printer: add --passthru flag

The --passthru flag causes ripgrep to print every line,
even if the line does not contain a match. This is a
response to the common pattern of `^|foo` to match every
line, while still highlighting things like `foo`.

Fixes #740

src/app.rs

@@ -166,6 +166,8 @@ pub fn app() -> App<'static, 'static> {
         .arg(flag("no-ignore-vcs"))
         .arg(flag("null").short("0"))
         .arg(flag("only-matching").short("o"))
+        .arg(flag("passthru").alias("passthrough")
+            .conflicts_with_all(&["only-matching", "replace"]))
         .arg(flag("path-separator").value_name("SEPARATOR").takes_value(true))
         .arg(flag("pretty").short("p"))
         .arg(flag("replace").short("r")
@@ -499,6 +501,8 @@ lazy_static! {
              "Print only matched parts of a line.",
              "Print only the matched (non-empty) parts of a matching line, \
               with each such part on a separate output line.");
+        doc!(h, "passthru",
+             "Show both matching and non-matching lines.");
         doc!(h, "path-separator",
              "Path separator to use when printing file paths.",
              "The path separator to use when printing file paths. This \

src/args.rs

@@ -433,7 +433,9 @@ impl<'a> ArgMatches<'a> {
     /// Note that if -F/--fixed-strings is set, then all patterns will be
     /// escaped. Similarly, if -w/--word-regexp is set, then all patterns
     /// are surrounded by `\b`, and if -x/--line-regexp is set, then all
-    /// patterns are surrounded by `^...$`.
+    /// patterns are surrounded by `^...$`. Finally, if --passthru is set,
+    /// the pattern `^` is added to the end (to ensure that it works as
+    /// expected with multiple -e/-f patterns).
     ///
     /// If any pattern is invalid UTF-8, then an error is returned.
     fn patterns(&self) -> Result<Vec<String>> {
@@ -470,7 +472,11 @@ impl<'a> ArgMatches<'a> {
                 }
             }
         }
-        if pats.is_empty() {
+        // It's important that this be at the end; otherwise it would always
+        // match first, and we wouldn't get colours in the output
+        if self.is_present("passthru") && !self.is_present("count") {
+            pats.push("^".to_string())
+        } else if pats.is_empty() {
             pats.push(self.empty_pattern())
         }
         Ok(pats)