Move some test-only code to test files

This also relaxes the bounds on some structs and moves them to the impl
block instead.

compiler/rustc_span/src/source_map/tests.rs

@@ -10,6 +10,50 @@ fn init_source_map() -> SourceMap {
     sm
 }
 
+impl SourceMap {
+    /// 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,
+    ///
+    ///    * the syntax contexts of both spans much match,
+    ///    * the LHS span needs to end on the same line the RHS span begins,
+    ///    * the LHS span must start at or before the RHS span.
+    fn merge_spans(&self, sp_lhs: Span, sp_rhs: Span) -> Option<Span> {
+        // Ensure we're at the same expansion ID.
+        if sp_lhs.ctxt() != sp_rhs.ctxt() {
+            return None;
+        }
+
+        let lhs_end = match self.lookup_line(sp_lhs.hi()) {
+            Ok(x) => x,
+            Err(_) => return None,
+        };
+        let rhs_begin = match self.lookup_line(sp_rhs.lo()) {
+            Ok(x) => x,
+            Err(_) => return None,
+        };
+
+        // If we must cross lines to merge, don't merge.
+        if lhs_end.line != rhs_begin.line {
+            return None;
+        }
+
+        // Ensure these follow the expected order and that we don't overlap.
+        if (sp_lhs.lo() <= sp_rhs.lo()) && (sp_lhs.hi() <= sp_rhs.lo()) {
+            Some(sp_lhs.to(sp_rhs))
+        } else {
+            None
+        }
+    }
+
+    /// Converts an absolute `BytePos` to a `CharPos` relative to the `SourceFile`.
+    fn bytepos_to_file_charpos(&self, bpos: BytePos) -> CharPos {
+        let idx = self.lookup_source_file_idx(bpos);
+        let sf = &(*self.files.borrow().source_files)[idx];
+        sf.bytepos_to_file_charpos(bpos)
+    }
+}
+
 /// Tests `lookup_byte_offset`.
 #[test]
 fn t3() {