rb_file_join_fastpath: optimize searching for separators

`chompdirsep` searches from the start of the string each time, which
perhaps is necessary for certain encodings (not even sure?) but for
the common encodings it's very wasteful. Instead we can start from the
back of the string and only compare one or two characters in most cases.

Also replace `StringValueCStr` for the simpler `rb_str_null_check`
as we only care about whether the string contains `NULL` bytes, we
don't care whether it is NULL terminated or not.

We also only check the final string for NULLs.

```
compare-ruby: ruby 4.1.0dev (2026-01-17T14:40:03Z master 00a3b71) +PRISM [arm64-darwin25]
built-ruby: ruby 4.1.0dev (2026-01-18T12:55:15Z spedup-file-join 5948e92e03) +PRISM [arm64-darwin25]
warming up....

|              |compare-ruby|built-ruby|
|:-------------|-----------:|---------:|
|two_strings   |      2.477M|   19.317M|
|              |           -|     7.80x|
|many_strings  |    547.577k|   10.298M|
|              |           -|    18.81x|
|array         |    515.280k|  523.291k|
|              |           -|     1.02x|
|mixed         |    621.840k|  635.422k|
|              |           -|     1.02x|
```

Author: byroot

file.c

@@ -3714,22 +3714,6 @@ chompdirsep(const char *path, const char *end, rb_encoding *enc)
     return (char *)path;
 }
 
-static char *
-single_byte_chompdirsep(const char *path, const char *end)
-{
-    while (path < end) {
-        if (isdirsep(*path)) {
-            const char *last = path++;
-            while (path < end && isdirsep(*path)) path++;
-            if (path >= end) return (char *)last;
-        }
-        else {
-            path++;
-        }
-    }
-    return (char *)path;
-}
-
 char *
 rb_enc_path_end(const char *path, const char *end, rb_encoding *enc)
 {
@@ -5374,14 +5358,22 @@ rb_file_join_fastpath(long argc, VALUE *args)
     const char *name = RSTRING_PTR(result);
     for (i = 1; i < argc; i++) {
         VALUE tmp = args[i];
-        StringValueCStr(tmp);
         long len = RSTRING_LEN(result);
 
-        const char *tail = single_byte_chompdirsep(name, name + len);
-        if (RSTRING_PTR(tmp) && isdirsep(RSTRING_PTR(tmp)[0])) {
-            rb_str_set_len(result, tail - name);
+        const char *tmp_s;
+        long tmp_len;
+        RSTRING_GETMEM(tmp, tmp_s, tmp_len);
+
+        if (isdirsep(tmp_s[0])) {
+            // right side has a leading separator, remove left side separators.
+            long trailing_seps = 0;
+            while (isdirsep(name[len - trailing_seps - 1])) {
+                trailing_seps++;
+            }
+            rb_str_set_len(result, len - trailing_seps);
         }
-        else if (!*tail) {
+        else if (!isdirsep(name[len - 1])) {
+            // neither side have a separator, append one;
             rb_str_cat(result, "/", 1);
         }
 
@@ -5391,9 +5383,10 @@ rb_file_join_fastpath(long argc, VALUE *args)
             encidx = rb_enc_to_index(new_enc);
         }
 
-        rb_str_buf_append(result, tmp);
+        rb_str_buf_cat(result, tmp_s, tmp_len);
     }
 
+    rb_str_null_check(result);
     return result;
 }
 

internal/string.h

@@ -84,6 +84,7 @@ bool rb_str_reembeddable_p(VALUE);
 VALUE rb_str_upto_endless_each(VALUE, int (*each)(VALUE, VALUE), VALUE);
 VALUE rb_str_with_debug_created_info(VALUE, VALUE, int);
 VALUE rb_str_frozen_bare_string(VALUE);
+const char *rb_str_null_check(VALUE);
 
 /* error.c */
 void rb_warn_unchilled_literal(VALUE str);

string.c

@@ -2902,6 +2902,34 @@ str_null_check(VALUE str, int *w)
     return s;
 }
 
+const char *
+rb_str_null_check(VALUE str)
+{
+    RUBY_ASSERT(RB_TYPE_P(str, T_STRING));
+
+    char *s;
+    long len;
+    RSTRING_GETMEM(str, s, len);
+
+    if (RB_LIKELY(rb_str_enc_fastpath(str))) {
+        if (!s || memchr(s, 0, len)) {
+            rb_raise(rb_eArgError, "string contains null byte");
+        }
+    }
+    else {
+        int w;
+        const char *s = str_null_check(str, &w);
+        if (!s) {
+            if (w) {
+                rb_raise(rb_eArgError, "string contains null char");
+            }
+            rb_raise(rb_eArgError, "string contains null byte");
+        }
+    }
+
+    return s;
+}
+
 char *
 rb_str_to_cstr(VALUE str)
 {