printer: support previews for long lines

This commit adds support for showing a preview of long lines. While the
default still remains as completely suppressing the entire line, this
new functionality will show the first N graphemes of a matching line,
including the number of matches that are suppressed.

This was unfortunately a fairly invasive change to the printer that
required a bit of refactoring. On the bright side, the single line
and multi-line coloring are now more unified than they were before.

Closes #1078

src/app.rs

@@ -582,6 +582,7 @@ pub fn all_args_and_flags() -> Vec<RGArg> {
     flag_line_number(&mut args);
     flag_line_regexp(&mut args);
     flag_max_columns(&mut args);
+    flag_max_column_preview(&mut args);
     flag_max_count(&mut args);
     flag_max_depth(&mut args);
     flag_max_filesize(&mut args);
@@ -1443,6 +1444,30 @@ When this flag is omitted or is set to 0, then it has no effect.
     args.push(arg);
 }
 
+fn flag_max_column_preview(args: &mut Vec<RGArg>) {
+    const SHORT: &str = "Print a preview for lines exceeding the limit.";
+    const LONG: &str = long!("\
+When the '--max-columns' flag is used, ripgrep will by default completely
+replace any line that is too long with a message indicating that a matching
+line was removed. When this flag is combined with '--max-columns', a preview
+of the line (corresponding to the limit size) is shown instead, where the part
+of the line exceeding the limit is not shown.
+
+If the '--max-columns' flag is not set, then this has no effect.
+
+This flag can be disabled with '--no-max-column-preview'.
+");
+    let arg = RGArg::switch("max-column-preview")
+        .help(SHORT).long_help(LONG)
+        .overrides("no-max-column-preview");
+    args.push(arg);
+
+    let arg = RGArg::switch("no-max-column-preview")
+        .hidden()
+        .overrides("max-column-preview");
+    args.push(arg);
+}
+
 fn flag_max_count(args: &mut Vec<RGArg>) {
     const SHORT: &str = "Limit the number of matches.";
     const LONG: &str = long!("\

src/args.rs

@@ -746,6 +746,7 @@ impl ArgMatches {
             .per_match(self.is_present("vimgrep"))
             .replacement(self.replacement())
             .max_columns(self.max_columns()?)
+            .max_column_preview(self.max_column_preview())
             .max_matches(self.max_count()?)
             .column(self.column())
             .byte_offset(self.is_present("byte-offset"))
@@ -1142,6 +1143,12 @@ impl ArgMatches {
         Ok(self.usize_of_nonzero("max-columns")?.map(|n| n as u64))
     }
 
+    /// Returns true if and only if a preview should be shown for lines that
+    /// exceed the maximum column limit.
+    fn max_column_preview(&self) -> bool {
+        self.is_present("max-column-preview")
+    }
+
     /// The maximum number of matches permitted.
     fn max_count(&self) -> Result<Option<u64>> {
         Ok(self.usize_of("max-count")?.map(|n| n as u64))