Make some std::path::Prefix methods const

Insta-stabilize the method `std::path::Prefix::is_verbatim` as const.
Also make private methods `is_drive` and `has_implicit_root` const.

Possible because of the recent stabilization of const control flow.

Also adds a test for `is_verbatim` in a const context.

Author: CDirkx

library/std/src/path.rs

@@ -214,19 +214,20 @@ impl<'a> Prefix<'a> {
     /// assert!(!Disk(b'C').is_verbatim());
     /// ```
     #[inline]
+    #[rustc_const_stable(feature = "const_ip", since = "1.48.0")]
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn is_verbatim(&self) -> bool {
+    pub const fn is_verbatim(&self) -> bool {
         use self::Prefix::*;
         matches!(*self, Verbatim(_) | VerbatimDisk(_) | VerbatimUNC(..))
     }
 
     #[inline]
-    fn is_drive(&self) -> bool {
+    const fn is_drive(&self) -> bool {
         matches!(*self, Prefix::Disk(_))
     }
 
     #[inline]
-    fn has_implicit_root(&self) -> bool {
+    const fn has_implicit_root(&self) -> bool {
         !self.is_drive()
     }
 }

src/test/ui/consts/std/path.rs

@@ -0,0 +1,10 @@
+// run-pass
+
+use std::path::Prefix;
+
+fn main() {
+    const PREFIX : Prefix = Prefix::Disk(2);
+
+    const IS_VERBATIM : bool = PREFIX.is_verbatim();
+    assert!(!IS_VERBATIM);
+}