diff --git a/compiler/rustc_span/src/lib.rs b/compiler/rustc_span/src/lib.rs
index 65702f76fdac7..1b0ab2dcb1056 100644
--- a/compiler/rustc_span/src/lib.rs
+++ b/compiler/rustc_span/src/lib.rs
@@ -142,10 +142,10 @@ pub fn set_session_globals_then<R>(session_globals: &SessionGlobals, f: impl FnO
     SESSION_GLOBALS.set(session_globals, f)
 }
 
-pub fn create_session_if_not_set_then<R, F>(edition: Edition, f: F) -> R
-where
-    F: FnOnce(&SessionGlobals) -> R,
-{
+pub fn create_session_if_not_set_then<R>(
+    edition: Edition,
+    f: impl FnOnce(&SessionGlobals) -> R,
+) -> R {
     if !SESSION_GLOBALS.is_set() {
         let session_globals = SessionGlobals::new(edition);
         SESSION_GLOBALS.set(&session_globals, || SESSION_GLOBALS.with(f))
@@ -160,7 +160,14 @@ where
 {
     SESSION_GLOBALS.with(f)
 }
+pub struct NotSet;
+
+#[inline]
+pub fn try_with_session_globals<R>(f: impl FnOnce(&SessionGlobals) -> R) -> Result<R, NotSet> {
+    if SESSION_GLOBALS.is_set() { Ok(SESSION_GLOBALS.with(f)) } else { Err(NotSet) }
+}
 
+#[inline]
 pub fn create_default_session_globals_then<R>(f: impl FnOnce() -> R) -> R {
     create_session_globals_then(edition::DEFAULT_EDITION, f)
 }
diff --git a/compiler/rustc_span/src/source_map.rs b/compiler/rustc_span/src/source_map.rs
index 8253ffefcaa7d..349b310dceecf 100644
--- a/compiler/rustc_span/src/source_map.rs
+++ b/compiler/rustc_span/src/source_map.rs
@@ -824,7 +824,12 @@ impl SourceMap {
     }
 
     /// Returns a new span representing just the first character of the given span.
+    /// When the span is empty, the same empty span is returned.
     pub fn start_point(&self, sp: Span) -> Span {
+        if sp.is_empty() {
+            return sp;
+        }
+
         let width = {
             let sp = sp.data();
             let local_begin = self.lookup_byte_offset(sp.lo);
diff --git a/compiler/rustc_span/src/span_encoding.rs b/compiler/rustc_span/src/span_encoding.rs
index e162695a13beb..c65e2bd0e6f18 100644
--- a/compiler/rustc_span/src/span_encoding.rs
+++ b/compiler/rustc_span/src/span_encoding.rs
@@ -1,6 +1,6 @@
 use crate::def_id::{DefIndex, LocalDefId};
 use crate::hygiene::SyntaxContext;
-use crate::SPAN_TRACK;
+use crate::{try_with_session_globals, NotSet, Pos, SPAN_TRACK};
 use crate::{BytePos, SpanData};
 
 use rustc_data_structures::fx::FxIndexSet;
@@ -103,6 +103,40 @@ impl Span {
         ctxt: SyntaxContext,
         parent: Option<LocalDefId>,
     ) -> Self {
+        // Make sure that the byte positions are at char boundaries.
+        // Only do this if the session globals are set correctly, which they aren't in some unit tests.
+        if cfg!(debug_assertions) {
+            let _: Result<(), NotSet> = try_with_session_globals(|sess| {
+                let sm = sess.source_map.lock();
+                if let Some(sm) = &*sm {
+                    let offset = sm.lookup_byte_offset(lo);
+                    if let Some(file) = &offset.sf.src {
+                        // `is_char_boundary` already checks this, but asserting it seperately gives a better panic message.
+                        assert!(
+                            file.len() >= offset.pos.to_usize(),
+                            "start of span is out of bounds"
+                        );
+                        assert!(
+                            file.is_char_boundary(offset.pos.to_usize()),
+                            "start of span not on char boundary"
+                        );
+                    }
+
+                    let offset = sm.lookup_byte_offset(hi);
+                    if let Some(file) = &offset.sf.src {
+                        assert!(
+                            file.len() >= offset.pos.to_usize(),
+                            "end of span is out of bounds"
+                        );
+                        assert!(
+                            file.is_char_boundary(offset.pos.to_usize()),
+                            "end of span not on char boundary"
+                        );
+                    }
+                }
+            });
+        }
+
         if lo > hi {
             std::mem::swap(&mut lo, &mut hi);
         }