Trait LifetimeFree

source

pub unsafe trait LifetimeFree: 'static { }

Expand description

A marker trait for types that do not contain any lifetime parameters.

Such types are safe to cast from non-static type parameters if their types are equal. This trait is used to determine safe specialization methods in TrySpecialize trait and safe constructor for Specialization. The LifetimeFree bound is stricter than 'static bound. All LifetimeFree types are also 'static, but the reverse is not true.

This trait is not automatically generated for all lifetime free types. Use Specialization::rev to specialize from a LifetimeFree type to an unconstrained type.

§Safety

When implementing this trait for a struct, enum or union type, you must ensure that the type does not contain any lifetime parameters. When implementing this trait for a type alias, you must ensure that the underlying struct, enum or union type does not contain any lifetime parameters.

Note, however, that the use of 'static values in type fields is allowed. So:

struct First<'a, T>( /* ... */ ) is not a lifetime free type.
type Second<T> = First<'static, T> is not a lifetime free type.
type Third = Second<u32> is not a lifetime free type, because the underlying type is still First<'static, T>.

Whereas for T: LifetimeFree the struct Fourth<T>(&'static str, Cow<'static, T>) and its aliases can be considered as lifetime free types, since the underlying struct has no lifetime parameters. It only has fields with lifetimes, which that is allowed. Therefore it is safe to impl LifetimeFree<T> for Fourth<T> where T: LifetimeFree {}.

Implementing this trait for types which uses lifetimes may result to undefined behavior.

§Examples

use try_specialize::LifetimeFree;

struct MarkerType;
struct U32Pair(u32, u32);
struct CustomVec<T> {
    ptr: *mut T,
    cap: usize,
    len: usize,
}
struct Data {
    // ...
}
enum Either<T1, T2> {
    Left(T1),
    Right(T2),
}

unsafe impl LifetimeFree for MarkerType {}
unsafe impl LifetimeFree for U32Pair {}
unsafe impl<T: LifetimeFree> LifetimeFree for CustomVec<T> {}
unsafe impl LifetimeFree for Data {}
unsafe impl<T1, T2> LifetimeFree for Either<T1, T2>
where
    T1: LifetimeFree,
    T2: LifetimeFree,
{
}

Safe examples with 'static used in fields:

use std::borrow::Cow;

use try_specialize::LifetimeFree;

// Original type must not implement `LifetimeFree` because it has lifetime parameters.
// However its wrapper without lifetime parameters can implement `LifetimeFree`.
struct WithLifetimes<'a, 'b, T, U>(&'a str, &'b [u8], T, U);

struct StaticStrWrapper(pub &'static str);
struct StaticRefWrapper<T: LifetimeFree>(pub &'static T);
struct StaticCowWrapper<T: LifetimeFree + ToOwned>(pub Cow<'static, T>);
struct WithLifetimesWrapper<T, U>(pub WithLifetimes<'static, 'static, T, U>);

unsafe impl LifetimeFree for StaticStrWrapper {}
unsafe impl<T: LifetimeFree> LifetimeFree for StaticRefWrapper<T> {}
unsafe impl<T: LifetimeFree + ToOwned> LifetimeFree for StaticCowWrapper<T> {}
unsafe impl<T: LifetimeFree, U: LifetimeFree> LifetimeFree for WithLifetimesWrapper<T, U> {}

Invalid usage examples with explicit or implicit 'static parameters:

use std::borrow::Cow;

use try_specialize::LifetimeFree;

struct WithLifetimes<'a, 'b, T, U>(&'a str, &'b [u8], T, U);

type StaticStrAlias = &'static str;
type StaticRefAlias<T> = &'static T;
type StaticCowAlias<T> = Cow<'static, T>;
type WithLifetimesAlias<T, U> = WithLifetimes<'static, 'static, T, U>;

// Do not implement use it!
// Any implementation below may result to undefined behavior!
UB!{ unsafe impl LifetimeFree for StaticStrAlias {} }
UB!{ unsafe impl<T: LifetimeFree> LifetimeFree for StaticRefAlias<T> {} }
UB!{ unsafe impl<T: LifetimeFree> LifetimeFree for StaticCowAlias<T> {} }
UB!{
    unsafe impl<T: LifetimeFree, U: LifetimeFree> LifetimeFree
        for WithLifetimes<'static, 'static, T, U> {}
}
UB!{
    unsafe impl<T: LifetimeFree, U: LifetimeFree> LifetimeFree
        for WithLifetimesAlias<T, U> {}
}

Trait LifetimeFreeCopy item path

§Safety

§Examples

Implementations on Foreign Types§

impl LifetimeFree for Ordering

impl LifetimeFree for Infallible

impl LifetimeFree for c_void

impl LifetimeFree for Alignment

impl LifetimeFree for IpAddr

impl LifetimeFree for SocketAddr

impl LifetimeFree for FpCategory

impl LifetimeFree for IntErrorKind

impl LifetimeFree for Ordering

impl LifetimeFree for BacktraceStatus

impl LifetimeFree for VarError

impl LifetimeFree for SeekFrom

impl LifetimeFree for ErrorKind

impl LifetimeFree for Shutdown

impl LifetimeFree for RecvTimeoutError

impl LifetimeFree for TryRecvError

impl LifetimeFree for bool

impl LifetimeFree for char

impl LifetimeFree for f32

impl LifetimeFree for f64

impl LifetimeFree for i8

impl LifetimeFree for i16

impl LifetimeFree for i32

impl LifetimeFree for i64

impl LifetimeFree for i128

impl LifetimeFree for isize

impl LifetimeFree for str

impl LifetimeFree for u8

impl LifetimeFree for u16

impl LifetimeFree for u32

impl LifetimeFree for u64

impl LifetimeFree for u128

impl LifetimeFree for ()

impl LifetimeFree for usize

impl LifetimeFree for TryReserveError

impl LifetimeFree for CString

impl LifetimeFree for FromUtf8Error

impl LifetimeFree for FromUtf16Error

impl LifetimeFree for String

impl LifetimeFree for Layout

impl LifetimeFree for LayoutError

impl LifetimeFree for TypeId

impl LifetimeFree for TryFromSliceError

impl LifetimeFree for EscapeDefault

impl LifetimeFree for BorrowError

impl LifetimeFree for BorrowMutError

impl LifetimeFree for CharTryFromError

impl LifetimeFree for ParseCharError

impl LifetimeFree for DecodeUtf16Error

impl LifetimeFree for EscapeDebug

impl LifetimeFree for EscapeDefault

impl LifetimeFree for EscapeUnicode

impl LifetimeFree for ToLowercase

impl LifetimeFree for ToUppercase

impl LifetimeFree for TryFromCharError

impl LifetimeFree for CpuidResult

impl LifetimeFree for __m128

impl LifetimeFree for __m128d

impl LifetimeFree for __m128i

impl LifetimeFree for __m256

impl LifetimeFree for __m256d

impl LifetimeFree for __m256i

impl LifetimeFree for __m512

impl LifetimeFree for __m512d

impl LifetimeFree for __m512i

impl LifetimeFree for CStr

impl LifetimeFree for Error

impl LifetimeFree for PhantomPinned

impl LifetimeFree for Ipv4Addr

impl LifetimeFree for Ipv6Addr

impl LifetimeFree for AddrParseError

impl LifetimeFree for SocketAddrV4

impl LifetimeFree for SocketAddrV6

impl LifetimeFree for ParseFloatError

impl LifetimeFree for ParseIntError

impl LifetimeFree for TryFromIntError

Trait LifetimeFree