context_lines_before: usize,
-        context_lines_after: usize,
+        context_lines_before: Option<usize>,
+        context_lines_after: Option<usize>,
     ) -> Result<Box<dyn SpanContents<'a> + 'a>, MietteError>;
 }