Add suggestions for possible missing fn, struct, or enum keywords

Author: trevyn

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -620,6 +620,55 @@ impl<'a> Parser<'a> {
             );
         }
 
+        // Possible missing `struct` or `enum` keyword
+        if let TokenKind::Ident(symbol, _) = &self.prev_token.kind
+            && let TokenKind::OpenDelim(Delimiter::Brace) = self.token.kind
+            && let [TokenType::Token(token::Not), TokenType::Token(token::PathSep)] = expected[..]
+        {
+            let mut lookahead = self.clone();
+            for position in 1..1000 {
+                lookahead.bump();
+                if lookahead.token == token::CloseDelim(Delimiter::Brace)
+                    || lookahead.token == token::Colon
+                {
+                    let (article, kw) = if lookahead.token == token::Colon || position == 1 {
+                        ("a", "struct")
+                    } else {
+                        ("an", "enum")
+                    };
+                    err.span_suggestion(
+                        self.prev_token.span,
+                        format!("if this is {article} {kw} definition, use the `{kw}` keyword"),
+                        format!("{kw} {symbol}"),
+                        Applicability::MaybeIncorrect,
+                    );
+                    break;
+                }
+            }
+        }
+
+        // Possible missing `fn` keyword
+        if let TokenKind::Ident(symbol, _) = &self.prev_token.kind
+            && let TokenKind::OpenDelim(Delimiter::Parenthesis) = self.token.kind
+        {
+            let mut lookahead = self.clone();
+            for _ in 1..100 {
+                lookahead.bump();
+                if lookahead.token == token::Semi {
+                    break;
+                }
+                if lookahead.token == token::OpenDelim(Delimiter::Brace) {
+                    err.span_suggestion(
+                        self.prev_token.span,
+                        "if this is a function definition, use the `fn` keyword",
+                        format!("fn {symbol}"),
+                        Applicability::MaybeIncorrect,
+                    );
+                    break;
+                }
+            }
+        }
+
         if let TokenKind::Ident(symbol, _) = &self.prev_token.kind {
             if ["def", "fun", "func", "function"].contains(&symbol.as_str()) {
                 err.span_suggestion_short(

tests/ui/did_you_mean/issue-40006.stderr

@@ -21,6 +21,11 @@ LL |     X() {}
 ...
 LL | }
    | - the item list ends here
+   |
+help: if this is a function definition, use the `fn` keyword
+   |
+LL |     fn X() {}
+   |     ~~~~
 
 error: expected one of `!` or `::`, found `(`
   --> $DIR/issue-40006.rs:16:6
@@ -31,6 +36,11 @@ LL |     X() {}
    |      ^ expected one of `!` or `::`
 LL | }
    | - the item list ends here
+   |
+help: if this is a function definition, use the `fn` keyword
+   |
+LL |     fn X() {}
+   |     ~~~~
 
 error: expected one of `!` or `[`, found `#`
   --> $DIR/issue-40006.rs:19:17

tests/ui/parser/missing-enum-issue-125446.rs

@@ -0,0 +1,5 @@
+Whoops {
+//~^ ERROR expected one of `!` or `::`, found `{`
+    OptionA,
+    OptionB,
+}

tests/ui/parser/missing-enum-issue-125446.stderr

@@ -0,0 +1,13 @@
+error: expected one of `!` or `::`, found `{`
+  --> $DIR/missing-enum-issue-125446.rs:1:8
+   |
+LL | Whoops {
+   |        ^ expected one of `!` or `::`
+   |
+help: if this is an enum definition, use the `enum` keyword
+   |
+LL | enum Whoops {
+   | ~~~~~~~~~~~
+
+error: aborting due to 1 previous error
+

tests/ui/parser/missing-fn-issue-125446.rs

@@ -0,0 +1,6 @@
+whoops() {}
+//~^ ERROR expected one of `!` or `::`, found `(`
+
+fn main() {
+    whoops();
+}

tests/ui/parser/missing-fn-issue-125446.stderr

@@ -0,0 +1,13 @@
+error: expected one of `!` or `::`, found `(`
+  --> $DIR/missing-fn-issue-125446.rs:1:7
+   |
+LL | whoops() {}
+   |       ^ expected one of `!` or `::`
+   |
+help: if this is a function definition, use the `fn` keyword
+   |
+LL | fn whoops() {}
+   | ~~~~~~~~~
+
+error: aborting due to 1 previous error
+

tests/ui/parser/missing-fn-issue-65381-1.rs

@@ -0,0 +1,3 @@
+main() {
+//~^ ERROR expected one of `!` or `::`, found `(`
+}

tests/ui/parser/missing-fn-issue-65381-1.stderr

@@ -0,0 +1,13 @@
+error: expected one of `!` or `::`, found `(`
+  --> $DIR/missing-fn-issue-65381-1.rs:1:5
+   |
+LL | main() {
+   |     ^ expected one of `!` or `::`
+   |
+help: if this is a function definition, use the `fn` keyword
+   |
+LL | fn main() {
+   | ~~~~~~~
+
+error: aborting due to 1 previous error
+

tests/ui/parser/missing-fn-issue-65381-2.rs

@@ -0,0 +1,2 @@
+main();
+//~^ ERROR expected one of `!` or `::`, found `(`

tests/ui/parser/missing-fn-issue-65381-2.stderr

@@ -0,0 +1,8 @@
+error: expected one of `!` or `::`, found `(`
+  --> $DIR/missing-fn-issue-65381-2.rs:1:5
+   |
+LL | main();
+   |     ^ expected one of `!` or `::`
+
+error: aborting due to 1 previous error
+