Diagnostics for let mut in item context

Author: kornelski

compiler/rustc_parse/src/parser/item.rs

@@ -77,18 +77,25 @@ impl<'a> Parser<'a> {
         if !self.eat(term) {
             let token_str = super::token_descr(&self.token);
             if !self.maybe_consume_incorrect_semicolon(items.last().map(|x| &**x)) {
+                let is_let = self.token.is_keyword(kw::Let);
+                let is_let_mut = is_let && self.look_ahead(1, |t| t.is_keyword(kw::Mut));
+
                 let msg = format!("expected item, found {token_str}");
                 let mut err = self.dcx().struct_span_err(self.token.span, msg);
-                let span = self.token.span;
-                if self.is_kw_followed_by_ident(kw::Let) {
-                    err.span_label(
-                        span,
-                        "consider using `const` or `static` instead of `let` for global variables",
-                    );
+
+                let label = if is_let {
+                    "`let` cannot be used for global variables"
                 } else {
-                    err.span_label(span, "expected item")
-                        .note("for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>");
+                    "expected item"
+                };
+                err.span_label(self.token.span, label);
+
+                if is_let_mut {
+                    err.help("consider using `static` and a `Mutex` instead of `let mut`");
+                } else if is_let {
+                    err.help("consider using `const` or `static` instead of `let`");
                 };
+                err.note("for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>");
                 return Err(err);
             }
         }

tests/ui/parser/suggest-const-for-global-var.stderr

@@ -2,7 +2,10 @@ 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
+   | ^^^ `let` cannot be used for global variables
+   |
+   = help: consider using `const` or `static` instead of `let`
+   = note: for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>
 
 error: aborting due to 1 previous error
 

tests/ui/parser/suggest-static-for-global-var-mut.rs

@@ -0,0 +1,5 @@
+let mut _data = vec![1,2,3];
+//~^ ERROR expected item, found keyword `let`
+
+fn main() {
+}

tests/ui/parser/suggest-static-for-global-var-mut.stderr

@@ -0,0 +1,11 @@
+error: expected item, found keyword `let`
+  --> $DIR/suggest-static-for-global-var-mut.rs:1:1
+   |
+LL | let mut _data = vec![1,2,3];
+   | ^^^ `let` cannot be used for global variables
+   |
+   = help: consider using `static` and a `Mutex` instead of `let mut`
+   = note: for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>
+
+error: aborting due to 1 previous error
+