Auto merge of #97575 - nnethercote:lazify-SourceFile-lines, r=Mark-Simulacrum

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 `RefCell` 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.

r? `@Mark-Simulacrum`

1 files changed, 1 insertions, 1 deletions

diff --git a/clippy_utils/src/lib.rs b/clippy_utils/src/lib.rs
index adb37cc9d75..833f8cde63a 100644
--- a/clippy_utils/src/lib.rs
+++ b/clippy_utils/src/lib.rs
@@ -1149,7 +1149,7 @@ fn line_span<T: LintContext>(cx: &T, span: Span) -> Span {
     let span = original_sp(span, DUMMY_SP);
     let source_map_and_line = cx.sess().source_map().lookup_line(span.lo()).unwrap();
     let line_no = source_map_and_line.line;
-    let line_start = source_map_and_line.sf.lines[line_no];
+    let line_start = source_map_and_line.sf.lines(|lines| lines[line_no]);
     span.with_lo(line_start)
 }