not use offset when there is not ends with brace

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -211,11 +211,18 @@ fn missing_items_err(
         .collect::<Vec<_>>()
         .join("`, `");
 
+    let sugg_sp = if let Ok(snippet) = tcx.sess.source_map().span_to_snippet(full_impl_span)
+        && snippet.ends_with("}")
+    {
         // `Span` before impl block closing brace.
         let hi = full_impl_span.hi() - BytePos(1);
         // Point at the place right before the closing brace of the relevant `impl` to suggest
         // adding the associated item at the end of its body.
-    let sugg_sp = full_impl_span.with_lo(hi).with_hi(hi);
+        full_impl_span.with_lo(hi).with_hi(hi)
+    } else {
+        full_impl_span.shrink_to_hi()
+    };
+
     // Obtain the level of indentation ending in `sugg_sp`.
     let padding =
         tcx.sess.source_map().indentation_before(sugg_sp).unwrap_or_else(|| String::new());

tests/ui/suggestions/missing-impl-trait-block-but-not-ascii.rs

@@ -0,0 +1,13 @@
+// issue#126764
+
+struct S;
+
+trait T {
+    fn f();
+}
+impl T for S；
+//~^ ERROR: unknown start of token
+//~| ERROR: expected `{}`
+//~| ERROR: not all trait items implemented, missing: `f`
+
+fn main() {}

tests/ui/suggestions/missing-impl-trait-block-but-not-ascii.stderr

@@ -0,0 +1,31 @@
+error: unknown start of token: \u{ff1b}
+  --> $DIR/missing-impl-trait-block-but-not-ascii.rs:8:13
+   |
+LL | impl T for S；
+   |             ^^
+   |
+help: Unicode character '；' (Fullwidth Semicolon) looks like ';' (Semicolon), but it is not
+   |
+LL | impl T for S;
+   |             ~
+
+error: expected `{}`, found `;`
+  --> $DIR/missing-impl-trait-block-but-not-ascii.rs:8:13
+   |
+LL | impl T for S；
+   |             ^^
+   |
+   = help: try using `{}` instead
+
+error[E0046]: not all trait items implemented, missing: `f`
+  --> $DIR/missing-impl-trait-block-but-not-ascii.rs:8:1
+   |
+LL |     fn f();
+   |     ------- `f` from trait
+LL | }
+LL | impl T for S；
+   | ^^^^^^^^^^^^ missing `f` in implementation
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0046`.