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`

Author: bors

clippy_lints/src/undocumented_unsafe_blocks.rs

@@ -187,11 +187,13 @@ fn item_has_safety_comment(cx: &LateContext<'_>, item: &hir::Item<'_>) -> bool {
                 && Lrc::ptr_eq(&unsafe_line.sf, &comment_start_line.sf)
                 && let Some(src) = unsafe_line.sf.src.as_deref()
             {
-                comment_start_line.line < unsafe_line.line && text_has_safety_comment(
-                    src,
-                    &unsafe_line.sf.lines[comment_start_line.line + 1..=unsafe_line.line],
-                    unsafe_line.sf.start_pos.to_usize(),
-                )
+                unsafe_line.sf.lines(|lines| {
+                    comment_start_line.line < unsafe_line.line && text_has_safety_comment(
+                        src,
+                        &lines[comment_start_line.line + 1..=unsafe_line.line],
+                        unsafe_line.sf.start_pos.to_usize(),
+                    )
+                })
             } else {
                 // Problem getting source text. Pretend a comment was found.
                 true
@@ -249,11 +251,13 @@ fn span_from_macro_expansion_has_safety_comment(cx: &LateContext<'_>, span: Span
             && Lrc::ptr_eq(&unsafe_line.sf, &macro_line.sf)
             && let Some(src) = unsafe_line.sf.src.as_deref()
         {
-            macro_line.line < unsafe_line.line && text_has_safety_comment(
-                src,
-                &unsafe_line.sf.lines[macro_line.line + 1..=unsafe_line.line],
-                unsafe_line.sf.start_pos.to_usize(),
-            )
+            unsafe_line.sf.lines(|lines| {
+                macro_line.line < unsafe_line.line && text_has_safety_comment(
+                    src,
+                    &lines[macro_line.line + 1..=unsafe_line.line],
+                    unsafe_line.sf.start_pos.to_usize(),
+                )
+            })
         } else {
             // Problem getting source text. Pretend a comment was found.
             true
@@ -276,11 +280,13 @@ fn span_in_body_has_safety_comment(cx: &LateContext<'_>, span: Span) -> bool {
             // Get the text from the start of function body to the unsafe block.
             //     fn foo() { some_stuff; unsafe { stuff }; other_stuff; }
             //              ^-------------^
-            body_line.line < unsafe_line.line && text_has_safety_comment(
-                src,
-                &unsafe_line.sf.lines[body_line.line + 1..=unsafe_line.line],
-                unsafe_line.sf.start_pos.to_usize(),
-            )
+            unsafe_line.sf.lines(|lines| {
+                body_line.line < unsafe_line.line && text_has_safety_comment(
+                    src,
+                    &lines[body_line.line + 1..=unsafe_line.line],
+                    unsafe_line.sf.start_pos.to_usize(),
+                )
+            })
         } else {
             // Problem getting source text. Pretend a comment was found.
             true

clippy_utils/src/diagnostics.rs

@@ -283,10 +283,10 @@ pub fn span_lint_and_sugg_for_edges(
             {
                 let split_idx = MAX_SUGGESTION_HIGHLIGHT_LINES / 2;
                 let span_upper = sm.span_until_char(
-                    sp.with_hi(line_upper.sf.lines[line_upper.line + split_idx]),
+                    sp.with_hi(line_upper.sf.lines(|lines| lines[line_upper.line + split_idx])),
                     '\n',
                 );
-                let span_bottom = sp.with_lo(line_bottom.sf.lines[line_bottom.line - split_idx]);
+                let span_bottom = sp.with_lo(line_bottom.sf.lines(|lines| lines[line_bottom.line - split_idx]));
 
                 let sugg_lines_vec = sugg.lines().collect::<Vec<&str>>();
                 let sugg_upper = sugg_lines_vec[..split_idx].join("\n");

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)
 }