fix(forge): clarify empty coverage

crates/forge/src/cmd/coverage.rs

@@ -40,7 +40,12 @@ foundry_config::impl_figment_convert!(CoverageArgs, test);
 /// option in `foundry.toml`. CLI flags take precedence over config; the helper
 /// `resolve_with` merges them after the config is loaded.
 #[derive(Parser)]
-#[command(after_long_help = r#"Compatibility:
+#[command(after_long_help = r#"Source attribution:
+  Coverage follows compiler source maps. Inherited modifier code is reported under the
+  source where the modifier is declared. Dependency sources are excluded by default;
+  use `--include-libs` to include their coverage.
+
+Compatibility:
   `forge coverage` supports test filters and `--watch`, but not test-only output or
   execution modes such as `--json`, `--junit`, `--list`, `--debug`, flame profiles,
   symbolic artifact replay, showmap replay, brutalization, or mutation testing. Use
@@ -91,7 +96,7 @@ pub struct CoverageArgs {
     )]
     report_file: Option<PathBuf>,
 
-    /// Whether to include libraries in the coverage report.
+    /// Include dependency sources in the coverage report.
     #[arg(long)]
     include_libs: bool,
 

crates/forge/src/coverage.rs

@@ -90,20 +90,31 @@ impl CoverageReporter for CoverageSummaryReporter {
 }
 
 fn format_cell(hits: usize, total: usize) -> Cell {
-    let percentage = if total == 0 { 1. } else { hits as f64 / total as f64 };
+    if total == 0 {
+        return Cell::new(format!("N/A ({hits}/{total})"))
+            .fg(Color::Grey)
+            .add_attribute(Attribute::Dim);
+    }
 
-    let mut cell =
-        Cell::new(format!("{:.2}% ({hits}/{total})", percentage * 100.)).fg(match percentage {
-            _ if total == 0 => Color::Grey,
-            _ if percentage < 0.5 => Color::Red,
-            _ if percentage < 0.75 => Color::Yellow,
-            _ => Color::Green,
-        });
+    let percentage = hits as f64 / total as f64;
+    Cell::new(format!("{:.2}% ({hits}/{total})", percentage * 100.)).fg(match percentage {
+        _ if percentage < 0.5 => Color::Red,
+        _ if percentage < 0.75 => Color::Yellow,
+        _ => Color::Green,
+    })
+}
 
-    if total == 0 {
-        cell = cell.add_attribute(Attribute::Dim);
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn empty_summary_cell_is_not_applicable() {
+        assert_eq!(
+            format_cell(0, 0),
+            Cell::new("N/A (0/0)").fg(Color::Grey).add_attribute(Attribute::Dim)
+        );
     }
-    cell
 }
 
 /// Writes the coverage report in [LCOV]'s [tracefile format].