ripgrep: suggest -U/--multiline

When a "\n literal is not allowed" error is reported, ripgrep will now
suggest the use of the -U/--multiline flag, which enables matching
newlines.

Fixes #1055

src/args.rs

@@ -615,7 +615,10 @@ impl ArgMatches {
         if let Some(limit) = self.dfa_size_limit()? {
             builder.dfa_size_limit(limit);
         }
-        Ok(builder.build(&patterns.join("|"))?)
+        match builder.build(&patterns.join("|")) {
+            Ok(m) => Ok(m),
+            Err(err) => Err(From::from(suggest_multiline(err.to_string()))),
+        }
     }
 
     /// Build a matcher using PCRE2.
@@ -1546,6 +1549,17 @@ and look-around.", msg)
     }
 }
 
+fn suggest_multiline(msg: String) -> String {
+    if msg.contains("the literal") && msg.contains("not allowed") {
+        format!("{}
+
+Consider enabling multiline mode with the --multiline flag (or -U for short).
+When multiline mode is enabled, new line characters can be matched.", msg)
+    } else {
+        msg
+    }
+}
+
 /// Convert the result of parsing a human readable file size to a `usize`,
 /// failing if the type does not fit.
 fn u64_to_usize(