rustc_span: improve bounds checks in byte_pos_to_line_and_col

compiler/rustc_span/src/caching_source_map_view.rs

@@ -1,13 +1,25 @@
 use crate::source_map::SourceMap;
 use crate::{BytePos, SourceFile};
 use rustc_data_structures::sync::Lrc;
+use std::ops::Range;
 
 #[derive(Clone)]
 struct CacheEntry {
     time_stamp: usize,
     line_number: usize,
-    line_start: BytePos,
-    line_end: BytePos,
+    // The line's byte position range in the `SourceMap`. This range will fail to contain a valid
+    // position in certain edge cases. Spans often start/end one past something, and when that
+    // something is the last character of a file (this can happen when a file doesn't end in a
+    // newline, for example), we'd still like for the position to be considered within the last
+    // line. However, it isn't according to the exclusive upper bound of this range. We cannot
+    // change the upper bound to be inclusive, because for most lines, the upper bound is the same
+    // as the lower bound of the next line, so there would be an ambiguity.
+    //
+    // Since the containment aspect of this range is only used to see whether or not the cache
+    // entry contains a position, the only ramification of the above is that we will get cache
+    // misses for these rare positions. A line lookup for the position via `SourceMap::lookup_line`
+    // after a cache miss will produce the last line number, as desired.
+    line: Range<BytePos>,
     file: Lrc<SourceFile>,
     file_index: usize,
 }
@@ -26,8 +38,7 @@ impl<'sm> CachingSourceMapView<'sm> {
         let entry = CacheEntry {
             time_stamp: 0,
             line_number: 0,
-            line_start: BytePos(0),
-            line_end: BytePos(0),
+            line: BytePos(0)..BytePos(0),
             file: first_file,
             file_index: 0,
         };
@@ -47,13 +58,13 @@ impl<'sm> CachingSourceMapView<'sm> {
 
         // Check if the position is in one of the cached lines
         for cache_entry in self.line_cache.iter_mut() {
-            if pos >= cache_entry.line_start && pos < cache_entry.line_end {
+            if cache_entry.line.contains(&pos) {
                 cache_entry.time_stamp = self.time_stamp;
 
                 return Some((
                     cache_entry.file.clone(),
                     cache_entry.line_number,
-                    pos - cache_entry.line_start,
+                    pos - cache_entry.line.start,
                 ));
             }
         }
@@ -69,13 +80,13 @@ impl<'sm> CachingSourceMapView<'sm> {
         let cache_entry = &mut self.line_cache[oldest];
 
         // If the entry doesn't point to the correct file, fix it up
-        if pos < cache_entry.file.start_pos || pos >= cache_entry.file.end_pos {
+        if !file_contains(&cache_entry.file, pos) {
             let file_valid;
             if self.source_map.files().len() > 0 {
                 let file_index = self.source_map.lookup_source_file_idx(pos);
                 let file = self.source_map.files()[file_index].clone();
 
-                if pos >= file.start_pos && pos < file.end_pos {
+                if file_contains(&file, pos) {
                     cache_entry.file = file;
                     cache_entry.file_index = file_index;
                     file_valid = true;
@@ -95,10 +106,19 @@ impl<'sm> CachingSourceMapView<'sm> {
         let line_bounds = cache_entry.file.line_bounds(line_index);
 
         cache_entry.line_number = line_index + 1;
-        cache_entry.line_start = line_bounds.0;
-        cache_entry.line_end = line_bounds.1;
+        cache_entry.line = line_bounds;
         cache_entry.time_stamp = self.time_stamp;
 
-        Some((cache_entry.file.clone(), cache_entry.line_number, pos - cache_entry.line_start))
+        Some((cache_entry.file.clone(), cache_entry.line_number, pos - cache_entry.line.start))
     }
 }
+
+#[inline]
+fn file_contains(file: &SourceFile, pos: BytePos) -> bool {
+    // `SourceMap::lookup_source_file_idx` and `SourceFile::contains` both consider the position
+    // one past the end of a file to belong to it. Normally, that's what we want. But for the
+    // purposes of converting a byte position to a line and column number, we can't come up with a
+    // line and column number if the file is empty, because an empty file doesn't contain any
+    // lines. So for our purposes, we don't consider empty files to contain any byte position.
+    file.contains(pos) && !file.is_empty()
+}

compiler/rustc_span/src/lib.rs

@@ -52,7 +52,7 @@ use std::cell::RefCell;
 use std::cmp::{self, Ordering};
 use std::fmt;
 use std::hash::Hash;
-use std::ops::{Add, Sub};
+use std::ops::{Add, Range, Sub};
 use std::path::{Path, PathBuf};
 use std::str::FromStr;
 
@@ -1426,24 +1426,33 @@ impl SourceFile {
         if line_index >= 0 { Some(line_index as usize) } else { None }
     }
 
-    pub fn line_bounds(&self, line_index: usize) -> (BytePos, BytePos) {
-        if self.start_pos == self.end_pos {
-            return (self.start_pos, self.end_pos);
+    pub fn line_bounds(&self, line_index: usize) -> Range<BytePos> {
+        if self.is_empty() {
+            return self.start_pos..self.end_pos;
         }
 
         assert!(line_index < self.lines.len());
         if line_index == (self.lines.len() - 1) {
-            (self.lines[line_index], self.end_pos)
+            self.lines[line_index]..self.end_pos
         } else {
-            (self.lines[line_index], self.lines[line_index + 1])
+            self.lines[line_index]..self.lines[line_index + 1]
         }
     }
 
+    /// Returns whether or not the file contains the given `SourceMap` byte
+    /// position. The position one past the end of the file is considered to be
+    /// contained by the file. This implies that files for which `is_empty`
+    /// returns true still contain one byte position according to this function.
     #[inline]
     pub fn contains(&self, byte_pos: BytePos) -> bool {
         byte_pos >= self.start_pos && byte_pos <= self.end_pos
     }
 
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.start_pos == self.end_pos
+    }
+
     /// Calculates the original byte position relative to the start of the file
     /// based on the given byte position.
     pub fn original_relative_byte_pos(&self, pos: BytePos) -> BytePos {