Instead of modifying the item's span synthesize it

src/librustc_resolve/lib.rs

@@ -4000,7 +4000,13 @@ impl<'a> Resolver<'a> {
                                            binding.is_renamed_extern_crate()) {
                 err.span_suggestion(binding.span,
                                     rename_msg,
-                                    format!("{} as Other{}", snippet, name));
+                                    if snippet.ends_with(';') {
+                                        format!("{} as Other{};",
+                                                &snippet[..snippet.len()-1],
+                                                name)
+                                    } else {
+                                        format!("{} as Other{}", snippet, name)
+                                    });
             } else {
                 err.span_label(binding.span, rename_msg);
             }

src/libsyntax/parse/parser.rs

@@ -6128,11 +6128,10 @@ impl<'a> Parser<'a> {
         } else {
             (None, crate_name)
         };
-
-        // We grab this before expecting the `;` so it's not a part of the span
-        let prev_span = self.prev_span;
         self.expect(&token::Semi)?;
 
+        let prev_span = self.prev_span;
+
         Ok(self.mk_item(lo.to(prev_span),
                         ident,
                         ItemKind::ExternCrate(maybe_path),

src/test/ui/suggestions/issue-45799-bad-extern-crate-rename-suggestion-formatting.rs

@@ -10,3 +10,4 @@
 
 extern crate std;
 fn main() {}
+//~^^ ERROR the name `std` is defined multiple times [E0259]

src/test/ui/suggestions/issue-45799-bad-extern-crate-rename-suggestion-formatting.stderr

@@ -2,13 +2,13 @@ error[E0259]: the name `std` is defined multiple times
   --> $DIR/issue-45799-bad-extern-crate-rename-suggestion-formatting.rs:11:1
    |
 11 | extern crate std;
-   | ^^^^^^^^^^^^^^^^ `std` reimported here
+   | ^^^^^^^^^^^^^^^^^ `std` reimported here
    |
    = note: `std` must be defined only once in the type namespace of this module
 help: You can use `as` to change the binding name of the import
    |
 11 | extern crate std as Otherstd;
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
 
 error: aborting due to previous error