Stabilize all string functions working with search patterns around a new generic API that provides a unified way to define and use those patterns.


Right now, string slices define a couple of methods for string manipulation that work with user provided values that act as search patterns. For example, split() takes an type implementing CharEq to split the slice at all codepoints that match that predicate.

Among these methods, the notion of what exactly is being used as a search pattern varies inconsistently: Many work with the generic CharEq, which only looks at a single codepoint at a time; and some work with char or &str directly, sometimes duplicating a method to provide operations for both.

This presents a couple of issues:

  • The API is inconsistent.
  • The API duplicates similar operations on different types. (contains vs contains_char)
  • The API does not provide all operations for all types. (For example, no rsplit for &str patterns)
  • The API is not extensible, eg to allow splitting at regex matches.
  • The API offers no way to explicitly decide between different search algorithms for the same pattern, for example to use Boyer-Moore string searching.

At the moment, the full set of relevant string methods roughly looks like this:

pub trait StrExt for ?Sized {
    fn contains(&self, needle: &str) -> bool;
    fn contains_char(&self, needle: char) -> bool;

    fn split<Sep: CharEq>(&self, sep: Sep) -> CharSplits<Sep>;
    fn splitn<Sep: CharEq>(&self, sep: Sep, count: uint) -> CharSplitsN<Sep>;
    fn rsplitn<Sep: CharEq>(&self, sep: Sep, count: uint) -> CharSplitsN<Sep>;
    fn split_terminator<Sep: CharEq>(&self, sep: Sep) -> CharSplits<Sep>;
    fn split_str<'a>(&'a self, &'a str) -> StrSplits<'a>;

    fn match_indices<'a>(&'a self, sep: &'a str) -> MatchIndices<'a>;

    fn starts_with(&self, needle: &str) -> bool;
    fn ends_with(&self, needle: &str) -> bool;

    fn trim_chars<C: CharEq>(&self, to_trim: C) -> &'a str;
    fn trim_left_chars<C: CharEq>(&self, to_trim: C) -> &'a str;
    fn trim_right_chars<C: CharEq>(&self, to_trim: C) -> &'a str;

    fn find<C: CharEq>(&self, search: C) -> Option<uint>;
    fn rfind<C: CharEq>(&self, search: C) -> Option<uint>;
    fn find_str(&self, &str) -> Option<uint>;

    // ...

This RFC proposes to fix those issues by providing a unified Pattern trait that all “string pattern” types would implement, and that would be used by the string API exclusively.

This fixes the duplication, consistency, and extensibility problems, and also allows to define newtype wrappers for the same pattern types that use different or specific search implementations.

As an additional design goal, the new abstractions should also not pose a problem for optimization - like for iterators, a concrete instance should produce similar machine code to a hardcoded optimized loop written in C.

Detailed design

New traits

First, new traits will be added to the str module in the std library:

trait Pattern<'a> {
    type Searcher: Searcher<'a>;
    fn into_matcher(self, haystack: &'a str) -> Self::Searcher;

    fn is_contained_in(self, haystack: &'a str) -> bool { /* default*/ }
    fn match_starts_at(self, haystack: &'a str, idx: usize) -> bool { /* default*/ }
    fn match_ends_at(self, haystack: &'a str, idx: usize) -> bool
        where Self::Searcher: ReverseSearcher<'a> { /* default*/ }

A Pattern represents a builder for an associated type implementing a family of Searcher traits (see below), and will be implemented by all types that represent string patterns, which includes:

  • &str
  • char, and everything else implementing CharEq
  • Third party types like &Regex or Ascii
  • Alternative algorithm wrappers like struct BoyerMoore(&str)
impl<'a>     Pattern<'a> for char       { /* ... */ }
impl<'a, 'b> Pattern<'a> for &'b str    { /* ... */ }

impl<'a, 'b> Pattern<'a> for &'b [char] { /* ... */ }
impl<'a, F>  Pattern<'a> for F where F: FnMut(char) -> bool { /* ... */ }

impl<'a, 'b> Pattern<'a> for &'b Regex  { /* ... */ }

The lifetime parameter on Pattern exists in order to allow threading the lifetime of the haystack (the string to be searched through) through the API, and is a workaround for not having associated higher kinded types yet.

Consumers of this API can then call into_searcher() on the pattern to convert it into a type implementing a family of Searcher traits:

pub enum SearchStep {
    Match(usize, usize),
    Reject(usize, usize),
pub unsafe trait Searcher<'a> {
    fn haystack(&self) -> &'a str;
    fn next(&mut self) -> SearchStep;

    fn next_match(&mut self) -> Option<(usize, usize)> { /* default*/ }
    fn next_reject(&mut self) -> Option<(usize, usize)> { /* default*/ }
pub unsafe trait ReverseSearcher<'a>: Searcher<'a> {
    fn next_back(&mut self) -> SearchStep;

    fn next_match_back(&mut self) -> Option<(usize, usize)> { /* default*/ }
    fn next_reject_back(&mut self) -> Option<(usize, usize)> { /* default*/ }
pub trait DoubleEndedSearcher<'a>: ReverseSearcher<'a> {}

The basic idea of a Searcher is to expose a interface for iterating through all connected string fragments of the haystack while classifying them as either a match, or a reject.

This happens in form of the returned enum value. A Match needs to contain the start and end indices of a complete non-overlapping match, while a Rejects may be emitted for arbitrary non-overlapping rejected parts of the string, as long as the start and end indices lie on valid utf8 boundaries.

Similar to iterators, depending on the concrete implementation a searcher can have additional capabilities that build on each other, which is why they will be defined in terms of a three-tier hierarchy:

  • Searcher<'a> is the basic trait that all searchers need to implement. It contains a next() method that returns the start and end indices of the next match or reject in the haystack, with the search beginning at the front (left) of the string. It also contains a haystack() getter for returning the actual haystack, which is the source of the 'a lifetime on the hierarchy. The reason for this getter being made part of the trait is twofold:
    • Every searcher needs to store some reference to the haystack anyway.
    • Users of this trait will need access to the haystack in order for the individual match results to be useful.
  • ReverseSearcher<'a> adds an next_back() method, for also allowing to efficiently search in reverse (starting from the right). However, the results are not required to be equal to the results of next() in reverse, (as would be the case for the DoubleEndedIterator trait) because that can not be efficiently guaranteed for all searchers. (For an example, see further below)
  • Instead DoubleEndedSearcher<'a> is provided as an marker trait for expressing that guarantee - If a searcher implements this trait, all results found from the left need to be equal to all results found from the right in reverse order.

As an important last detail, both Searcher and ReverseSearcher are marked as unsafe traits, even though the actual methods aren’t. This is because every implementation of these traits need to ensure that all indices returned by next() and next_back() lie on valid utf8 boundaries in the haystack.

Without that guarantee, every single match returned by a matcher would need to be double-checked for validity, which would be unnecessary and most likely unoptimizable work.

This is in contrast to the current hardcoded implementations, which can make use of such guarantees because the concrete types are known and all unsafe code needed for such optimizations is contained inside a single safe impl.

Given that most implementations of these traits will likely live in the std library anyway, and are thoroughly tested, marking these traits unsafe doesn’t seem like a huge burden to bear for good, optimizable performance.

The role of the additional default methods

Pattern, Searcher and ReverseSearcher each offer a few additional default methods that give better optimization opportunities.

Most consumers of the pattern API will use them to more narrowly constraint how they are looking for a pattern, which given an optimized implementantion, should lead to mostly optimal code being generated.

Example for the issue with double-ended searching

Let the haystack be the string "fooaaaaabar", and let the pattern be the string "aa".

Then a efficient, lazy implementation of the matcher searching from the left would find these matches:


However, the same algorithm searching from the right would find these matches:


This discrepancy can not be avoided without additional overhead or even allocations for caching in the reverse matcher, and thus “matching from the front” needs to be considered a different operation than “matching from the back”.

Why (uint, uint) instead of &str

Note: This section is a bit outdated now

It would be possible to define next and next_back to return &strs instead of (uint, uint) tuples.

A concrete searcher impl could then make use of unsafe code to construct such an slice cheaply, and by its very nature it is guaranteed to lie on utf8 boundaries, which would also allow not marking the traits as unsafe.

However, this approach has a couple of issues. For one, not every consumer of this API cares about only the matched slice itself:

  • The split() family of operations cares about the slices between matches.
  • Operations like match_indices() and find() need to actually return the offset to the start of the string as part of their definition.
  • The trim() and Xs_with() family of operations need to compare individual match offsets with each other and the start and end of the string.

In order for these use cases to work with a &str match, the concrete adapters would need to unsafely calculate the offset of a match &str to the start of the haystack &str.

But that in turn would require matcher implementors to only return actual sub slices into the haystack, and not random static string slices, as the API defined with &str would allow.

In order to resolve that issue, you’d have to do one of:

  • Add the uncheckable API constraint of only requiring true subslices, which would make the traits unsafe again, negating much of the benefit.
  • Return a more complex custom slice type that still contains the haystack offset. (This is listed as an alternative at the end of this RFC.)

In both cases, the API does not really improve significantly, so uint indices have been chosen as the “simple” default design.

New methods on StrExt

With the Pattern and Searcher traits defined and implemented, the actual str methods will be changed to make use of them:

pub trait StrExt for ?Sized {
    fn contains<'a, P>(&'a self, pat: P) -> bool where P: Pattern<'a>;

    fn split<'a, P>(&'a self, pat: P) -> Splits<P> where P: Pattern<'a>;
    fn rsplit<'a, P>(&'a self, pat: P) -> RSplits<P> where P: Pattern<'a>;
    fn split_terminator<'a, P>(&'a self, pat: P) -> TermSplits<P> where P: Pattern<'a>;
    fn rsplit_terminator<'a, P>(&'a self, pat: P) -> RTermSplits<P> where P: Pattern<'a>;
    fn splitn<'a, P>(&'a self, pat: P, n: uint) -> NSplits<P> where P: Pattern<'a>;
    fn rsplitn<'a, P>(&'a self, pat: P, n: uint) -> RNSplits<P> where P: Pattern<'a>;

    fn matches<'a, P>(&'a self, pat: P) -> Matches<P> where P: Pattern<'a>;
    fn rmatches<'a, P>(&'a self, pat: P) -> RMatches<P> where P: Pattern<'a>;
    fn match_indices<'a, P>(&'a self, pat: P) -> MatchIndices<P> where P: Pattern<'a>;
    fn rmatch_indices<'a, P>(&'a self, pat: P) -> RMatchIndices<P> where P: Pattern<'a>;

    fn starts_with<'a, P>(&'a self, pat: P) -> bool where P: Pattern<'a>;
    fn ends_with<'a, P>(&'a self, pat: P) -> bool where P: Pattern<'a>,
                                                        P::Searcher: ReverseSearcher<'a>;

    fn trim_matches<'a, P>(&'a self, pat: P) -> &'a str where P: Pattern<'a>,
                                                              P::Searcher: DoubleEndedSearcher<'a>;
    fn trim_left_matches<'a, P>(&'a self, pat: P) -> &'a str where P: Pattern<'a>;
    fn trim_right_matches<'a, P>(&'a self, pat: P) -> &'a str where P: Pattern<'a>,
                                                                    P::Searcher: ReverseSearcher<'a>;

    fn find<'a, P>(&'a self, pat: P) -> Option<uint> where P: Pattern<'a>;
    fn rfind<'a, P>(&'a self, pat: P) -> Option<uint> where P: Pattern<'a>,
                                                            P::Searcher: ReverseSearcher<'a>;

    // ...

These are mainly the same pattern-using methods as currently existing, only changed to uniformly use the new pattern API. The main differences are:

  • Duplicates like contains(char) and contains_str(&str) got merged into single generic methods.
  • CharEq-centric naming got changed to Pattern-centric naming by changing chars to matches in a few method names.
  • A Matches iterator has been added, that just returns the pattern matches as &str slices. Its uninteresting for patterns that look for a single string fragment, like the char and &str matcher, but useful for advanced patterns like predicates over codepoints, or regular expressions.
  • All operations that can work from both the front and the back consistently exist in two versions, the regular front version, and a r prefixed reverse versions. As explained above, this is because both represent different operations, and thus need to be handled as such. To be more precise, the two can not be abstracted over by providing a DoubleEndedIterator implementations, as the different results would break the requirement for double ended iterators to behave like a double ended queues where you just pop elements from both sides.

However, all iterators will still implement DoubleEndedIterator if the underlying matcher implements DoubleEndedSearcher, to keep the ability to do things like foo.split('a').rev().

Transition and deprecation plans

Most changes in this RFC can be made in such a way that code using the old hardcoded or CharEq-using methods will still compile, or give deprecation warning.

It would even be possible to generically implement Pattern for all CharEq types, making the transition more painless.

Long-term, post 1.0, it would be possible to define new sets of Pattern and Searcher without a lifetime parameter by making use of higher kinded types in order to simplify the string APIs. Eg, instead of fn starts_with<'a, P>(&'a self, pat: P) -> bool where P: Pattern<'a>; you’d have fn starts_with<P>(&self, pat: P) -> bool where P: Pattern;.

In order to not break backwards-compatibility, these can use the same generic-impl trick to forward to the old traits, which would roughly look like this:

unsafe trait NewPattern {
    type Searcher<'a> where Searcher: NewSearcher;

    fn into_matcher<'a>(self, s: &'a str) -> Self::Searcher<'a>;

unsafe impl<'a, P> Pattern<'a> for P where P: NewPattern {
    type Searcher = <Self as NewPattern>::Searcher<'a>;

    fn into_matcher(self, haystack: &'a str) -> Self::Searcher {
        <Self as NewPattern>::into_matcher(self, haystack)

unsafe trait NewSearcher for Self<'_> {
    fn haystack<'a>(self: &Self<'a>) -> &'a str;
    fn next_match<'a>(self: &mut Self<'a>) -> Option<(uint, uint)>;

unsafe impl<'a, M> Searcher<'a> for M<'a> where M: NewSearcher {
    fn haystack(&self) -> &'a str {
        <M as NewSearcher>::haystack(self)
    fn next_match(&mut self) -> Option<(uint, uint)> {
        <M as NewSearcher>::next_match(self)

Based on coherency experiments and assumptions about how future HKT will work, the author is assuming that the above implementation will work, but can not experimentally prove it.

Note: There might be still an issue with this upgrade path on the concrete iterator types. That is, Split<P> might turn into Split<'a, P>… Maybe require the 'a from the beginning?

In order for these new traits to fully replace the old ones without getting in their way, the old ones need to not be defined in a way that makes them “final”. That is, they should be defined in their own submodule, like str::pattern that can grow a sister module like str::newpattern, and not be exported in a global place like str or even the prelude (which would be unneeded anyway).


  • It complicates the whole machinery and API behind the implementation of matching on string patterns.
  • The no-HKT-lifetime-workaround wart might be to confusing for something as commonplace as the string API.
  • This add a few layers of generics, so compilation times and micro optimizations might suffer.


Note: This section is not updated to the new naming scheme

In general:

  • Keep status quo, with all issues listed at the beginning.
  • Stabilize on hardcoded variants, eg providing both contains and contains_str. Similar to status quo, but no CharEq and thus no generics.

Under the assumption that the lifetime parameter on the traits in this proposal is too big a wart to have in the release string API, there is an primary alternative that would avoid it:

  • Stabilize on a variant around CharEq - This would mean hardcoded _str methods, generic CharEq methods, and no extensibility to types like Regex, but has a upgrade path for later upgrading CharEq to a full-fledged, HKT-using Pattern API, by providing back-comp generic impls.

Next, there are alternatives that might make a positive difference in the authors opinion, but still have some negative trade-offs:

  • With the Matcher traits having the unsafe constraint of returning results unique to the current haystack already, they could just directly return a (*const u8, *const u8) pointing into it. This would allow a few more micro-optimizations, as now the matcher -> match -> final slice pipeline would no longer need to keep adding and subtracting the start address of the haystack for immediate results.
  • Extend Pattern into Pattern and ReversePattern, starting the forward-reverse split at the level of patterns directly. The two would still be in a inherits-from relationship like Matcher and ReverseSearcher, and be interchangeable if the later also implement DoubleEndedSearcher, but on the str API where clauses like where P: Pattern<'a>, P::Searcher: ReverseSearcher<'a> would turn into where P: ReversePattern<'a>.

Lastly, there are alternatives that don’t seem very favorable, but are listed for completeness sake:

  • Remove unsafe from the API by returning a special SubSlice<'a> type instead of (uint, uint) in each match, that wraps the haystack and the current match as a (*start, *match_start, *match_end, *end) pointer quad. It is unclear whether those two additional words per match end up being an issue after monomorphization, but two of them will be constant for the duration of the iteration, so changes are they won’t matter. The haystack() could also be removed that way, as each match already returns the haystack. However, this still prevents removal of the lifetime parameters without HKT.
  • Remove the lifetimes on Matcher and Pattern by requiring users of the API to store the haystack slice themselves, duplicating it in the in-memory representation. However, this still runs into HKT issues with the impl of Pattern.
  • Remove the lifetime parameter on Pattern and Matcher by making them fully unsafe API’s, and require implementations to unsafely transmuting back the lifetime of the haystack slice.
  • Remove unsafe from the API by not marking the Matcher traits as unsafe, requiring users of the API to explicitly check every match on validity in regard to utf8 boundaries.
  • Allow to opt-in the unsafe traits by providing parallel safe and unsafe Matcher traits or methods, with the one per default implemented in terms of the other.

Unresolved questions

  • Concrete performance is untested compared to the current situation.
  • Should the API split in regard to forward-reverse matching be as symmetrical as possible, or as minimal as possible? In the first case, iterators like Matches and RMatches could both implement DoubleEndedIterator if a DoubleEndedSearcher exists, in the latter only Matches would, with RMatches only providing the minimum to support reverse operation. A ruling in favor of symmetry would also speak for the ReversePattern alternative.

Additional extensions

A similar abstraction system could be implemented for String APIs, so that for example string.push("foo"), string.push('f'), string.push('f'.to_ascii()) all work by using something like a StringSource trait.

This would allow operations like s.replace(&regex!(...), "foo"), which would be a method generic over both the pattern matched and the string fragment it gets replaced with:

fn replace<P, S>(&mut self, pat: P, with: S) where P: Pattern, S: StringSource { /* ... */ }