fix: Span.trim_remainder gave incorrect result if after was incom…

…plete subset of `self` (#147)

src/span/primitives.rs

@@ -95,14 +95,13 @@ impl<'src> Span<'src> {
     ///
     /// Note that the trailing remainder condition is not enforced.
     pub(crate) fn trim_remainder(self, after: Span<'src>) -> Span<'src> {
-        // Sanity check: If after is longer than source, we can't trim.
-        let rlen = after.len();
-        let slen = self.len();
+        let offset = (self.offset + self.len()).min(after.offset);
 
-        if rlen >= slen {
+        if offset <= self.offset {
+            // Invalid input: We'll respond with an empty slice.
             self.slice(0..0)
         } else {
-            let trim_len = slen - rlen;
+            let trim_len = offset - self.offset;
             self.slice(0..trim_len)
         }
     }

src/tests/span/primitives.rs

@@ -567,4 +567,21 @@ mod trim_remainder {
             }
         );
     }
+
+    #[test]
+    fn rem_is_incomplete_subset_of_source() {
+        let source = Span::new("abc\ndef\n");
+        let line1 = source.take_normalized_line();
+        let line2 = line1.after.take_line();
+
+        assert_eq!(
+            source.trim_remainder(line2.item),
+            TSpan {
+                data: "abc\n",
+                line: 1,
+                col: 1,
+                offset: 0
+            }
+        );
+    }
 }