Suggest using NameValue syntax for malformed deprecated attribute

Author: ariagivens

compiler/rustc_attr_parsing/src/attributes/deprecation.rs

@@ -76,6 +76,16 @@ impl SingleAttributeParser for DeprecatedParser {
                 // ok
             }
             ArgParser::List(list) => {
+                // If the argument list contains a single string literal, suggest using NameValue syntax
+                if let Some(elem) = list.as_single() && let Some(lit) = elem.as_lit() && lit.kind.is_str() {
+                    let mut adcx = cx.adcx();
+                    if let Some(span) = args.span() {
+                        adcx.push_suggestion(String::from("try using `=` instead"), span, format!(" = {}", lit.kind));
+                    }
+                    adcx.expected_not_literal(elem.span());
+                    return None;
+                }
+
                 for param in list.mixed() {
                     let Some(param) = param.meta_item() else {
                         cx.adcx().expected_not_literal(param.span());

tests/ui/deprecation/deprecation-sanity.stderr

@@ -55,6 +55,12 @@ LL |     #[deprecated("test")]
    |     ^^^^^^^^^^^^^------^^
    |                  |
    |                  didn't expect a literal here
+   |
+help: try using `=` instead
+   |
+LL -     #[deprecated("test")]
+LL +     #[deprecated = "test"]
+   |
 
 error: multiple `deprecated` attributes
   --> $DIR/deprecation-sanity.rs:29:1

tests/ui/error-codes/E0565-1.stderr

@@ -5,6 +5,12 @@ LL | #[deprecated("since")]
    | ^^^^^^^^^^^^^-------^^
    |              |
    |              didn't expect a literal here
+   |
+help: try using `=` instead
+   |
+LL - #[deprecated("since")]
+LL + #[deprecated = "since"]
+   |
 
 error: aborting due to 1 previous error