Allow custom highlight mapping beyond file extensions (#15808)

Co-authored-by: Lauris BH <lauris@nix.lv>

modules/highlight/highlight.go

@@ -16,6 +16,7 @@ import (
 	"code.gitea.io/gitea/modules/analyze"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
+	"github.com/alecthomas/chroma"
 	"github.com/alecthomas/chroma/formatters/html"
 	"github.com/alecthomas/chroma/lexers"
 	"github.com/alecthomas/chroma/styles"
@@ -66,15 +67,18 @@ func Code(fileName, code string) string {
 	htmlbuf := bytes.Buffer{}
 	htmlw := bufio.NewWriter(&htmlbuf)
 
+	var lexer chroma.Lexer
 	if val, ok := highlightMapping[filepath.Ext(fileName)]; ok {
-		//change file name to one with mapped extension so we look that up instead
-		fileName = "mapped." + val
+		//use mapped value to find lexer
+		lexer = lexers.Get(val)
 	}
 
-	lexer := lexers.Match(fileName)
+	if lexer == nil {
+		lexer = lexers.Match(fileName)
 		if lexer == nil {
 			lexer = lexers.Fallback
 		}
+	}
 
 	iterator, err := lexer.Tokenise(nil, string(code))
 	if err != nil {
@@ -114,19 +118,22 @@ func File(numLines int, fileName string, code []byte) map[int]string {
 	htmlbuf := bytes.Buffer{}
 	htmlw := bufio.NewWriter(&htmlbuf)
 
+	var lexer chroma.Lexer
 	if val, ok := highlightMapping[filepath.Ext(fileName)]; ok {
-		fileName = "test." + val
+		lexer = lexers.Get(val)
 	}
 
+	if lexer == nil {
 		language := analyze.GetCodeLanguage(fileName, code)
 
-	lexer := lexers.Get(language)
+		lexer = lexers.Get(language)
 		if lexer == nil {
 			lexer = lexers.Match(fileName)
 			if lexer == nil {
 				lexer = lexers.Fallback
 			}
 		}
+	}
 
 	iterator, err := lexer.Tokenise(nil, string(code))
 	if err != nil {