Do not ICE when synthesizing spans falling inside unicode chars

src/libsyntax/source_map.rs

@@ -554,8 +554,14 @@ impl SourceMap {
             }
 
             if let Some(ref src) = local_begin.sf.src {
+                if !src.is_char_boundary(start_index) || !src.is_char_boundary(end_index) {
+                    return Err(SpanSnippetError::IllFormedSpan(sp));
+                }
                 return Ok(extract_source(src, start_index, end_index));
             } else if let Some(src) = local_begin.sf.external_src.borrow().get_source() {
+                if !src.is_char_boundary(start_index) || !src.is_char_boundary(end_index) {
+                    return Err(SpanSnippetError::IllFormedSpan(sp));
+                }
                 return Ok(extract_source(src, start_index, end_index));
             } else {
                 return Err(SpanSnippetError::SourceNotAvailable {

src/test/ui/suggestions/issue-61226.rs

@@ -0,0 +1,5 @@
+struct X {}
+fn main() {
+    vec![X]; //…
+    //~^ ERROR expected value, found struct `X`
+}

src/test/ui/suggestions/issue-61226.stderr

@@ -0,0 +1,9 @@
+error[E0423]: expected value, found struct `X`
+  --> $DIR/issue-61226.rs:3:10
+   |
+LL |     vec![X]; //…
+   |          ^ did you mean `X { /* fields */ }`?
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0423`.