Rollup merge of #141467 - cyrgani:const-empty-stringlikes, r=Amanieu

make `OsString::new` and `PathBuf::new` unstably const

Since #129041, `String::into_bytes` is `const`, which allows making `OsString::new` and `PathBuf::new` unstably const now.
Not sure what the exact process for this is; does it need an ACP?

library/std/src/ffi/os_str.rs

@@ -137,7 +137,8 @@ impl OsString {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[must_use]
     #[inline]
-    pub fn new() -> OsString {
+    #[rustc_const_unstable(feature = "const_pathbuf_osstring_new", issue = "141520")]
+    pub const fn new() -> OsString {
         OsString { inner: Buf::from_string(String::new()) }
     }
 

library/std/src/path.rs

@@ -1191,7 +1191,8 @@ impl PathBuf {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[must_use]
     #[inline]
-    pub fn new() -> PathBuf {
+    #[rustc_const_unstable(feature = "const_pathbuf_osstring_new", issue = "141520")]
+    pub const fn new() -> PathBuf {
         PathBuf { inner: OsString::new() }
     }
 

library/std/src/sys/os_str/bytes.rs

@@ -115,7 +115,7 @@ impl Buf {
     }
 
     #[inline]
-    pub fn from_string(s: String) -> Buf {
+    pub const fn from_string(s: String) -> Buf {
         Buf { inner: s.into_bytes() }
     }
 

library/std/src/sys/os_str/wtf8.rs

@@ -92,7 +92,7 @@ impl Buf {
     }
 
     #[inline]
-    pub fn from_string(s: String) -> Buf {
+    pub const fn from_string(s: String) -> Buf {
         Buf { inner: Wtf8Buf::from_string(s) }
     }
 

library/std/src/sys_common/wtf8.rs

@@ -209,7 +209,7 @@ impl Wtf8Buf {
     ///
     /// Since WTF-8 is a superset of UTF-8, this always succeeds.
     #[inline]
-    pub fn from_string(string: String) -> Wtf8Buf {
+    pub const fn from_string(string: String) -> Wtf8Buf {
         Wtf8Buf { bytes: string.into_bytes(), is_known_utf8: true }
     }