pub struct OnceCell<T> { /* private fields */ }
__ecma
and __utils
only.Expand description
A cell which can be written to only once. It is not thread safe.
Unlike std::cell::RefCell
, a OnceCell
provides simple &
references to the contents.
§Example
use once_cell::unsync::OnceCell;
let cell = OnceCell::new();
assert!(cell.get().is_none());
let value: &String = cell.get_or_init(|| {
"Hello, World!".to_string()
});
assert_eq!(value, "Hello, World!");
assert!(cell.get().is_some());
Implementations§
§impl<T> OnceCell<T>
impl<T> OnceCell<T>
pub const fn with_value(value: T) -> OnceCell<T>
Available on crate feature ecma_ast
only.
pub const fn with_value(value: T) -> OnceCell<T>
ecma_ast
only.Creates a new initialized cell.
pub fn get(&self) -> Option<&T>
Available on crate feature ecma_ast
only.
pub fn get(&self) -> Option<&T>
ecma_ast
only.Gets a reference to the underlying value.
Returns None
if the cell is empty.
pub fn get_mut(&mut self) -> Option<&mut T>
Available on crate feature ecma_ast
only.
pub fn get_mut(&mut self) -> Option<&mut T>
ecma_ast
only.Gets a mutable reference to the underlying value.
Returns None
if the cell is empty.
This method is allowed to violate the invariant of writing to a OnceCell
at most once because it requires &mut
access to self
. As with all
interior mutability, &mut
access permits arbitrary modification:
use once_cell::unsync::OnceCell;
let mut cell: OnceCell<u32> = OnceCell::new();
cell.set(92).unwrap();
*cell.get_mut().unwrap() = 93;
assert_eq!(cell.get(), Some(&93));
pub fn set(&self, value: T) -> Result<(), T>
Available on crate feature ecma_ast
only.
pub fn set(&self, value: T) -> Result<(), T>
ecma_ast
only.Sets the contents of this cell to value
.
Returns Ok(())
if the cell was empty and Err(value)
if it was
full.
§Example
use once_cell::unsync::OnceCell;
let cell = OnceCell::new();
assert!(cell.get().is_none());
assert_eq!(cell.set(92), Ok(()));
assert_eq!(cell.set(62), Err(62));
assert!(cell.get().is_some());
pub fn try_insert(&self, value: T) -> Result<&T, (&T, T)>
Available on crate feature ecma_ast
only.
pub fn try_insert(&self, value: T) -> Result<&T, (&T, T)>
ecma_ast
only.pub fn get_or_init<F>(&self, f: F) -> &Twhere
F: FnOnce() -> T,
Available on crate feature ecma_ast
only.
pub fn get_or_init<F>(&self, f: F) -> &Twhere
F: FnOnce() -> T,
ecma_ast
only.Gets the contents of the cell, initializing it with f
if the cell was empty.
§Panics
If f
panics, the panic is propagated to the caller, and the cell
remains uninitialized.
It is an error to reentrantly initialize the cell from f
. Doing
so results in a panic.
§Example
use once_cell::unsync::OnceCell;
let cell = OnceCell::new();
let value = cell.get_or_init(|| 92);
assert_eq!(value, &92);
let value = cell.get_or_init(|| unreachable!());
assert_eq!(value, &92);
pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
Available on crate feature ecma_ast
only.
pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
ecma_ast
only.Gets the contents of the cell, initializing it with f
if
the cell was empty. If the cell was empty and f
failed, an
error is returned.
§Panics
If f
panics, the panic is propagated to the caller, and the cell
remains uninitialized.
It is an error to reentrantly initialize the cell from f
. Doing
so results in a panic.
§Example
use once_cell::unsync::OnceCell;
let cell = OnceCell::new();
assert_eq!(cell.get_or_try_init(|| Err(())), Err(()));
assert!(cell.get().is_none());
let value = cell.get_or_try_init(|| -> Result<i32, ()> {
Ok(92)
});
assert_eq!(value, Ok(&92));
assert_eq!(cell.get(), Some(&92))
pub fn take(&mut self) -> Option<T>
Available on crate feature ecma_ast
only.
pub fn take(&mut self) -> Option<T>
ecma_ast
only.Takes the value out of this OnceCell
, moving it back to an uninitialized state.
Has no effect and returns None
if the OnceCell
hasn’t been initialized.
§Examples
use once_cell::unsync::OnceCell;
let mut cell: OnceCell<String> = OnceCell::new();
assert_eq!(cell.take(), None);
let mut cell = OnceCell::new();
cell.set("hello".to_string()).unwrap();
assert_eq!(cell.take(), Some("hello".to_string()));
assert_eq!(cell.get(), None);
This method is allowed to violate the invariant of writing to a OnceCell
at most once because it requires &mut
access to self
. As with all
interior mutability, &mut
access permits arbitrary modification:
use once_cell::unsync::OnceCell;
let mut cell: OnceCell<u32> = OnceCell::new();
cell.set(92).unwrap();
cell = OnceCell::new();
pub fn into_inner(self) -> Option<T>
Available on crate feature ecma_ast
only.
pub fn into_inner(self) -> Option<T>
ecma_ast
only.Consumes the OnceCell
, returning the wrapped value.
Returns None
if the cell was empty.
§Examples
use once_cell::unsync::OnceCell;
let cell: OnceCell<String> = OnceCell::new();
assert_eq!(cell.into_inner(), None);
let cell = OnceCell::new();
cell.set("hello".to_string()).unwrap();
assert_eq!(cell.into_inner(), Some("hello".to_string()));
Trait Implementations§
§impl<T> PartialEq for OnceCell<T>where
T: PartialEq,
impl<T> PartialEq for OnceCell<T>where
T: PartialEq,
impl<T> Eq for OnceCell<T>where
T: Eq,
impl<T> RefUnwindSafe for OnceCell<T>where
T: RefUnwindSafe + UnwindSafe,
impl<T> UnwindSafe for OnceCell<T>where
T: UnwindSafe,
Auto Trait Implementations§
impl<T> !Freeze for OnceCell<T>
impl<T> Send for OnceCell<T>where
T: Send,
impl<T> !Sync for OnceCell<T>
impl<T> Unpin for OnceCell<T>where
T: Unpin,
Blanket Implementations§
§impl<T> ArchivePointee for T
impl<T> ArchivePointee for T
§type ArchivedMetadata = ()
type ArchivedMetadata = ()
§fn pointer_metadata(
_: &<T as ArchivePointee>::ArchivedMetadata,
) -> <T as Pointee>::Metadata
fn pointer_metadata( _: &<T as ArchivePointee>::ArchivedMetadata, ) -> <T as Pointee>::Metadata
source§impl<T> BorrowMut<T> for Twhere
T: ?Sized,
impl<T> BorrowMut<T> for Twhere
T: ?Sized,
source§fn borrow_mut(&mut self) -> &mut T
fn borrow_mut(&mut self) -> &mut T
source§impl<T> CloneToUninit for Twhere
T: Clone,
impl<T> CloneToUninit for Twhere
T: Clone,
source§default unsafe fn clone_to_uninit(&self, dst: *mut T)
default unsafe fn clone_to_uninit(&self, dst: *mut T)
clone_to_uninit
)§impl<T> Conv for T
impl<T> Conv for T
§impl<F, W, T, D> Deserialize<With<T, W>, D> for F
impl<F, W, T, D> Deserialize<With<T, W>, D> for F
§fn deserialize(
&self,
deserializer: &mut D,
) -> Result<With<T, W>, <D as Fallible>::Error>
fn deserialize( &self, deserializer: &mut D, ) -> Result<With<T, W>, <D as Fallible>::Error>
§impl<Q, K> Equivalent<K> for Q
impl<Q, K> Equivalent<K> for Q
§fn equivalent(&self, key: &K) -> bool
fn equivalent(&self, key: &K) -> bool
source§impl<Q, K> Equivalent<K> for Q
impl<Q, K> Equivalent<K> for Q
source§fn equivalent(&self, key: &K) -> bool
fn equivalent(&self, key: &K) -> bool
key
and return true
if they are equal.§impl<Q, K> Equivalent<K> for Q
impl<Q, K> Equivalent<K> for Q
§fn equivalent(&self, key: &K) -> bool
fn equivalent(&self, key: &K) -> bool
key
and return true
if they are equal.§impl<Q, K> Equivalent<K> for Q
impl<Q, K> Equivalent<K> for Q
§fn equivalent(&self, key: &K) -> bool
fn equivalent(&self, key: &K) -> bool
§impl<T> FmtForward for T
impl<T> FmtForward for T
§fn fmt_binary(self) -> FmtBinary<Self>where
Self: Binary,
fn fmt_binary(self) -> FmtBinary<Self>where
Self: Binary,
self
to use its Binary
implementation when Debug
-formatted.§fn fmt_display(self) -> FmtDisplay<Self>where
Self: Display,
fn fmt_display(self) -> FmtDisplay<Self>where
Self: Display,
self
to use its Display
implementation when
Debug
-formatted.§fn fmt_lower_exp(self) -> FmtLowerExp<Self>where
Self: LowerExp,
fn fmt_lower_exp(self) -> FmtLowerExp<Self>where
Self: LowerExp,
self
to use its LowerExp
implementation when
Debug
-formatted.§fn fmt_lower_hex(self) -> FmtLowerHex<Self>where
Self: LowerHex,
fn fmt_lower_hex(self) -> FmtLowerHex<Self>where
Self: LowerHex,
self
to use its LowerHex
implementation when
Debug
-formatted.§fn fmt_octal(self) -> FmtOctal<Self>where
Self: Octal,
fn fmt_octal(self) -> FmtOctal<Self>where
Self: Octal,
self
to use its Octal
implementation when Debug
-formatted.§fn fmt_pointer(self) -> FmtPointer<Self>where
Self: Pointer,
fn fmt_pointer(self) -> FmtPointer<Self>where
Self: Pointer,
self
to use its Pointer
implementation when
Debug
-formatted.§fn fmt_upper_exp(self) -> FmtUpperExp<Self>where
Self: UpperExp,
fn fmt_upper_exp(self) -> FmtUpperExp<Self>where
Self: UpperExp,
self
to use its UpperExp
implementation when
Debug
-formatted.§fn fmt_upper_hex(self) -> FmtUpperHex<Self>where
Self: UpperHex,
fn fmt_upper_hex(self) -> FmtUpperHex<Self>where
Self: UpperHex,
self
to use its UpperHex
implementation when
Debug
-formatted.§fn fmt_list(self) -> FmtList<Self>where
&'a Self: for<'a> IntoIterator,
fn fmt_list(self) -> FmtList<Self>where
&'a Self: for<'a> IntoIterator,
§impl<T> FromFd for T
impl<T> FromFd for T
§impl<T> FromFilelike for T
impl<T> FromFilelike for T
§fn from_filelike(owned: OwnedFd) -> T
fn from_filelike(owned: OwnedFd) -> T
Self
from the given filelike object. Read more§fn from_into_filelike<Owned>(owned: Owned) -> Twhere
Owned: IntoFilelike,
fn from_into_filelike<Owned>(owned: Owned) -> Twhere
Owned: IntoFilelike,
Self
from the given filelike object
converted from into_owned
. Read more§impl<T> FromSocketlike for T
impl<T> FromSocketlike for T
§fn from_socketlike(owned: OwnedFd) -> T
fn from_socketlike(owned: OwnedFd) -> T
Self
from the given socketlike object.§fn from_into_socketlike<Owned>(owned: Owned) -> Twhere
Owned: IntoSocketlike,
fn from_into_socketlike<Owned>(owned: Owned) -> Twhere
Owned: IntoSocketlike,
Self
from the given socketlike object
converted from into_owned
.source§impl<T> ImplicitClone for Twhere
T: Clone,
impl<T> ImplicitClone for Twhere
T: Clone,
source§fn clone_quote_var(&self) -> Self
fn clone_quote_var(&self) -> Self
ecma_quote
only.§impl<T> Instrument for T
impl<T> Instrument for T
§fn instrument(self, span: Span) -> Instrumented<Self>
fn instrument(self, span: Span) -> Instrumented<Self>
§fn in_current_span(self) -> Instrumented<Self>
fn in_current_span(self) -> Instrumented<Self>
source§impl<T> IntoEither for T
impl<T> IntoEither for T
source§fn into_either(self, into_left: bool) -> Either<Self, Self> ⓘ
fn into_either(self, into_left: bool) -> Either<Self, Self> ⓘ
self
into a Left
variant of Either<Self, Self>
if into_left
is true
.
Converts self
into a Right
variant of Either<Self, Self>
otherwise. Read moresource§fn into_either_with<F>(self, into_left: F) -> Either<Self, Self> ⓘ
fn into_either_with<F>(self, into_left: F) -> Either<Self, Self> ⓘ
self
into a Left
variant of Either<Self, Self>
if into_left(&self)
returns true
.
Converts self
into a Right
variant of Either<Self, Self>
otherwise. Read more§impl<T> LayoutRaw for T
impl<T> LayoutRaw for T
§fn layout_raw(_: <T as Pointee>::Metadata) -> Result<Layout, LayoutError>
fn layout_raw(_: <T as Pointee>::Metadata) -> Result<Layout, LayoutError>
§impl<D> OwoColorize for D
impl<D> OwoColorize for D
§fn fg<C>(&self) -> FgColorDisplay<'_, C, Self>where
C: Color,
fn fg<C>(&self) -> FgColorDisplay<'_, C, Self>where
C: Color,
§fn bg<C>(&self) -> BgColorDisplay<'_, C, Self>where
C: Color,
fn bg<C>(&self) -> BgColorDisplay<'_, C, Self>where
C: Color,
§fn on_magenta(&self) -> BgColorDisplay<'_, Magenta, Self>
fn on_magenta(&self) -> BgColorDisplay<'_, Magenta, Self>
§fn default_color(&self) -> FgColorDisplay<'_, Default, Self>
fn default_color(&self) -> FgColorDisplay<'_, Default, Self>
§fn on_default_color(&self) -> BgColorDisplay<'_, Default, Self>
fn on_default_color(&self) -> BgColorDisplay<'_, Default, Self>
§fn bright_black(&self) -> FgColorDisplay<'_, BrightBlack, Self>
fn bright_black(&self) -> FgColorDisplay<'_, BrightBlack, Self>
§fn on_bright_black(&self) -> BgColorDisplay<'_, BrightBlack, Self>
fn on_bright_black(&self) -> BgColorDisplay<'_, BrightBlack, Self>
§fn bright_red(&self) -> FgColorDisplay<'_, BrightRed, Self>
fn bright_red(&self) -> FgColorDisplay<'_, BrightRed, Self>
§fn on_bright_red(&self) -> BgColorDisplay<'_, BrightRed, Self>
fn on_bright_red(&self) -> BgColorDisplay<'_, BrightRed, Self>
§fn bright_green(&self) -> FgColorDisplay<'_, BrightGreen, Self>
fn bright_green(&self) -> FgColorDisplay<'_, BrightGreen, Self>
§fn on_bright_green(&self) -> BgColorDisplay<'_, BrightGreen, Self>
fn on_bright_green(&self) -> BgColorDisplay<'_, BrightGreen, Self>
§fn bright_yellow(&self) -> FgColorDisplay<'_, BrightYellow, Self>
fn bright_yellow(&self) -> FgColorDisplay<'_, BrightYellow, Self>
§fn on_bright_yellow(&self) -> BgColorDisplay<'_, BrightYellow, Self>
fn on_bright_yellow(&self) -> BgColorDisplay<'_, BrightYellow, Self>
§fn bright_blue(&self) -> FgColorDisplay<'_, BrightBlue, Self>
fn bright_blue(&self) -> FgColorDisplay<'_, BrightBlue, Self>
§fn on_bright_blue(&self) -> BgColorDisplay<'_, BrightBlue, Self>
fn on_bright_blue(&self) -> BgColorDisplay<'_, BrightBlue, Self>
§fn bright_magenta(&self) -> FgColorDisplay<'_, BrightMagenta, Self>
fn bright_magenta(&self) -> FgColorDisplay<'_, BrightMagenta, Self>
§fn on_bright_magenta(&self) -> BgColorDisplay<'_, BrightMagenta, Self>
fn on_bright_magenta(&self) -> BgColorDisplay<'_, BrightMagenta, Self>
§fn bright_purple(&self) -> FgColorDisplay<'_, BrightMagenta, Self>
fn bright_purple(&self) -> FgColorDisplay<'_, BrightMagenta, Self>
§fn on_bright_purple(&self) -> BgColorDisplay<'_, BrightMagenta, Self>
fn on_bright_purple(&self) -> BgColorDisplay<'_, BrightMagenta, Self>
§fn bright_cyan(&self) -> FgColorDisplay<'_, BrightCyan, Self>
fn bright_cyan(&self) -> FgColorDisplay<'_, BrightCyan, Self>
§fn on_bright_cyan(&self) -> BgColorDisplay<'_, BrightCyan, Self>
fn on_bright_cyan(&self) -> BgColorDisplay<'_, BrightCyan, Self>
§fn bright_white(&self) -> FgColorDisplay<'_, BrightWhite, Self>
fn bright_white(&self) -> FgColorDisplay<'_, BrightWhite, Self>
§fn on_bright_white(&self) -> BgColorDisplay<'_, BrightWhite, Self>
fn on_bright_white(&self) -> BgColorDisplay<'_, BrightWhite, Self>
§fn blink_fast(&self) -> BlinkFastDisplay<'_, Self>
fn blink_fast(&self) -> BlinkFastDisplay<'_, Self>
§fn strikethrough(&self) -> StrikeThroughDisplay<'_, Self>
fn strikethrough(&self) -> StrikeThroughDisplay<'_, Self>
§fn color<Color>(&self, color: Color) -> FgDynColorDisplay<'_, Color, Self>where
Color: DynColor,
fn color<Color>(&self, color: Color) -> FgDynColorDisplay<'_, Color, Self>where
Color: DynColor,
OwoColorize::fg
or
a color-specific method, such as OwoColorize::green
, Read more§fn on_color<Color>(&self, color: Color) -> BgDynColorDisplay<'_, Color, Self>where
Color: DynColor,
fn on_color<Color>(&self, color: Color) -> BgDynColorDisplay<'_, Color, Self>where
Color: DynColor,
OwoColorize::bg
or
a color-specific method, such as OwoColorize::on_yellow
, Read more§fn fg_rgb<const R: u8, const G: u8, const B: u8>(
&self,
) -> FgColorDisplay<'_, CustomColor<R, G, B>, Self>
fn fg_rgb<const R: u8, const G: u8, const B: u8>( &self, ) -> FgColorDisplay<'_, CustomColor<R, G, B>, Self>
§fn bg_rgb<const R: u8, const G: u8, const B: u8>(
&self,
) -> BgColorDisplay<'_, CustomColor<R, G, B>, Self>
fn bg_rgb<const R: u8, const G: u8, const B: u8>( &self, ) -> BgColorDisplay<'_, CustomColor<R, G, B>, Self>
§fn truecolor(&self, r: u8, g: u8, b: u8) -> FgDynColorDisplay<'_, Rgb, Self>
fn truecolor(&self, r: u8, g: u8, b: u8) -> FgDynColorDisplay<'_, Rgb, Self>
§fn on_truecolor(&self, r: u8, g: u8, b: u8) -> BgDynColorDisplay<'_, Rgb, Self>
fn on_truecolor(&self, r: u8, g: u8, b: u8) -> BgDynColorDisplay<'_, Rgb, Self>
§impl<T> Pipe for Twhere
T: ?Sized,
impl<T> Pipe for Twhere
T: ?Sized,
§fn pipe<R>(self, func: impl FnOnce(Self) -> R) -> Rwhere
Self: Sized,
fn pipe<R>(self, func: impl FnOnce(Self) -> R) -> Rwhere
Self: Sized,
§fn pipe_ref<'a, R>(&'a self, func: impl FnOnce(&'a Self) -> R) -> Rwhere
R: 'a,
fn pipe_ref<'a, R>(&'a self, func: impl FnOnce(&'a Self) -> R) -> Rwhere
R: 'a,
self
and passes that borrow into the pipe function. Read more§fn pipe_ref_mut<'a, R>(&'a mut self, func: impl FnOnce(&'a mut Self) -> R) -> Rwhere
R: 'a,
fn pipe_ref_mut<'a, R>(&'a mut self, func: impl FnOnce(&'a mut Self) -> R) -> Rwhere
R: 'a,
self
and passes that borrow into the pipe function. Read more§fn pipe_borrow<'a, B, R>(&'a self, func: impl FnOnce(&'a B) -> R) -> R
fn pipe_borrow<'a, B, R>(&'a self, func: impl FnOnce(&'a B) -> R) -> R
§fn pipe_borrow_mut<'a, B, R>(
&'a mut self,
func: impl FnOnce(&'a mut B) -> R,
) -> R
fn pipe_borrow_mut<'a, B, R>( &'a mut self, func: impl FnOnce(&'a mut B) -> R, ) -> R
§fn pipe_as_ref<'a, U, R>(&'a self, func: impl FnOnce(&'a U) -> R) -> R
fn pipe_as_ref<'a, U, R>(&'a self, func: impl FnOnce(&'a U) -> R) -> R
self
, then passes self.as_ref()
into the pipe function.§fn pipe_as_mut<'a, U, R>(&'a mut self, func: impl FnOnce(&'a mut U) -> R) -> R
fn pipe_as_mut<'a, U, R>(&'a mut self, func: impl FnOnce(&'a mut U) -> R) -> R
self
, then passes self.as_mut()
into the pipe
function.§fn pipe_deref<'a, T, R>(&'a self, func: impl FnOnce(&'a T) -> R) -> R
fn pipe_deref<'a, T, R>(&'a self, func: impl FnOnce(&'a T) -> R) -> R
self
, then passes self.deref()
into the pipe function.§impl<T> Pointable for T
impl<T> Pointable for T
§impl<T> Tap for T
impl<T> Tap for T
§fn tap_borrow<B>(self, func: impl FnOnce(&B)) -> Self
fn tap_borrow<B>(self, func: impl FnOnce(&B)) -> Self
Borrow<B>
of a value. Read more§fn tap_borrow_mut<B>(self, func: impl FnOnce(&mut B)) -> Self
fn tap_borrow_mut<B>(self, func: impl FnOnce(&mut B)) -> Self
BorrowMut<B>
of a value. Read more§fn tap_ref<R>(self, func: impl FnOnce(&R)) -> Self
fn tap_ref<R>(self, func: impl FnOnce(&R)) -> Self
AsRef<R>
view of a value. Read more§fn tap_ref_mut<R>(self, func: impl FnOnce(&mut R)) -> Self
fn tap_ref_mut<R>(self, func: impl FnOnce(&mut R)) -> Self
AsMut<R>
view of a value. Read more§fn tap_deref<T>(self, func: impl FnOnce(&T)) -> Self
fn tap_deref<T>(self, func: impl FnOnce(&T)) -> Self
Deref::Target
of a value. Read more§fn tap_deref_mut<T>(self, func: impl FnOnce(&mut T)) -> Self
fn tap_deref_mut<T>(self, func: impl FnOnce(&mut T)) -> Self
Deref::Target
of a value. Read more§fn tap_dbg(self, func: impl FnOnce(&Self)) -> Self
fn tap_dbg(self, func: impl FnOnce(&Self)) -> Self
.tap()
only in debug builds, and is erased in release builds.§fn tap_mut_dbg(self, func: impl FnOnce(&mut Self)) -> Self
fn tap_mut_dbg(self, func: impl FnOnce(&mut Self)) -> Self
.tap_mut()
only in debug builds, and is erased in release
builds.§fn tap_borrow_dbg<B>(self, func: impl FnOnce(&B)) -> Self
fn tap_borrow_dbg<B>(self, func: impl FnOnce(&B)) -> Self
.tap_borrow()
only in debug builds, and is erased in release
builds.§fn tap_borrow_mut_dbg<B>(self, func: impl FnOnce(&mut B)) -> Self
fn tap_borrow_mut_dbg<B>(self, func: impl FnOnce(&mut B)) -> Self
.tap_borrow_mut()
only in debug builds, and is erased in release
builds.§fn tap_ref_dbg<R>(self, func: impl FnOnce(&R)) -> Self
fn tap_ref_dbg<R>(self, func: impl FnOnce(&R)) -> Self
.tap_ref()
only in debug builds, and is erased in release
builds.§fn tap_ref_mut_dbg<R>(self, func: impl FnOnce(&mut R)) -> Self
fn tap_ref_mut_dbg<R>(self, func: impl FnOnce(&mut R)) -> Self
.tap_ref_mut()
only in debug builds, and is erased in release
builds.§fn tap_deref_dbg<T>(self, func: impl FnOnce(&T)) -> Self
fn tap_deref_dbg<T>(self, func: impl FnOnce(&T)) -> Self
.tap_deref()
only in debug builds, and is erased in release
builds.