api: improve Debug impl for Match

This makes it so the Debug impl for Match only shows the actual matched
text. Otherwise, the Match shows the entire haystack, which is likely to
be misleading.

Fixes #514

Author: BurntSushi

src/re_bytes.rs

@@ -17,7 +17,7 @@ use crate::re_trait::{self, RegularExpression, SubCapturesPosIter};
 /// Match represents a single match of a regex in a haystack.
 ///
 /// The lifetime parameter `'t` refers to the lifetime of the matched text.
-#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+#[derive(Copy, Clone, Eq, PartialEq)]
 pub struct Match<'t> {
     text: &'t [u8],
     start: usize,
@@ -69,6 +69,24 @@ impl<'t> Match<'t> {
     }
 }
 
+impl<'t> std::fmt::Debug for Match<'t> {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        let mut fmt = f.debug_struct("Match");
+        fmt.field("start", &self.start).field("end", &self.end);
+        if let Ok(s) = std::str::from_utf8(self.as_bytes()) {
+            fmt.field("bytes", &s);
+        } else {
+            // FIXME: It would be nice if this could be printed as a string
+            // with invalid UTF-8 replaced with hex escapes. A alloc would
+            // probably okay if that makes it easier, but regex-automata does
+            // (at time of writing) have internal routines that do this. So
+            // maybe we should expose them.
+            fmt.field("bytes", &self.as_bytes());
+        }
+        fmt.finish()
+    }
+}
+
 impl<'t> From<Match<'t>> for Range<usize> {
     fn from(m: Match<'t>) -> Range<usize> {
         m.range()

src/re_unicode.rs

@@ -25,7 +25,7 @@ pub fn escape(text: &str) -> String {
 /// Match represents a single match of a regex in a haystack.
 ///
 /// The lifetime parameter `'t` refers to the lifetime of the matched text.
-#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+#[derive(Copy, Clone, Eq, PartialEq)]
 pub struct Match<'t> {
     text: &'t str,
     start: usize,
@@ -77,6 +77,16 @@ impl<'t> Match<'t> {
     }
 }
 
+impl<'t> std::fmt::Debug for Match<'t> {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        f.debug_struct("Match")
+            .field("start", &self.start)
+            .field("end", &self.end)
+            .field("string", &self.as_str())
+            .finish()
+    }
+}
+
 impl<'t> From<Match<'t>> for &'t str {
     fn from(m: Match<'t>) -> &'t str {
         m.as_str()