Crate span_lang 

span-lang

The source-position substrate for language tooling. It defines the small, copyable coordinate types that a lexer, a parser, and a diagnostic renderer all share: a BytePos (a byte offset), a Span (a half-open byte range), a resolved LineCol, and the LineIndex that maps between them — correctly over UTF-8, across both \n and \r\n line endings.

It owns positions and nothing else. Loading source text is the job of a separate layer; rendering an error that points at a span is another. Keeping this crate to coordinates alone is what lets every layer above it depend on it without pulling in I/O or rendering.

Design

• A BytePos is a 32-bit byte offset, so a single source is addressable up to 4 GiB — the same envelope established compiler front-ends use.
• A Span is two packed offsets (eight bytes), Copy, with the invariant that start <= end. An empty span (start == end) is a legal zero-width point.
• A LineIndex is built once per source and answers byte → LineCol in O(log lines) (a binary search over line starts), and the inverse, without allocating on the lookup path.

Quickstart

use span_lang::{LineIndex, Span};

let src = "fn main() {\n    work();\n}\n";

// A span is a half-open byte range; merging covers both inputs.
let a = Span::new(16, 20);
let b = Span::new(18, 22);
assert_eq!(a.merge(b), Span::new(16, 22));

// Resolve a byte offset to a human (line, column) coordinate.
let index = LineIndex::new(src);
let lc = index.line_col(a.start());
assert_eq!((lc.line, lc.col), (2, 5));

// And back again — the mapping round-trips.
assert_eq!(index.offset(lc), Some(a.start()));

Structs

BytePos

A zero-based byte offset into a single source buffer.

LineCol

A resolved human coordinate: a 1-based line and a 1-based column.

LineIndex

An index over a single source string that maps byte offsets to line/column coordinates and back.

Span

A half-open byte range start..end into a single source.

Spanned

A value together with the source Span it was parsed from.