diff --git a/compiler/rustc_span/src/lib.rs b/compiler/rustc_span/src/lib.rs
index f6ae748560750..ac7c959302562 100644
--- a/compiler/rustc_span/src/lib.rs
+++ b/compiler/rustc_span/src/lib.rs
@@ -1173,17 +1173,7 @@ impl Span {
         }
     }
 
-    /// Returns a `Span` that would enclose both `self` and `end`.
-    ///
-    /// Note that this can also be used to extend the span "backwards":
-    /// `start.to(end)` and `end.to(start)` return the same `Span`.
-    ///
-    /// ```text
-    ///     ____             ___
-    ///     self lorem ipsum end
-    ///     ^^^^^^^^^^^^^^^^^^^^
-    /// ```
-    pub fn to(self, end: Span) -> Span {
+    fn to_general(self, end: Span) -> Span {
         match Span::prepare_to_combine(self, end) {
             Ok((from, to, parent)) => {
                 Span::new(cmp::min(from.lo, to.lo), cmp::max(from.hi, to.hi), from.ctxt, parent)
diff --git a/compiler/rustc_span/src/span_encoding.rs b/compiler/rustc_span/src/span_encoding.rs
index 64280d5ce4b5e..d6a7653d1c5f7 100644
--- a/compiler/rustc_span/src/span_encoding.rs
+++ b/compiler/rustc_span/src/span_encoding.rs
@@ -281,6 +281,36 @@ impl Span {
         }
     }
 
+    /// Returns a `Span` that would enclose both `self` and `end`.
+    ///
+    /// Note that this can also be used to extend the span "backwards":
+    /// `start.to(end)` and `end.to(start)` return the same `Span`.
+    ///
+    /// ```text
+    ///     ____             ___
+    ///     self lorem ipsum end
+    ///     ^^^^^^^^^^^^^^^^^^^^
+    /// ```
+    #[inline]
+    pub fn to(self, end: Span) -> Span {
+        // Inline-context spans have no parent dependency to record.
+        if u32::from(self.len_with_tag_or_marker) <= MAX_LEN
+            && u32::from(end.len_with_tag_or_marker) <= MAX_LEN
+            && self.ctxt_or_parent_or_marker == end.ctxt_or_parent_or_marker
+        {
+            let lo = self.lo_or_index.min(end.lo_or_index);
+            let hi = self
+                .lo_or_index
+                .debug_strict_add(u32::from(self.len_with_tag_or_marker))
+                .max(end.lo_or_index.debug_strict_add(u32::from(end.len_with_tag_or_marker)));
+            let len = hi - lo;
+            if len <= MAX_LEN {
+                return InlineCtxt::span(lo, len as u16, self.ctxt_or_parent_or_marker);
+            }
+        }
+        self.to_general(end)
+    }
+
     #[inline]
     pub fn data(self) -> SpanData {
         let data = self.data_untracked();
diff --git a/compiler/rustc_span/src/tests.rs b/compiler/rustc_span/src/tests.rs
index 64c40e6116250..1ca87b6a88700 100644
--- a/compiler/rustc_span/src/tests.rs
+++ b/compiler/rustc_span/src/tests.rs
@@ -1,5 +1,40 @@
 use super::*;
 
+#[test]
+fn test_span_union_encoding_boundaries() {
+    create_default_session_globals_then(|| {
+        let ranges = [
+            (0, 0),
+            (3, 10),
+            (8, 14),
+            (40_000, 40_001),
+            (1, 32_766),
+            (1, 32_767),
+            (1, 32_768),
+            (u32::MAX - 10, u32::MAX),
+        ];
+        for ctxt in [0, 7, 65_534, 65_535, 70_000] {
+            let ctxt = SyntaxContext::from_u32(ctxt);
+            for parent in [None, Some(crate::def_id::CRATE_DEF_ID)] {
+                for (lo_a, hi_a) in ranges {
+                    for (lo_b, hi_b) in ranges {
+                        let a = Span::new(BytePos(lo_a), BytePos(hi_a), ctxt, parent);
+                        let b = Span::new(BytePos(lo_b), BytePos(hi_b), ctxt, parent);
+                        let expected = Span::new(
+                            BytePos(lo_a.min(lo_b)),
+                            BytePos(hi_a.max(hi_b)),
+                            ctxt,
+                            parent,
+                        );
+                        assert_eq!(a.to(b), expected);
+                        assert_eq!(b.to(a), expected);
+                    }
+                }
+            }
+        }
+    });
+}
+
 #[test]
 fn test_lookup_line() {
     let source = "abcdefghijklm\nabcdefghij\n...".to_owned();