Lazify `SourceFile::lines`.

`SourceFile::lines` is a big part of metadata. It's stored in a compressed form
(a difference list) to save disk space. Decoding it is a big fraction of
compile time for very small crates/programs.

This commit introduces a new type `SourceFileLines` which has a `Lines`
form and a `Diffs` form. The latter is used when the metadata is first
read, and it is only decoded into the `Lines` form when line data is
actually needed. This avoids the decoding cost for many files,
especially in `std`. It's a performance win of up to 15% for tiny
crates/programs where metadata decoding is a high part of compilation
costs.

A `Lock` is needed because the methods that access lines data (which can
trigger decoding) take `&self` rather than `&mut self`. To allow for this,
`SourceFile::lines` now takes a `FnMut` that operates on the lines slice rather
than returning the lines slice.

1 files changed, 1 insertions, 1 deletions

diff --git a/compiler/rustc_query_impl/src/on_disk_cache.rs b/compiler/rustc_query_impl/src/on_disk_cache.rs
index 8e4b3269402..5a915933dc5 100644
--- a/compiler/rustc_query_impl/src/on_disk_cache.rs
+++ b/compiler/rustc_query_impl/src/on_disk_cache.rs
@@ -713,7 +713,7 @@ impl<'a, 'tcx> Decodable<CacheDecoder<'a, 'tcx>> for Span {
         let len = BytePos::decode(decoder);
 
         let file_lo = decoder.file_index_to_file(file_lo_index);
-        let lo = file_lo.lines[line_lo - 1] + col_lo;
+        let lo = file_lo.lines(|lines| lines[line_lo - 1] + col_lo);
         let hi = lo + len;
 
         Span::new(lo, hi, ctxt, parent)