Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Jump to bottom

Document From implementations #51753

Merged
merged 7 commits into from
Dec 6, 2018
Merged

Document From implementations #51753

Changes from 6 commits
Commits
Show all changes
7 commits
Select commit Hold shift + click to select a range
700c83b
Document `From` implementations
Jun 24, 2018
072bca3
Remove 'unsafe' comments
Jun 25, 2018
88a708d
Update comments
Jun 25, 2018
5c747eb
Update style of comments
Jul 26, 2018
e8dafba
Adjust doc comments
Nov 21, 2018
7933628
Remove trailing whitespace
Nov 21, 2018
450a8a6
Add extra comment slash
Dec 4, 2018
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
20 changes: 20 additions & 0 deletions src/libstd/path.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -1397,13 +1397,20 @@ impl<'a> From<&'a Path> for Box<Path> {

#[stable(feature = "path_buf_from_box", since = "1.18.0")]
impl From<Box<Path>> for PathBuf {
/// Converts a `Box<Path>` into a `PathBuf`
///
/// This conversion does not allocate or copy memory.
fn from(boxed: Box<Path>) -> PathBuf {
boxed.into_path_buf()
}
}

#[stable(feature = "box_from_path_buf", since = "1.20.0")]
impl From<PathBuf> for Box<Path> {
/// Converts a `PathBuf` into a `Box<Path>`
///
/// This conversion currently should not allocate memory,
// but this behavior is not guaranteed on all platforms or in all future versions.
QuietMisdreavus marked this conversation as resolved.
Show resolved Hide resolved
fn from(p: PathBuf) -> Box<Path> {
p.into_boxed_path()
}
Expand All @@ -1426,20 +1433,29 @@ impl<'a, T: ?Sized + AsRef<OsStr>> From<&'a T> for PathBuf {

#[stable(feature = "rust1", since = "1.0.0")]
impl From<OsString> for PathBuf {
/// Converts a `OsString` into a `PathBuf`
///
/// This conversion does not allocate or copy memory.
fn from(s: OsString) -> PathBuf {
PathBuf { inner: s }
Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Interestingly, this does not allocate memory. This is free.

PathBuf is a wrapper around OsString. If you have a struct with just one field, its memory structure is the same as the field itself. The Compiler "sees" through this and knows that turning OsString into a PathBuf is effectively a no-op.

See https://godbolt.org/g/1PBCTY (using String, as an example, but the principle is the same). If you have a close look at the blue and orange part, this just sets up for returning the previously allocated String. It doesn't even call the implementation!

Copy link
Contributor Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Thanks a lot for the explanation, was a while since I looked at Assembler. Do you have a general set of tools you use for investigating memory allocation? I sort of did all over the place with no real plan.

}
}

#[stable(feature = "from_path_buf_for_os_string", since = "1.14.0")]
impl From<PathBuf> for OsString {
/// Converts a `PathBuf` into a `OsString`
///
/// This conversion does not allocate or copy memory.
fn from(path_buf : PathBuf) -> OsString {
path_buf.inner
}
}

#[stable(feature = "rust1", since = "1.0.0")]
impl From<String> for PathBuf {
/// Converts a `String` into a `PathBuf`
///
/// This conversion does not allocate or copy memory.
fn from(s: String) -> PathBuf {
PathBuf::from(OsString::from(s))
}
Expand Down Expand Up @@ -1536,6 +1552,7 @@ impl<'a> From<Cow<'a, Path>> for PathBuf {

#[stable(feature = "shared_from_slice2", since = "1.24.0")]
impl From<PathBuf> for Arc<Path> {
/// Converts a Path into a Rc by copying the Path data into a new Rc buffer.
#[inline]
fn from(s: PathBuf) -> Arc<Path> {
let arc: Arc<OsStr> = Arc::from(s.into_os_string());
Expand All @@ -1545,6 +1562,7 @@ impl From<PathBuf> for Arc<Path> {

#[stable(feature = "shared_from_slice2", since = "1.24.0")]
impl<'a> From<&'a Path> for Arc<Path> {
/// Converts a Path into a Rc by copying the Path data into a new Rc buffer.
#[inline]
fn from(s: &Path) -> Arc<Path> {
let arc: Arc<OsStr> = Arc::from(s.as_os_str());
Expand All @@ -1554,6 +1572,7 @@ impl<'a> From<&'a Path> for Arc<Path> {

#[stable(feature = "shared_from_slice2", since = "1.24.0")]
impl From<PathBuf> for Rc<Path> {
/// Converts a Path into a Rc by copying the Path data into a new Rc buffer.
#[inline]
fn from(s: PathBuf) -> Rc<Path> {
let rc: Rc<OsStr> = Rc::from(s.into_os_string());
Expand All @@ -1563,6 +1582,7 @@ impl From<PathBuf> for Rc<Path> {

#[stable(feature = "shared_from_slice2", since = "1.24.0")]
impl<'a> From<&'a Path> for Rc<Path> {
/// Converts a Path into a Rc by copying the Path data into a new Rc buffer.
#[inline]
fn from(s: &Path) -> Rc<Path> {
let rc: Rc<OsStr> = Rc::from(s.as_os_str());
Expand Down