# Struct rustc_metadata::rmeta::SpanTag

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    struct SpanTag(u8);

Expand description

A span tag byte encodes a bunch of data, so that we can cut out a few extra bytes from span encodings (which are very common, for example, libcore has ~650,000 unique spans and over 1.1 million references to prior-written spans).

The byte format is split into several parts:

[ a a a a a c d d ]

`a` bits represent the span length. We have 5 bits, so we can store lengths up to 30 inline, with an all-1s pattern representing that the length is stored separately.

`c` represents whether the span context is zero (and then it is not stored as a separate varint) for direct span encodings, and whether the offset is absolute or relative otherwise (zero for absolute).

d bits represent the kind of span we are storing (local, foreign, partial, indirect).

## Tuple Fields§

§`0: u8`

## Implementations§

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### impl SpanTag

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#### fn new(kind: SpanKind, context: SyntaxContext, length: usize) -> SpanTag

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#### fn indirect(relative: bool) -> SpanTag

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#### fn kind(self) -> SpanKind

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#### fn is_relative_offset(self) -> bool

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#### fn context(self) -> Option<SyntaxContext>

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#### fn length(self) -> Option<BytePos>

## Trait Implementations§

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### impl Clone for SpanTag

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#### fn clone(&self) -> SpanTag

Returns a copy of the value. Read more
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#### fn clone_from(&mut self, source: &Self)

Performs copy-assignment from `source`. Read more
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### impl<__D: SpanDecoder> Decodable<__D> for SpanTag

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#### fn decode(__decoder: &mut __D) -> Self

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### impl<__E: SpanEncoder> Encodable<__E> for SpanTag

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#### fn encode(&self, __encoder: &mut __E)

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### impl Copy for SpanTag

## Auto Trait Implementations§

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### impl RefUnwindSafe for SpanTag

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### impl Send for SpanTag

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### impl Sync for SpanTag

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### impl Unpin for SpanTag

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### impl UnwindSafe for SpanTag

## Blanket Implementations§

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### impl<T> Any for T
where T: 'static + ?Sized,

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
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### impl<T> Borrow<T> for T
where T: ?Sized,

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#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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### impl<T> BorrowMut<T> for T
where T: ?Sized,

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#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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### impl<T> From<T> for T

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#### fn from(t: T) -> T

Returns the argument unchanged.

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### impl<T, U> Into<U> for T
where U: From<T>,

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#### fn into(self) -> U

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

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### impl<T> ToOwned for T
where T: Clone,

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#### type Owned = T

The resulting type after obtaining ownership.
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#### fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
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#### fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more
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### impl<T, U> TryFrom<U> for T
where U: Into<T>,

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#### type Error = Infallible

The type returned in the event of a conversion error.
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#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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### impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

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#### type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.

## Layout§

Note: Most layout information is completely unstable and may even differ between compilations. The only exception is types with certain `repr(...)` attributes. Please see the Rust Reference's “Type Layout” chapter for details on type layout guarantees.

Size: 1 byte