From 3e2206a29a2f8b93932a56768516845ec4379bb4 Mon Sep 17 00:00:00 2001
From: Michael Hall <michael@mbh.sh>
Date: Tue, 11 May 2021 10:29:57 +1000
Subject: [PATCH 1/7] add file_prefix method

---
 library/std/src/path.rs       |  53 ++++++++++-
 library/std/src/path/tests.rs | 174 +++++++++++++++++++++++++++-------
 2 files changed, 191 insertions(+), 36 deletions(-)

diff --git a/library/std/src/path.rs b/library/std/src/path.rs
index ed0987064e8f7..cec8010fe24be 100644
--- a/library/std/src/path.rs
+++ b/library/std/src/path.rs
@@ -315,7 +315,7 @@ fn has_physical_root(s: &[u8], prefix: Option<Prefix<'_>>) -> bool {
 }
 
 // basic workhorse for splitting stem and extension
-fn split_file_at_dot(file: &OsStr) -> (Option<&OsStr>, Option<&OsStr>) {
+fn rsplit_file_at_dot(file: &OsStr) -> (Option<&OsStr>, Option<&OsStr>) {
     if os_str_as_u8_slice(file) == b".." {
         return (Some(file), None);
     }
@@ -334,6 +334,29 @@ fn split_file_at_dot(file: &OsStr) -> (Option<&OsStr>, Option<&OsStr>) {
     }
 }
 
+fn split_file_at_dot(file: &OsStr) -> (Option<&OsStr>, Option<&OsStr>) {
+    let slice = os_str_as_u8_slice(file);
+    if slice == b".." {
+        return (Some(file), None);
+    }
+
+    // The unsafety here stems from converting between &OsStr and &[u8]
+    // and back. This is safe to do because (1) we only look at ASCII
+    // contents of the encoding and (2) new &OsStr values are produced
+    // only from ASCII-bounded slices of existing &OsStr values.
+    let i = match slice[1..].iter().position(|b| *b == b'.') {
+        Some(i) => i + 1,
+        None => slice.len(),
+    };
+    if i == slice.len() {
+        (Some(file), None)
+    } else {
+        let before = Some(&slice[..i]);
+        let after = Some(&slice[i + 1..]);
+        unsafe { (before.map(|s| u8_slice_as_os_str(s)), after.map(|s| u8_slice_as_os_str(s))) }
+    }
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // The core iterators
 ////////////////////////////////////////////////////////////////////////////////
@@ -2158,6 +2181,32 @@ impl Path {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn file_stem(&self) -> Option<&OsStr> {
+        self.file_name().map(rsplit_file_at_dot).and_then(|(before, after)| before.or(after))
+    }
+
+    /// Extracts the prefix (non-extension(s)) portion of [`self.file_name`]. This is a "left"
+    /// variant of `file_stem` - meaning it takes the portion of the file name before the *first* `.`
+    ///
+    /// [`self.file_name`]: Path::file_name
+    ///
+    /// The prefix is:
+    ///
+    /// * [`None`], if there is no file name;
+    /// * The entire file name if there is no embedded `.`;
+    /// * The entire file name if the file name begins with `.` and has no other `.`s within;
+    /// * Otherwise, the portion of the file name before the first `.`
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #![feature(path_file_prefix)]
+    /// use std::path::Path;
+    ///
+    /// assert_eq!("foo", Path::new("foo.rs").file_prefix().unwrap());
+    /// assert_eq!("foo", Path::new("foo.tar.gz").file_prefix().unwrap());
+    /// ```
+    #[unstable(feature = "path_file_prefix", issue = "none")]
+    pub fn file_prefix(&self) -> Option<&OsStr> {
         self.file_name().map(split_file_at_dot).and_then(|(before, after)| before.or(after))
     }
 
@@ -2182,7 +2231,7 @@ impl Path {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn extension(&self) -> Option<&OsStr> {
-        self.file_name().map(split_file_at_dot).and_then(|(before, after)| before.and(after))
+        self.file_name().map(rsplit_file_at_dot).and_then(|(before, after)| before.and(after))
     }
 
     /// Creates an owned [`PathBuf`] with `path` adjoined to `self`.
diff --git a/library/std/src/path/tests.rs b/library/std/src/path/tests.rs
index 896d6c2a64c60..14dcab450acbe 100644
--- a/library/std/src/path/tests.rs
+++ b/library/std/src/path/tests.rs
@@ -3,7 +3,7 @@ use super::*;
 use crate::rc::Rc;
 use crate::sync::Arc;
 
-macro_rules! t(
+macro_rules! t (
     ($path:expr, iter: $iter:expr) => (
         {
             let path = Path::new($path);
@@ -73,15 +73,33 @@ macro_rules! t(
         }
     );
 
+    ($path:expr, file_prefix: $file_prefix:expr, extension: $extension:expr) => (
+        {
+            let path = Path::new($path);
+
+            let prefix = path.file_prefix().map(|p| p.to_str().unwrap());
+            let exp_prefix: Option<&str> = $file_prefix;
+            assert!(prefix == exp_prefix, "file_prefix: Expected {:?}, found {:?}",
+                    exp_prefix, prefix);
+
+            let ext = path.extension().map(|p| p.to_str().unwrap());
+            let exp_ext: Option<&str> = $extension;
+            assert!(ext == exp_ext, "extension: Expected {:?}, found {:?}",
+                    exp_ext, ext);
+        }
+    );
+
     ($path:expr, iter: $iter:expr,
                  has_root: $has_root:expr, is_absolute: $is_absolute:expr,
                  parent: $parent:expr, file_name: $file:expr,
-                 file_stem: $file_stem:expr, extension: $extension:expr) => (
+                 file_stem: $file_stem:expr, extension: $extension:expr,
+                 file_prefix: $file_prefix:expr) => (
         {
             t!($path, iter: $iter);
             t!($path, has_root: $has_root, is_absolute: $is_absolute);
             t!($path, parent: $parent, file_name: $file);
             t!($path, file_stem: $file_stem, extension: $extension);
+            t!($path, file_prefix: $file_prefix, extension: $extension);
         }
     );
 );
@@ -116,7 +134,8 @@ pub fn test_decompositions_unix() {
     parent: None,
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("foo",
@@ -126,7 +145,8 @@ pub fn test_decompositions_unix() {
     parent: Some(""),
     file_name: Some("foo"),
     file_stem: Some("foo"),
-    extension: None
+    extension: None,
+    file_prefix: Some("foo")
     );
 
     t!("/",
@@ -136,7 +156,8 @@ pub fn test_decompositions_unix() {
     parent: None,
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("/foo",
@@ -146,7 +167,8 @@ pub fn test_decompositions_unix() {
     parent: Some("/"),
     file_name: Some("foo"),
     file_stem: Some("foo"),
-    extension: None
+    extension: None,
+    file_prefix: Some("foo")
     );
 
     t!("foo/",
@@ -156,7 +178,8 @@ pub fn test_decompositions_unix() {
     parent: Some(""),
     file_name: Some("foo"),
     file_stem: Some("foo"),
-    extension: None
+    extension: None,
+    file_prefix: Some("foo")
     );
 
     t!("/foo/",
@@ -166,7 +189,8 @@ pub fn test_decompositions_unix() {
     parent: Some("/"),
     file_name: Some("foo"),
     file_stem: Some("foo"),
-    extension: None
+    extension: None,
+    file_prefix: Some("foo")
     );
 
     t!("foo/bar",
@@ -176,7 +200,8 @@ pub fn test_decompositions_unix() {
     parent: Some("foo"),
     file_name: Some("bar"),
     file_stem: Some("bar"),
-    extension: None
+    extension: None,
+    file_prefix: Some("bar")
     );
 
     t!("/foo/bar",
@@ -186,7 +211,8 @@ pub fn test_decompositions_unix() {
     parent: Some("/foo"),
     file_name: Some("bar"),
     file_stem: Some("bar"),
-    extension: None
+    extension: None,
+    file_prefix: Some("bar")
     );
 
     t!("///foo///",
@@ -196,7 +222,8 @@ pub fn test_decompositions_unix() {
     parent: Some("/"),
     file_name: Some("foo"),
     file_stem: Some("foo"),
-    extension: None
+    extension: None,
+    file_prefix: Some("foo")
     );
 
     t!("///foo///bar",
@@ -206,7 +233,8 @@ pub fn test_decompositions_unix() {
     parent: Some("///foo"),
     file_name: Some("bar"),
     file_stem: Some("bar"),
-    extension: None
+    extension: None,
+    file_prefix: Some("bar")
     );
 
     t!("./.",
@@ -216,7 +244,8 @@ pub fn test_decompositions_unix() {
     parent: Some(""),
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("/..",
@@ -226,7 +255,8 @@ pub fn test_decompositions_unix() {
     parent: Some("/"),
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("../",
@@ -236,7 +266,8 @@ pub fn test_decompositions_unix() {
     parent: Some(""),
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("foo/.",
@@ -246,7 +277,8 @@ pub fn test_decompositions_unix() {
     parent: Some(""),
     file_name: Some("foo"),
     file_stem: Some("foo"),
-    extension: None
+    extension: None,
+    file_prefix: Some("foo")
     );
 
     t!("foo/..",
@@ -256,7 +288,8 @@ pub fn test_decompositions_unix() {
     parent: Some("foo"),
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("foo/./",
@@ -266,7 +299,8 @@ pub fn test_decompositions_unix() {
     parent: Some(""),
     file_name: Some("foo"),
     file_stem: Some("foo"),
-    extension: None
+    extension: None,
+    file_prefix: Some("foo")
     );
 
     t!("foo/./bar",
@@ -276,7 +310,8 @@ pub fn test_decompositions_unix() {
     parent: Some("foo"),
     file_name: Some("bar"),
     file_stem: Some("bar"),
-    extension: None
+    extension: None,
+    file_prefix: Some("bar")
     );
 
     t!("foo/../",
@@ -286,7 +321,8 @@ pub fn test_decompositions_unix() {
     parent: Some("foo"),
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("foo/../bar",
@@ -296,7 +332,8 @@ pub fn test_decompositions_unix() {
     parent: Some("foo/.."),
     file_name: Some("bar"),
     file_stem: Some("bar"),
-    extension: None
+    extension: None,
+    file_prefix: Some("bar")
     );
 
     t!("./a",
@@ -306,7 +343,8 @@ pub fn test_decompositions_unix() {
     parent: Some("."),
     file_name: Some("a"),
     file_stem: Some("a"),
-    extension: None
+    extension: None,
+    file_prefix: Some("a")
     );
 
     t!(".",
@@ -316,7 +354,8 @@ pub fn test_decompositions_unix() {
     parent: Some(""),
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("./",
@@ -326,7 +365,8 @@ pub fn test_decompositions_unix() {
     parent: Some(""),
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("a/b",
@@ -336,7 +376,8 @@ pub fn test_decompositions_unix() {
     parent: Some("a"),
     file_name: Some("b"),
     file_stem: Some("b"),
-    extension: None
+    extension: None,
+    file_prefix: Some("b")
     );
 
     t!("a//b",
@@ -346,7 +387,8 @@ pub fn test_decompositions_unix() {
     parent: Some("a"),
     file_name: Some("b"),
     file_stem: Some("b"),
-    extension: None
+    extension: None,
+    file_prefix: Some("b")
     );
 
     t!("a/./b",
@@ -356,7 +398,8 @@ pub fn test_decompositions_unix() {
     parent: Some("a"),
     file_name: Some("b"),
     file_stem: Some("b"),
-    extension: None
+    extension: None,
+    file_prefix: Some("b")
     );
 
     t!("a/b/c",
@@ -366,7 +409,8 @@ pub fn test_decompositions_unix() {
     parent: Some("a/b"),
     file_name: Some("c"),
     file_stem: Some("c"),
-    extension: None
+    extension: None,
+    file_prefix: Some("c")
     );
 
     t!(".foo",
@@ -376,7 +420,30 @@ pub fn test_decompositions_unix() {
     parent: Some(""),
     file_name: Some(".foo"),
     file_stem: Some(".foo"),
-    extension: None
+    extension: None,
+    file_prefix: Some(".foo")
+    );
+
+    t!("a/.foo",
+    iter: ["a", ".foo"],
+    has_root: false,
+    is_absolute: false,
+    parent: Some("a"),
+    file_name: Some(".foo"),
+    file_stem: Some(".foo"),
+    extension: None,
+    file_prefix: Some(".foo")
+    );
+
+    t!("a/.rustfmt.toml",
+    iter: ["a", ".rustfmt.toml"],
+    has_root: false,
+    is_absolute: false,
+    parent: Some("a"),
+    file_name: Some(".rustfmt.toml"),
+    file_stem: Some(".rustfmt"),
+    extension: Some("toml"),
+    file_prefix: Some(".rustfmt")
     );
 }
 
@@ -952,9 +1019,48 @@ pub fn test_stem_ext() {
     t!("", file_stem: None, extension: None);
 }
 
+#[test]
+pub fn test_prefix_ext() {
+    t!("foo",
+    file_prefix: Some("foo"),
+    extension: None
+    );
+
+    t!("foo.",
+    file_prefix: Some("foo"),
+    extension: Some("")
+    );
+
+    t!(".foo",
+    file_prefix: Some(".foo"),
+    extension: None
+    );
+
+    t!("foo.txt",
+    file_prefix: Some("foo"),
+    extension: Some("txt")
+    );
+
+    t!("foo.bar.txt",
+    file_prefix: Some("foo"),
+    extension: Some("txt")
+    );
+
+    t!("foo.bar.",
+    file_prefix: Some("foo"),
+    extension: Some("")
+    );
+
+    t!(".", file_prefix: None, extension: None);
+
+    t!("..", file_prefix: None, extension: None);
+
+    t!("", file_prefix: None, extension: None);
+}
+
 #[test]
 pub fn test_push() {
-    macro_rules! tp(
+    macro_rules! tp (
         ($path:expr, $push:expr, $expected:expr) => ( {
             let mut actual = PathBuf::from($path);
             actual.push($push);
@@ -1042,7 +1148,7 @@ pub fn test_push() {
 
 #[test]
 pub fn test_pop() {
-    macro_rules! tp(
+    macro_rules! tp (
         ($path:expr, $expected:expr, $output:expr) => ( {
             let mut actual = PathBuf::from($path);
             let output = actual.pop();
@@ -1096,7 +1202,7 @@ pub fn test_pop() {
 
 #[test]
 pub fn test_set_file_name() {
-    macro_rules! tfn(
+    macro_rules! tfn (
             ($path:expr, $file:expr, $expected:expr) => ( {
             let mut p = PathBuf::from($path);
             p.set_file_name($file);
@@ -1130,7 +1236,7 @@ pub fn test_set_file_name() {
 
 #[test]
 pub fn test_set_extension() {
-    macro_rules! tfe(
+    macro_rules! tfe (
             ($path:expr, $ext:expr, $expected:expr, $output:expr) => ( {
             let mut p = PathBuf::from($path);
             let output = p.set_extension($ext);
@@ -1192,7 +1298,7 @@ pub fn test_compare() {
         s.finish()
     }
 
-    macro_rules! tc(
+    macro_rules! tc (
         ($path1:expr, $path2:expr, eq: $eq:expr,
          starts_with: $starts_with:expr, ends_with: $ends_with:expr,
          relative_from: $relative_from:expr) => ({

From b1d84f721e3e423e4159e7f63729bd492d2cb77a Mon Sep 17 00:00:00 2001
From: Michael Hall <michael@mbh.sh>
Date: Tue, 11 May 2021 11:41:37 +1000
Subject: [PATCH 2/7] add missing windows tests

---
 library/std/src/path/tests.rs | 193 ++++++++++++++++++++++++----------
 1 file changed, 140 insertions(+), 53 deletions(-)

diff --git a/library/std/src/path/tests.rs b/library/std/src/path/tests.rs
index 14dcab450acbe..7ffc787b0a2b4 100644
--- a/library/std/src/path/tests.rs
+++ b/library/std/src/path/tests.rs
@@ -457,7 +457,8 @@ pub fn test_decompositions_windows() {
     parent: None,
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("foo",
@@ -467,7 +468,8 @@ pub fn test_decompositions_windows() {
     parent: Some(""),
     file_name: Some("foo"),
     file_stem: Some("foo"),
-    extension: None
+    extension: None,
+    file_prefix: Some("foo")
     );
 
     t!("/",
@@ -477,7 +479,8 @@ pub fn test_decompositions_windows() {
     parent: None,
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("\\",
@@ -487,7 +490,8 @@ pub fn test_decompositions_windows() {
     parent: None,
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("c:",
@@ -497,7 +501,8 @@ pub fn test_decompositions_windows() {
     parent: None,
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("c:\\",
@@ -507,7 +512,8 @@ pub fn test_decompositions_windows() {
     parent: None,
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("c:/",
@@ -517,7 +523,8 @@ pub fn test_decompositions_windows() {
     parent: None,
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("/foo",
@@ -527,7 +534,8 @@ pub fn test_decompositions_windows() {
     parent: Some("/"),
     file_name: Some("foo"),
     file_stem: Some("foo"),
-    extension: None
+    extension: None,
+    file_prefix: Some("foo")
     );
 
     t!("foo/",
@@ -537,7 +545,8 @@ pub fn test_decompositions_windows() {
     parent: Some(""),
     file_name: Some("foo"),
     file_stem: Some("foo"),
-    extension: None
+    extension: None,
+    file_prefix: Some("foo")
     );
 
     t!("/foo/",
@@ -547,7 +556,8 @@ pub fn test_decompositions_windows() {
     parent: Some("/"),
     file_name: Some("foo"),
     file_stem: Some("foo"),
-    extension: None
+    extension: None,
+    file_prefix: Some("foo")
     );
 
     t!("foo/bar",
@@ -557,7 +567,8 @@ pub fn test_decompositions_windows() {
     parent: Some("foo"),
     file_name: Some("bar"),
     file_stem: Some("bar"),
-    extension: None
+    extension: None,
+    file_prefix: Some("bar")
     );
 
     t!("/foo/bar",
@@ -567,7 +578,8 @@ pub fn test_decompositions_windows() {
     parent: Some("/foo"),
     file_name: Some("bar"),
     file_stem: Some("bar"),
-    extension: None
+    extension: None,
+    file_prefix: Some("bar")
     );
 
     t!("///foo///",
@@ -577,7 +589,8 @@ pub fn test_decompositions_windows() {
     parent: Some("/"),
     file_name: Some("foo"),
     file_stem: Some("foo"),
-    extension: None
+    extension: None,
+    file_prefix: Some("foo")
     );
 
     t!("///foo///bar",
@@ -587,7 +600,8 @@ pub fn test_decompositions_windows() {
     parent: Some("///foo"),
     file_name: Some("bar"),
     file_stem: Some("bar"),
-    extension: None
+    extension: None,
+    file_prefix: Some("bar")
     );
 
     t!("./.",
@@ -597,7 +611,8 @@ pub fn test_decompositions_windows() {
     parent: Some(""),
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("/..",
@@ -607,7 +622,8 @@ pub fn test_decompositions_windows() {
     parent: Some("/"),
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("../",
@@ -617,7 +633,8 @@ pub fn test_decompositions_windows() {
     parent: Some(""),
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("foo/.",
@@ -627,7 +644,8 @@ pub fn test_decompositions_windows() {
     parent: Some(""),
     file_name: Some("foo"),
     file_stem: Some("foo"),
-    extension: None
+    extension: None,
+    file_prefix: Some("foo")
     );
 
     t!("foo/..",
@@ -637,7 +655,8 @@ pub fn test_decompositions_windows() {
     parent: Some("foo"),
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("foo/./",
@@ -647,7 +666,8 @@ pub fn test_decompositions_windows() {
     parent: Some(""),
     file_name: Some("foo"),
     file_stem: Some("foo"),
-    extension: None
+    extension: None,
+    file_prefix: Some("foo")
     );
 
     t!("foo/./bar",
@@ -657,7 +677,8 @@ pub fn test_decompositions_windows() {
     parent: Some("foo"),
     file_name: Some("bar"),
     file_stem: Some("bar"),
-    extension: None
+    extension: None,
+    file_prefix: Some("bar")
     );
 
     t!("foo/../",
@@ -667,7 +688,8 @@ pub fn test_decompositions_windows() {
     parent: Some("foo"),
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("foo/../bar",
@@ -677,7 +699,8 @@ pub fn test_decompositions_windows() {
     parent: Some("foo/.."),
     file_name: Some("bar"),
     file_stem: Some("bar"),
-    extension: None
+    extension: None,
+    file_prefix: Some("bar")
     );
 
     t!("./a",
@@ -687,7 +710,8 @@ pub fn test_decompositions_windows() {
     parent: Some("."),
     file_name: Some("a"),
     file_stem: Some("a"),
-    extension: None
+    extension: None,
+    file_prefix: Some("a")
     );
 
     t!(".",
@@ -697,7 +721,8 @@ pub fn test_decompositions_windows() {
     parent: Some(""),
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("./",
@@ -707,7 +732,8 @@ pub fn test_decompositions_windows() {
     parent: Some(""),
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("a/b",
@@ -717,7 +743,8 @@ pub fn test_decompositions_windows() {
     parent: Some("a"),
     file_name: Some("b"),
     file_stem: Some("b"),
-    extension: None
+    extension: None,
+    file_prefix: Some("b")
     );
 
     t!("a//b",
@@ -727,7 +754,8 @@ pub fn test_decompositions_windows() {
     parent: Some("a"),
     file_name: Some("b"),
     file_stem: Some("b"),
-    extension: None
+    extension: None,
+    file_prefix: Some("b")
     );
 
     t!("a/./b",
@@ -737,7 +765,8 @@ pub fn test_decompositions_windows() {
     parent: Some("a"),
     file_name: Some("b"),
     file_stem: Some("b"),
-    extension: None
+    extension: None,
+    file_prefix: Some("b")
     );
 
     t!("a/b/c",
@@ -747,7 +776,9 @@ pub fn test_decompositions_windows() {
        parent: Some("a/b"),
        file_name: Some("c"),
        file_stem: Some("c"),
-       extension: None);
+       extension: None,
+       file_prefix: Some("c")
+    );
 
     t!("a\\b\\c",
     iter: ["a", "b", "c"],
@@ -756,7 +787,8 @@ pub fn test_decompositions_windows() {
     parent: Some("a\\b"),
     file_name: Some("c"),
     file_stem: Some("c"),
-    extension: None
+    extension: None,
+    file_prefix: Some("c")
     );
 
     t!("\\a",
@@ -766,7 +798,8 @@ pub fn test_decompositions_windows() {
     parent: Some("\\"),
     file_name: Some("a"),
     file_stem: Some("a"),
-    extension: None
+    extension: None,
+    file_prefix: Some("a")
     );
 
     t!("c:\\foo.txt",
@@ -776,7 +809,8 @@ pub fn test_decompositions_windows() {
     parent: Some("c:\\"),
     file_name: Some("foo.txt"),
     file_stem: Some("foo"),
-    extension: Some("txt")
+    extension: Some("txt"),
+    file_prefix: Some("foo")
     );
 
     t!("\\\\server\\share\\foo.txt",
@@ -786,7 +820,8 @@ pub fn test_decompositions_windows() {
     parent: Some("\\\\server\\share\\"),
     file_name: Some("foo.txt"),
     file_stem: Some("foo"),
-    extension: Some("txt")
+    extension: Some("txt"),
+    file_prefix: Some("foo")
     );
 
     t!("\\\\server\\share",
@@ -796,7 +831,8 @@ pub fn test_decompositions_windows() {
     parent: None,
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("\\\\server",
@@ -806,7 +842,8 @@ pub fn test_decompositions_windows() {
     parent: Some("\\"),
     file_name: Some("server"),
     file_stem: Some("server"),
-    extension: None
+    extension: None,
+    file_prefix: Some("server")
     );
 
     t!("\\\\?\\bar\\foo.txt",
@@ -816,7 +853,8 @@ pub fn test_decompositions_windows() {
     parent: Some("\\\\?\\bar\\"),
     file_name: Some("foo.txt"),
     file_stem: Some("foo"),
-    extension: Some("txt")
+    extension: Some("txt"),
+    file_prefix: Some("foo")
     );
 
     t!("\\\\?\\bar",
@@ -826,7 +864,8 @@ pub fn test_decompositions_windows() {
     parent: None,
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("\\\\?\\",
@@ -836,7 +875,8 @@ pub fn test_decompositions_windows() {
     parent: None,
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("\\\\?\\UNC\\server\\share\\foo.txt",
@@ -846,7 +886,8 @@ pub fn test_decompositions_windows() {
     parent: Some("\\\\?\\UNC\\server\\share\\"),
     file_name: Some("foo.txt"),
     file_stem: Some("foo"),
-    extension: Some("txt")
+    extension: Some("txt"),
+    file_prefix: Some("foo")
     );
 
     t!("\\\\?\\UNC\\server",
@@ -856,7 +897,8 @@ pub fn test_decompositions_windows() {
     parent: None,
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("\\\\?\\UNC\\",
@@ -866,7 +908,8 @@ pub fn test_decompositions_windows() {
     parent: None,
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("\\\\?\\C:\\foo.txt",
@@ -876,7 +919,8 @@ pub fn test_decompositions_windows() {
     parent: Some("\\\\?\\C:\\"),
     file_name: Some("foo.txt"),
     file_stem: Some("foo"),
-    extension: Some("txt")
+    extension: Some("txt"),
+    file_prefix: Some("foo")
     );
 
     t!("\\\\?\\C:\\",
@@ -886,7 +930,8 @@ pub fn test_decompositions_windows() {
     parent: None,
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("\\\\?\\C:",
@@ -896,7 +941,8 @@ pub fn test_decompositions_windows() {
     parent: None,
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("\\\\?\\foo/bar",
@@ -906,7 +952,8 @@ pub fn test_decompositions_windows() {
     parent: None,
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("\\\\?\\C:/foo",
@@ -916,7 +963,8 @@ pub fn test_decompositions_windows() {
     parent: None,
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("\\\\.\\foo\\bar",
@@ -926,7 +974,8 @@ pub fn test_decompositions_windows() {
     parent: Some("\\\\.\\foo\\"),
     file_name: Some("bar"),
     file_stem: Some("bar"),
-    extension: None
+    extension: None,
+    file_prefix: Some("bar")
     );
 
     t!("\\\\.\\foo",
@@ -936,7 +985,8 @@ pub fn test_decompositions_windows() {
     parent: None,
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("\\\\.\\foo/bar",
@@ -946,7 +996,8 @@ pub fn test_decompositions_windows() {
     parent: Some("\\\\.\\foo/"),
     file_name: Some("bar"),
     file_stem: Some("bar"),
-    extension: None
+    extension: None,
+    file_prefix: Some("bar")
     );
 
     t!("\\\\.\\foo\\bar/baz",
@@ -956,7 +1007,8 @@ pub fn test_decompositions_windows() {
     parent: Some("\\\\.\\foo\\bar"),
     file_name: Some("baz"),
     file_stem: Some("baz"),
-    extension: None
+    extension: None,
+    file_prefix: Some("baz")
     );
 
     t!("\\\\.\\",
@@ -966,7 +1018,8 @@ pub fn test_decompositions_windows() {
     parent: None,
     file_name: None,
     file_stem: None,
-    extension: None
+    extension: None,
+    file_prefix: None
     );
 
     t!("\\\\?\\a\\b\\",
@@ -976,7 +1029,41 @@ pub fn test_decompositions_windows() {
     parent: Some("\\\\?\\a\\"),
     file_name: Some("b"),
     file_stem: Some("b"),
-    extension: None
+    extension: None,
+    file_prefix: Some("b")
+    );
+
+    t!("\\\\?\\C:\\foo.txt.zip",
+    iter: ["\\\\?\\C:", "\\", "foo.txt.zip"],
+    has_root: true,
+    is_absolute: true,
+    parent: Some("\\\\?\\C:\\"),
+    file_name: Some("foo.txt.zip"),
+    file_stem: Some("foo.txt"),
+    extension: Some("zip"),
+    file_prefix: Some("foo")
+    );
+
+    t!("\\\\?\\C:\\.foo.txt.zip",
+    iter: ["\\\\?\\C:", "\\", ".foo.txt.zip"],
+    has_root: true,
+    is_absolute: true,
+    parent: Some("\\\\?\\C:\\"),
+    file_name: Some(".foo.txt.zip"),
+    file_stem: Some(".foo.txt"),
+    extension: Some("zip"),
+    file_prefix: Some(".foo")
+    );
+
+    t!("\\\\?\\C:\\.foo",
+    iter: ["\\\\?\\C:", "\\", ".foo"],
+    has_root: true,
+    is_absolute: true,
+    parent: Some("\\\\?\\C:\\"),
+    file_name: Some(".foo"),
+    file_stem: Some(".foo"),
+    extension: None,
+    file_prefix: Some(".foo")
     );
 }
 

From 7465192c7de96ac46a9c5b0200a34a6d010068ac Mon Sep 17 00:00:00 2001
From: Michael Hall <michael@mbh.sh>
Date: Tue, 15 Jun 2021 14:00:22 +1000
Subject: [PATCH 3/7] simplify logic for split_file_at_dot

---
 library/std/src/path.rs | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/library/std/src/path.rs b/library/std/src/path.rs
index cec8010fe24be..17e47b76cad73 100644
--- a/library/std/src/path.rs
+++ b/library/std/src/path.rs
@@ -346,15 +346,11 @@ fn split_file_at_dot(file: &OsStr) -> (Option<&OsStr>, Option<&OsStr>) {
     // only from ASCII-bounded slices of existing &OsStr values.
     let i = match slice[1..].iter().position(|b| *b == b'.') {
         Some(i) => i + 1,
-        None => slice.len(),
+        None => return (Some(file), None),
     };
-    if i == slice.len() {
-        (Some(file), None)
-    } else {
-        let before = Some(&slice[..i]);
-        let after = Some(&slice[i + 1..]);
-        unsafe { (before.map(|s| u8_slice_as_os_str(s)), after.map(|s| u8_slice_as_os_str(s))) }
-    }
+    let before = &slice[..i];
+    let after = &slice[i + 1..];
+    unsafe { (Some(u8_slice_as_os_str(before)), Some(u8_slice_as_os_str(after))) }
 }
 
 ////////////////////////////////////////////////////////////////////////////////

From a889529e984f1d477153e60c16fa4066d8b6c9d7 Mon Sep 17 00:00:00 2001
From: Michael Hall <michael@mbh.sh>
Date: Tue, 15 Jun 2021 14:47:23 +1000
Subject: [PATCH 4/7] add explicit hidden file name tests

---
 library/std/src/path/tests.rs | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/library/std/src/path/tests.rs b/library/std/src/path/tests.rs
index 7ffc787b0a2b4..d7ba478939169 100644
--- a/library/std/src/path/tests.rs
+++ b/library/std/src/path/tests.rs
@@ -445,6 +445,17 @@ pub fn test_decompositions_unix() {
     extension: Some("toml"),
     file_prefix: Some(".rustfmt")
     );
+
+    t!("a/.x.y.z",
+    iter: ["a", ".x.y.z"],
+    has_root: false,
+    is_absolute: false,
+    parent: Some("a"),
+    file_name: Some(".x.y.z"),
+    file_stem: Some(".x.y"),
+    extension: Some("z"),
+    file_prefix: Some(".x")
+    );
 }
 
 #[test]
@@ -1065,6 +1076,17 @@ pub fn test_decompositions_windows() {
     extension: None,
     file_prefix: Some(".foo")
     );
+
+    t!("a/.x.y.z",
+    iter: ["a", ".x.y.z"],
+    has_root: false,
+    is_absolute: false,
+    parent: Some("a"),
+    file_name: Some(".x.y.z"),
+    file_stem: Some(".x.y"),
+    extension: Some("z"),
+    file_prefix: Some(".x")
+    );
 }
 
 #[test]
@@ -1103,6 +1125,8 @@ pub fn test_stem_ext() {
 
     t!("..", file_stem: None, extension: None);
 
+    t!(".x.y.z", file_stem: Some(".x.y"), extension: Some("z"));
+
     t!("", file_stem: None, extension: None);
 }
 
@@ -1142,6 +1166,8 @@ pub fn test_prefix_ext() {
 
     t!("..", file_prefix: None, extension: None);
 
+    t!(".x.y.z", file_prefix: Some(".x"), extension: Some("z"));
+
     t!("", file_prefix: None, extension: None);
 }
 

From fcb1ebf1945c616365c74e95666dcdcfdc8bbd70 Mon Sep 17 00:00:00 2001
From: Michael Hall <michael@mbh.sh>
Date: Thu, 24 Jun 2021 13:45:56 +1000
Subject: [PATCH 5/7] change return signature for split_file_at_dot

---
 library/std/src/path.rs | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/library/std/src/path.rs b/library/std/src/path.rs
index 17e47b76cad73..1cb596d691c41 100644
--- a/library/std/src/path.rs
+++ b/library/std/src/path.rs
@@ -334,10 +334,10 @@ fn rsplit_file_at_dot(file: &OsStr) -> (Option<&OsStr>, Option<&OsStr>) {
     }
 }
 
-fn split_file_at_dot(file: &OsStr) -> (Option<&OsStr>, Option<&OsStr>) {
+fn split_file_at_dot(file: &OsStr) -> (&OsStr, Option<&OsStr>) {
     let slice = os_str_as_u8_slice(file);
     if slice == b".." {
-        return (Some(file), None);
+        return (file, None);
     }
 
     // The unsafety here stems from converting between &OsStr and &[u8]
@@ -346,11 +346,11 @@ fn split_file_at_dot(file: &OsStr) -> (Option<&OsStr>, Option<&OsStr>) {
     // only from ASCII-bounded slices of existing &OsStr values.
     let i = match slice[1..].iter().position(|b| *b == b'.') {
         Some(i) => i + 1,
-        None => return (Some(file), None),
+        None => return (file, None),
     };
     let before = &slice[..i];
     let after = &slice[i + 1..];
-    unsafe { (Some(u8_slice_as_os_str(before)), Some(u8_slice_as_os_str(after))) }
+    unsafe { (u8_slice_as_os_str(before), Some(u8_slice_as_os_str(after))) }
 }
 
 ////////////////////////////////////////////////////////////////////////////////
@@ -2201,9 +2201,11 @@ impl Path {
     /// assert_eq!("foo", Path::new("foo.rs").file_prefix().unwrap());
     /// assert_eq!("foo", Path::new("foo.tar.gz").file_prefix().unwrap());
     /// ```
-    #[unstable(feature = "path_file_prefix", issue = "none")]
+    #[unstable(feature = "path_file_prefix", issue = "86319")]
     pub fn file_prefix(&self) -> Option<&OsStr> {
-        self.file_name().map(split_file_at_dot).and_then(|(before, after)| before.or(after))
+        self.file_name()
+            .map(split_file_at_dot)
+            .and_then(|(before, after)| if before.is_empty() { after } else { Some(before) })
     }
 
     /// Extracts the extension of [`self.file_name`], if possible.

From 1e759bef919e3600481839324ba02c637ec13ee1 Mon Sep 17 00:00:00 2001
From: Michael Hall <michael@mbh.sh>
Date: Thu, 24 Jun 2021 14:26:10 +1000
Subject: [PATCH 6/7] make docs clearer about how hidden files are dealt with

---
 library/std/src/path.rs | 24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/library/std/src/path.rs b/library/std/src/path.rs
index 1cb596d691c41..9ab2f045d697e 100644
--- a/library/std/src/path.rs
+++ b/library/std/src/path.rs
@@ -2175,22 +2175,29 @@ impl Path {
     /// assert_eq!("foo", Path::new("foo.rs").file_stem().unwrap());
     /// assert_eq!("foo.tar", Path::new("foo.tar.gz").file_stem().unwrap());
     /// ```
+    ///
+    /// # See Also
+    /// This method is similar to [`Path::file_prefix`], which extracts the portion of the file name
+    /// before the *first* `.`
+    ///
+    /// [`Path::file_prefix`]: Path::file_prefix
+    ///
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn file_stem(&self) -> Option<&OsStr> {
         self.file_name().map(rsplit_file_at_dot).and_then(|(before, after)| before.or(after))
     }
 
-    /// Extracts the prefix (non-extension(s)) portion of [`self.file_name`]. This is a "left"
-    /// variant of `file_stem` - meaning it takes the portion of the file name before the *first* `.`
-    ///
-    /// [`self.file_name`]: Path::file_name
+    /// Extracts the prefix of [`self.file_name`].
     ///
     /// The prefix is:
     ///
     /// * [`None`], if there is no file name;
     /// * The entire file name if there is no embedded `.`;
+    /// * The portion of the file name before the first non-beginning `.`;
     /// * The entire file name if the file name begins with `.` and has no other `.`s within;
-    /// * Otherwise, the portion of the file name before the first `.`
+    /// * The portion of the file name before the second `.` if the file name begins with `.`
+    ///
+    /// [`self.file_name`]: Path::file_name
     ///
     /// # Examples
     ///
@@ -2201,6 +2208,13 @@ impl Path {
     /// assert_eq!("foo", Path::new("foo.rs").file_prefix().unwrap());
     /// assert_eq!("foo", Path::new("foo.tar.gz").file_prefix().unwrap());
     /// ```
+    ///
+    /// # See Also
+    /// This method is similar to [`Path::file_stem`], which extracts the portion of the file name
+    /// before the *last* `.`
+    ///
+    /// [`Path::file_stem`]: Path::file_stem
+    ///
     #[unstable(feature = "path_file_prefix", issue = "86319")]
     pub fn file_prefix(&self) -> Option<&OsStr> {
         self.file_name()

From 51cf318dbcdd38d6a7478413513a72097d9622c3 Mon Sep 17 00:00:00 2001
From: Michael Hall <michael@mbh.sh>
Date: Tue, 17 Aug 2021 12:24:28 +1000
Subject: [PATCH 7/7] remove unnecessary empty check

---
 library/std/src/path.rs       | 4 +---
 library/std/src/path/tests.rs | 4 ++++
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/library/std/src/path.rs b/library/std/src/path.rs
index 9ab2f045d697e..8a2450fcb887c 100644
--- a/library/std/src/path.rs
+++ b/library/std/src/path.rs
@@ -2217,9 +2217,7 @@ impl Path {
     ///
     #[unstable(feature = "path_file_prefix", issue = "86319")]
     pub fn file_prefix(&self) -> Option<&OsStr> {
-        self.file_name()
-            .map(split_file_at_dot)
-            .and_then(|(before, after)| if before.is_empty() { after } else { Some(before) })
+        self.file_name().map(split_file_at_dot).and_then(|(before, _after)| Some(before))
     }
 
     /// Extracts the extension of [`self.file_name`], if possible.
diff --git a/library/std/src/path/tests.rs b/library/std/src/path/tests.rs
index d7ba478939169..e8b960e6d7370 100644
--- a/library/std/src/path/tests.rs
+++ b/library/std/src/path/tests.rs
@@ -1127,6 +1127,8 @@ pub fn test_stem_ext() {
 
     t!(".x.y.z", file_stem: Some(".x.y"), extension: Some("z"));
 
+    t!("..x.y.z", file_stem: Some("..x.y"), extension: Some("z"));
+
     t!("", file_stem: None, extension: None);
 }
 
@@ -1168,6 +1170,8 @@ pub fn test_prefix_ext() {
 
     t!(".x.y.z", file_prefix: Some(".x"), extension: Some("z"));
 
+    t!("..x.y.z", file_prefix: Some("."), extension: Some("z"));
+
     t!("", file_prefix: None, extension: None);
 }