Highlighting tab errors

Added option `hltaberrors` which helps to spot sloppy whitespace errors
with tabs used instead of spaces or vice versa.

It uses the value of `tabstospaces` option as a criterion whether a
tab or space character is an error or not.
If `tabstospaces` is on, we probably expect that the file should contain
no tab characters, so any tab character is highlighted as an error.
If `tabstospaces` is off, we probably expect that the file uses
indentation with tabs, so space characters in the initial indent part
of lines are highlighted as errors.

internal/config/settings.go

@@ -289,6 +289,7 @@ var defaultCommonSettings = map[string]interface{}{
 	"fileformat":      defaultFileFormat(),
 	"filetype":        "unknown",
 	"hlsearch":        false,
+	"hltaberrors":     false,
 	"incsearch":       true,
 	"ignorecase":      true,
 	"indentchar":      " ",

internal/display/bufwindow.go

@@ -495,6 +495,8 @@ func (w *BufWindow) displayBuffer() {
 			vloc.X = w.gutterOffset
 		}
 
+		leadingwsEnd := len(util.GetLeadingWhitespace(b.LineBytes(bloc.Y)))
+
 		line, nColsBeforeStart, bslice, startStyle := w.getStartInfo(w.StartCol, bloc.Y)
 		if startStyle != nil {
 			curStyle = *startStyle
@@ -518,6 +520,18 @@ func (w *BufWindow) displayBuffer() {
 					// over cursor-line and color-column
 					dontOverrideBackground := origBg != defBg
 
+					if b.Settings["hltaberrors"].(bool) {
+						if s, ok := config.Colorscheme["tab-error"]; ok {
+							isTab := (r == '\t') || (r == ' ' && !showcursor)
+							if (b.Settings["tabstospaces"].(bool) && isTab) ||
+								(!b.Settings["tabstospaces"].(bool) && bloc.X < leadingwsEnd && r == ' ' && !isTab) {
+								fg, _, _ := s.Decompose()
+								style = style.Background(fg)
+								dontOverrideBackground = true
+							}
+						}
+					}
+
 					for _, c := range cursors {
 						if c.HasSelection() &&
 							(bloc.GreaterEqual(c.CurSelection[0]) && bloc.LessThan(c.CurSelection[1]) ||

runtime/help/options.md

@@ -174,6 +174,13 @@ Here are the available options:
 
 	default value: `false`
 
+* `hltaberrors`: highlight tabs when spaces are expected, and spaces when tabs
+   are expected. More precisely: if `tabstospaces` option is on, highlight
+   all tab characters; if `tabstospaces` is off, highlight space characters
+   in the initial indent part of the line.
+
+	default value: `false`
+
 * `incsearch`: enable incremental search in "Find" prompt (matching as you type).
 
 	default value: `true`