Struct Span

pub struct Span { /* private fields */ }

A compressed span.

Whereas SpanData is 12 bytes, which is a bit too big to stick everywhere, Span is a form that only takes up 8 bytes, with less space for the length and context. The vast majority (99.9%+) of SpanData instances will fit within those 8 bytes; any SpanData whose fields don’t fit into a Span are stored in a separate interner table, and the Span will index into that table. Interning is rare enough that the cost is low, but common enough that the code is exercised regularly.

An earlier version of this code used only 4 bytes for Span, but that was slower because only 80–90% of spans could be stored inline (even less in very large crates) and so the interner was used a lot more.

Inline (compressed) format:

• span.base_or_index == span_data.lo
• span.len_or_tag == len == span_data.hi - span_data.lo (must be <= MAX_LEN)
• span.ctxt == span_data.ctxt (must be <= MAX_CTXT)

Interned format:

• span.base_or_index == index (indexes into the interner table)
• span.len_or_tag == LEN_TAG (high bit set, all other bits are zero)
• span.ctxt == 0

The inline form uses 0 for the tag value (rather than 1) so that we don’t need to mask out the tag bit when getting the length, and so that the dummy span can be all zeroes.

Notes about the choice of field sizes:

• base is 32 bits in both Span and SpanData, which means that base values never cause interning. The number of bits needed for base depends on the crate size. 32 bits allows up to 4 GiB of code in a crate.
• len is 15 bits in Span (a u16, minus 1 bit for the tag) and 32 bits in SpanData, which means that large len values will cause interning. The number of bits needed for len does not depend on the crate size. The most common numbers of bits for len are from 0 to 7, with a peak usually at 3 or 4, and then it drops off quickly from 8 onwards. 15 bits is enough for 99.99%+ of cases, but larger values (sometimes 20+ bits) might occur dozens of times in a typical crate.
• ctxt is 16 bits in Span and 32 bits in SpanData, which means that large ctxt values will cause interning. The number of bits needed for ctxt values depend partly on the crate size and partly on the form of the code. No crates in rustc-perf need more than 15 bits for ctxt, but larger crates might need more than 16 bits.

In order to reliably use parented spans in incremental compilation, the dependency to the parent definition’s span. This is performed using the callback SPAN_TRACK to access the query engine.

Implementations

impl Span

pub fn new(lo: BytePos, hi: BytePos) -> Self

pub fn data(self) -> SpanData

pub fn data_untracked(self) -> SpanData

Internal function to translate between an encoded span and the expanded representation. This function must not be used outside the incremental engine.

impl Span

pub fn lo(self) -> BytePos

pub fn with_lo(self, lo: BytePos) -> Span

pub fn hi(self) -> BytePos

pub fn with_hi(self, hi: BytePos) -> Span

pub fn is_dummy(self) -> bool

Returns true if this is a dummy span with any hygienic context.

pub fn shrink_to_lo(self) -> Span

Returns a new span representing an empty span at the beginning of this span.

pub fn shrink_to_hi(self) -> Span

Returns a new span representing an empty span at the end of this span.

pub fn is_empty(self) -> bool

Returns true if hi == lo.

pub fn substitute_dummy(self, other: Span) -> Span

Returns self if self is not the dummy span, and other otherwise.

pub fn contains(self, other: Span) -> bool

Returns true if self fully encloses other.

pub fn overlaps(self, other: Span) -> bool

Returns true if self touches other.

pub fn source_equal(self, other: Span) -> bool

Returns true if the spans are equal with regards to the source text.

Use this instead of == when either span could be generated code, and you only care that they point to the same bytes of source text.

pub fn trim_start(self, other: Span) -> Option<Span>

Returns Some(span), where the start is trimmed by the end of other.

pub fn to(self, end: Span) -> Span

Returns a Span that would enclose both self and end.

    ____             ___
    self lorem ipsum end
    ^^^^^^^^^^^^^^^^^^^^

pub fn between(self, end: Span) -> Span

Returns a Span between the end of self to the beginning of end.

    ____             ___
    self lorem ipsum end
        ^^^^^^^^^^^^^

pub fn until(self, end: Span) -> Span

Returns a Span from the beginning of self until the beginning of end.

    ____             ___
    self lorem ipsum end
    ^^^^^^^^^^^^^^^^^

pub fn from_inner(self, inner: InnerSpan) -> Span

Trait Implementations

impl Clone for Span

fn clone(&self) -> Span

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for Span

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for Span

fn default() -> Self

Returns the “default value” for a type.

impl Hash for Span

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl Ord for Span

fn cmp(&self, rhs: &Self) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

where Self: Sized,

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

where Self: Sized,

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

where Self: Sized,

Restrict a value to a certain interval.

impl PartialEq for Span

fn eq(&self, other: &Span) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl PartialOrd for Span

fn partial_cmp(&self, rhs: &Self) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &Rhs) -> bool

Tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Rhs) -> bool

Tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &Rhs) -> bool

Tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Rhs) -> bool

Tests greater than or equal to (for self and other) and is used by the >= operator.

impl Copy for Span

impl Eq for Span

impl StructuralPartialEq for Span