tamer: diagnose::report: Add line of padding above footer

This adds a line of padding between the last line of a source marking and
the first line of a footer, making it easier to read.  This also matches the
behavior of Rust's error message.

This is something I intended to do previously, but didn't have the
time.  Not that I do now, but now that we'll be showing some more robust
diagnostics to users, it ought to look decent.

DEV-13430

tamer/src/diagnose/report.rs

@@ -124,6 +124,10 @@ impl<R: SpanResolver> Reporter for VisualReporter<R> {
         //   which is more aesthetically pleasing.
         report.normalize_gutters();
 
+        // Let sections perform any final formatting,
+        //   e.g. to trim or merge padding.
+        report.finalize_sections();
+
         if report.level.is_error() {
             // Not worried about overflow panic
             //   (you have bigger problems if there are that many errors).
@@ -298,6 +302,11 @@ impl<'d, D: Diagnostic> Report<'d, D> {
             sec.line_max = self.line_max;
         }
     }
+
+    /// Finalize section formatting before display to the user.
+    fn finalize_sections(&mut self) {
+        self.secs.iter_mut().for_each(Section::finalize)
+    }
 }
 
 impl<'d, D: Diagnostic> Extend<Section<'d>> for Report<'d, D> {
@@ -469,6 +478,7 @@ impl<'s, 'd> Section<'d> {
                 SectionLine::SourceLinePadding,
                 SectionLine::SourceLine(src.into()),
                 SectionLine::SourceLineMark(LineMark { col, level, label }),
+                SectionLine::SourceLinePadding,
             ]);
         } else {
             dest.extend(vec![
@@ -538,6 +548,27 @@ impl<'s, 'd> Section<'d> {
     fn gutter_text_width(&self) -> usize {
         self.line_max.ilog10().add(1).max(2) as usize
     }
+
+    /// Finalize formatting of this section before display to the user.
+    ///
+    /// This is the last chance to clean things up.
+    fn finalize(&mut self) {
+        use SectionLine::SourceLinePadding;
+
+        // Padding is added conservatively during generation,
+        //   which may lead to adjacent padding for multi-line spans.
+        // That padding can be merged into a single line.
+        self.body.dedup_by(|a, b| match (a, b) {
+            (SourceLinePadding, SourceLinePadding) => true,
+            _ => false,
+        });
+
+        // Padding at the very end of the section is not desirable since the
+        //   report already adds padding around sections.
+        if self.body.last() == Some(&SourceLinePadding) {
+            self.body.pop();
+        }
+    }
 }
 
 impl<'d, 'a, S> From<MaybeResolvedSpan<'d, S>> for Section<'d>

tamer/src/diagnose/report/test.rs

@@ -199,6 +199,8 @@ fn section_from_mspan_resolved() {
                     label: None,
                 }),
                 SectionLine::SourceLinePadding,
+                // Will be stripped during finalization
+                SectionLine::SourceLinePadding,
                 SectionLine::SourceLine(src_lines[1].clone().into()),
                 SectionLine::SourceLineMark(LineMark {
                     level: Level::Note,
@@ -206,6 +208,8 @@ fn section_from_mspan_resolved() {
                     // Label at last source line
                     label: Some("test label".into()),
                 }),
+                // Will be stripped during finalization
+                SectionLine::SourceLinePadding,
             ],
             line_max: 2.unwrap_into(),
         }

tamer/src/diagnose/report/test/integration.rs

@@ -241,6 +241,7 @@ error: multiple adjacent same span with labels
    |
  1 | bar/baz line 1
    |         ^^^^^^ error: A label
+   |
    = error: C label
 "
     );
@@ -275,12 +276,14 @@ error: eq context neq offset/len
    |
  1 | bar/baz line 1
    | ^^^
+   |
    = error: A, first label, after mark
 
   --> bar/baz:1:1
    |
  1 | bar/baz line 1
    | ^^^^^^^ error: B, different length
+   |
    = error: B, collapse
 
   --> bar/baz:2:1
@@ -335,6 +338,7 @@ error: multiple adjacent different context
    |
  1 | foo/bar line 1
    | ^^^^^^^
+   |
    = error: A, first, after marked
    = error: A, collapsed
 
@@ -342,6 +346,7 @@ error: multiple adjacent different context
    |
  1 | bar/baz line 1
    | ^^^^^^^ error: B, first, no marked
+   |
    = error: B, collapsed
 
   --> foo/bar:1:1
@@ -381,6 +386,7 @@ internal error: multiple spans with labels of different severity level
    |
  4 | foo/bar line 4
    |         !!!!!! internal error: an internal error
+   |
    = error: an error
    = note: a note
    = help: a help message