Adds two source span utility functions used in source-based coverage

compiler/rustc_span/src/lib.rs

@@ -400,6 +400,13 @@ impl Span {
         span.with_lo(span.hi)
     }
 
+    #[inline]
+    /// Returns true if hi == lo
+    pub fn is_empty(&self) -> bool {
+        let span = self.data();
+        span.hi == span.lo
+    }
+
     /// Returns `self` if `self` is not the dummy span, and `other` otherwise.
     pub fn substitute_dummy(self, other: Span) -> Span {
         if self.is_dummy() { other } else { self }

compiler/rustc_span/src/source_map.rs

@@ -487,6 +487,15 @@ impl SourceMap {
         }
     }
 
+    /// Returns a new `Span` covering the start and end `BytePos`s of the file containing the given
+    /// `pos`. This can be used to quickly determine if another `BytePos` or `Span` is from the same
+    /// file.
+    pub fn lookup_file_span(&self, pos: BytePos) -> Span {
+        let idx = self.lookup_source_file_idx(pos);
+        let SourceFile { start_pos, end_pos, .. } = *(*self.files.borrow().source_files)[idx];
+        Span::with_root_ctxt(start_pos, end_pos)
+    }
+
     /// Returns `Some(span)`, a union of the LHS and RHS span. The LHS must precede the RHS. If
     /// there are gaps between LHS and RHS, the resulting union will cross these gaps.
     /// For this to work,