expand: allocate fewer objects in Fields

          │     old     │             new              │
          │   sec/op    │   sec/op     vs base         │
    Run-8   1.494m ± 6%   1.487m ± 2%  ~ (p=0.645 n=8)

          │     old      │                new                 │
          │     B/op     │     B/op      vs base              │
    Run-8   71.03Ki ± 0%   70.83Ki ± 0%  -0.28% (p=0.016 n=8)

          │    old     │               new                │
          │ allocs/op  │ allocs/op   vs base              │
    Run-8   856.0 ± 0%   826.0 ± 0%  -3.50% (p=0.000 n=8)

expand/expand.go

@@ -468,17 +468,17 @@ func Fields(cfg *Config, words ...*syntax.Word) ([]string, error) {
 			for _, field := range wfields {
 				path, doGlob := cfg.escapedGlobField(field)
 				var matches []string
-				var syntaxError *pattern.SyntaxError
 				if doGlob && cfg.ReadDir2 != nil {
 					matches, err = cfg.glob(dir, path)
-					if !errors.As(err, &syntaxError) {
-						if err != nil {
+					if err != nil {
+						// We avoid [errors.As] as it allocates,
+						// and we know that [Config.glob] returns [pattern.Regexp] errors without wrapping.
+						if _, ok := err.(*pattern.SyntaxError); !ok {
 							return nil, err
 						}
-						if len(matches) > 0 || cfg.NullGlob {
-							fields = append(fields, matches...)
-							continue
-						}
+					} else if len(matches) > 0 || cfg.NullGlob {
+						fields = append(fields, matches...)
+						continue
 					}
 				}
 				fields = append(fields, cfg.fieldJoin(field))