markup: Reimplement pygmentsCodefencesGuessSyntax

Fixes #6565

markup/highlight/config.go

@@ -58,6 +58,8 @@ type Config struct {
 
 	// TabWidth sets the number of characters for a tab. Defaults to 4.
 	TabWidth int
+
+	GuessSyntax bool
 }
 
 func (cfg Config) ToHTMLOptions() []html.Option {
@@ -104,6 +106,10 @@ func ApplyLegacyConfig(cfg config.Provider, conf *Config) error {
 		conf.CodeFences = cfg.GetBool("pygmentsCodeFences")
 	}
 
+	if conf.GuessSyntax == DefaultConfig.GuessSyntax && cfg.IsSet("pygmentsCodefencesGuessSyntax") {
+		conf.GuessSyntax = cfg.GetBool("pygmentsCodefencesGuessSyntax")
+	}
+
 	if cfg.IsSet("pygmentsOptions") {
 		if err := applyOptionsFromString(cfg.GetString("pygmentsOptions"), conf); err != nil {
 			return err

markup/highlight/highlight.go

@@ -52,6 +52,14 @@ func highlight(code, lang string, cfg Config) (string, error) {
 		lexer = lexers.Get(lang)
 	}
 
+	if lexer == nil && cfg.GuessSyntax {
+		lexer = lexers.Analyse(code)
+		if lexer == nil {
+			lexer = lexers.Fallback
+		}
+		lang = strings.ToLower(lexer.Config().Name)
+	}
+
 	if lexer == nil {
 		wrapper := getPreWrapper(lang)
 		fmt.Fprint(w, wrapper.Start(true, ""))

markup/highlight/highlight_test.go

@@ -84,4 +84,26 @@ LINE5
 		result, _ = h.Highlight(lines, "bash", "linenos=table")
 		c.Assert(result, qt.Contains, "<span class=\"lnt\">1\n</span>")
 	})
+
+	c.Run("No language", func(c *qt.C) {
+		cfg := DefaultConfig
+		cfg.NoClasses = false
+		cfg.LineNos = true
+		h := New(cfg)
+
+		result, _ := h.Highlight(lines, "", "")
+		c.Assert(result, qt.Equals, "<pre><code>LINE1\nLINE2\nLINE3\nLINE4\nLINE5\n</code></pre>")
+	})
+
+	c.Run("No language, guess syntax", func(c *qt.C) {
+		cfg := DefaultConfig
+		cfg.NoClasses = false
+		cfg.GuessSyntax = true
+		cfg.LineNos = true
+		cfg.LineNumbersInTable = false
+		h := New(cfg)
+
+		result, _ := h.Highlight(lines, "", "")
+		c.Assert(result, qt.Contains, "<span class=\"ln\">2</span>LINE2\n<")
+	})
 }