rustc/vendor/kstring

KString

Key String: Optimized for map keys.

Background

Considerations:

• Large maps
• Most keys live and drop without being used in any other way
• Most keys are relatively small (single to double digit bytes)
• Keys are immutable
• Allow zero-cost abstractions between structs and maps (e.g. no allocating when dealing with struct field names)

Ramifications:

• Inline small strings rather than going to the heap.
• Preserve &'static str across strings (KString), references (KStringRef), and lifetime abstractions (KStringCow) to avoid allocating for struct field names.
• Use Box<str> rather than String to use less memory.

Features

• max_inline: Instead of aligning the inline-string for performance (15 bytes + length on 64-bit), use the full width (22 bytes on 64-bit)
• arc: Instead of using Box<str>, use Arc<str>. Note: allocations are fast enough that this can actually slow things down for small enough strings.

Alternatives

• smol_str
  • Size of String
  • Always uses Arc instead of Box
  • Always inlines 22 bytes
  • Whitespace-only optimizations
• smartstring
  • Size of String
  • Allows mutability at the cost of relying on implementation details of String
  • Always inlines 23 bytes
• compact_str
  • Size of String
  • Always uses Arc instead of Box
  • Inlines 22-23 bytes, depending on implementation

License

Licensed under either of

• Apache License, Version 2.0, (http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (http://opensource.org/licenses/MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.