Do not ICE when synthesizing spans falling inside unicode chars

src/libsyntax/source_map.rs

@@ -554,8 +554,14 @@ fn span_to_source<F>(&self, sp: Span, extract_source: F) -> Result<String, SpanS
             }
 
             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 f() {
+    vec![X]; //…
+    //~^ ERROR expected value, found struct `X`
+}

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

@@ -0,0 +1,17 @@
+error[E0423]: expected value, found struct `X`
+  --> $DIR/issue-61226.rs:3:10
+   |
+LL |     vec![X]; //…
+   |          ^
+   |          |
+   |          did you mean `X { /* fields */ }`?
+   |          help: a function with a similar name exists: `f`
+
+error[E0601]: `main` function not found in crate `issue_61226`
+   |
+   = note: consider adding a `main` function to `$DIR/issue-61226.rs`
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0423, E0601.
+For more information about an error, try `rustc --explain E0423`.