Auto merge of #50997 - michaelwoerister:pre-analyze-filemaps, r=Mark-Simulacrum

 Make FileMap::{lines, multibyte_chars, non_narrow_chars} non-mutable.

This PR removes most of the interior mutability from `FileMap`, which should be beneficial, especially in a multithreaded setting. This is achieved by initializing the state in question when the filemap is constructed instead of during lexing. Hopefully this doesn't degrade performance.

cc @wesleywiser

2 files changed, 5 insertions, 21 deletions

diff --git a/src/libsyntax/parse/lexer/comments.rs b/src/libsyntax/parse/lexer/comments.rs
index 7da0d816d0f..3995a9b8689 100644
--- a/src/libsyntax/parse/lexer/comments.rs
+++ b/src/libsyntax/parse/lexer/comments.rs
@@ -240,9 +240,11 @@ fn read_block_comment(rdr: &mut StringReader,
     let mut lines: Vec<String> = Vec::new();
 
     // Count the number of chars since the start of the line by rescanning.
-    let mut src_index = rdr.src_index(rdr.filemap.line_begin_pos());
+    let mut src_index = rdr.src_index(rdr.filemap.line_begin_pos(rdr.pos));
     let end_src_index = rdr.src_index(rdr.pos);
-    assert!(src_index <= end_src_index);
+    assert!(src_index <= end_src_index,
+        "src_index={}, end_src_index={}, line_begin_pos={}",
+        src_index, end_src_index, rdr.filemap.line_begin_pos(rdr.pos).to_u32());
     let mut n = 0;
     while src_index < end_src_index {
         let c = char_at(&rdr.src, src_index);
diff --git a/src/libsyntax/parse/lexer/mod.rs b/src/libsyntax/parse/lexer/mod.rs
index c09cfd910d2..dcc71e78778 100644
--- a/src/libsyntax/parse/lexer/mod.rs
+++ b/src/libsyntax/parse/lexer/mod.rs
@@ -51,11 +51,7 @@ pub struct StringReader<'a> {
     pub ch: Option<char>,
     pub filemap: Lrc<syntax_pos::FileMap>,
     /// Stop reading src at this index.
-    end_src_index: usize,
-    /// Whether to record new-lines and multibyte chars in filemap.
-    /// This is only necessary the first time a filemap is lexed.
-    /// If part of a filemap is being re-lexed, this should be set to false.
-    save_new_lines_and_multibyte: bool,
+    pub end_src_index: usize,
     // cached:
     peek_tok: token::Token,
     peek_span: Span,
@@ -188,7 +184,6 @@ impl<'a> StringReader<'a> {
             ch: Some('\n'),
             filemap,
             end_src_index: src.len(),
-            save_new_lines_and_multibyte: true,
             // dummy values; not read
             peek_tok: token::Eof,
             peek_span: syntax_pos::DUMMY_SP,
@@ -225,7 +220,6 @@ impl<'a> StringReader<'a> {
         let mut sr = StringReader::new_raw_internal(sess, begin.fm, None);
 
         // Seek the lexer to the right byte range.
-        sr.save_new_lines_and_multibyte = false;
         sr.next_pos = span.lo();
         sr.end_src_index = sr.src_index(span.hi());
 
@@ -458,18 +452,6 @@ impl<'a> StringReader<'a> {
             let next_ch = char_at(&self.src, next_src_index);
             let next_ch_len = next_ch.len_utf8();
 
-            if self.ch.unwrap() == '\n' {
-                if self.save_new_lines_and_multibyte {
-                    self.filemap.next_line(self.next_pos);
-                }
-            }
-            if next_ch_len > 1 {
-                if self.save_new_lines_and_multibyte {
-                    self.filemap.record_multibyte_char(self.next_pos, next_ch_len);
-                }
-            }
-            self.filemap.record_width(self.next_pos, next_ch);
-
             self.ch = Some(next_ch);
             self.pos = self.next_pos;
             self.next_pos = self.next_pos + Pos::from_usize(next_ch_len);