Rollup merge of #78423 - tgnottingham:caching_source_map_bounds_check, r=oli-obk

rustc_span: improve bounds checks in byte_pos_to_line_and_col

The effect of this change is to consider edge-case spans that start or
end at the position one past the end of a file to be valid during span
hashing and encoding. This change means that these spans will be
preserved across incremental compilation sessions when they are part of
a serialized query result, instead of causing the dummy span to be used.

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 {