suggest const or static for global variable

Author: chenyukang

compiler/rustc_parse/src/parser/item.rs

@@ -68,7 +68,12 @@ impl<'a> Parser<'a> {
             if !self.maybe_consume_incorrect_semicolon(&items) {
                 let msg = &format!("expected item, found {token_str}");
                 let mut err = self.struct_span_err(self.token.span, msg);
-                err.span_label(self.token.span, "expected item");
+                let label = if self.is_kw_followed_by_ident(kw::Let) {
+                    "consider using `const` or `static` instead of `let` for global variables"
+                } else {
+                    "expected item"
+                };
+                err.span_label(self.token.span, label);
                 return Err(err);
             }
         }

src/test/ui/parser/suggest-const-for-global-var.rs

@@ -0,0 +1,6 @@
+let X: i32 = 12;
+//~^ ERROR expected item, found keyword `let`
+
+fn main() {
+    println!("{}", X);
+}

src/test/ui/parser/suggest-const-for-global-var.stderr

@@ -0,0 +1,8 @@
+error: expected item, found keyword `let`
+  --> $DIR/suggest-const-for-global-var.rs:1:1
+   |
+LL | let X: i32 = 12;
+   | ^^^ consider using `const` or `static` instead of `let` for global variables
+
+error: aborting due to previous error
+