Suggest invalid expression when use `public`

[TheAwiteb]

When writing public let some = 3; it will suggest pub let some = 3; and I think nothing should be suggested before it is validated.

Note:
The statement to be suggested must be checked if it is true or not, if true it is suggested and if not it is not.

The issue from: #99903

Given the following code:

fn main() {
    public let some = 3;
}

The current output is:

   Compiling playground v0.0.1 (/playground)
error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found keyword `let`
 --> src/main.rs:2:12
  |
2 |     public let some = 3;
  |            ^^^ expected one of 8 possible tokens
  |
help: write `pub` instead of `public` to make the item public
  |
2 |     pub let some = 3;
  |     ~~~

error: could not compile `playground` due to previous error

Ideally the output should look like:

   Compiling playground v0.0.1 (/playground)
error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found keyword `let`
 --> src/main.rs:2:12
  |
2 |     public let some = 3;
  |            ^^^ expected one of 8 possible tokens

error: could not compile `playground` due to previous error

[TheAwiteb]

@rustbot claim

Oh dear, I didn't think of that while making the PR. 😅

A possible solution could be checking if the current token is for initializing an item? Like, struct/enum/macro, etc.

[TheAwiteb]

A possible solution could be checking if the current token is for initializing an item? Like, struct/enum/macro, etc.

I wrote this solution, but I don't know if I can use it in rustc, it's currently building 😁

let is_valid_stmt: bool = syn::parse_str::<syn::Stmt>("pub let some = 3;")
    .is_ok(); // false

[TheAwiteb]

I can't work on the problem unfortunately, whoever will assign it to him you can solve it this way

can_be_public = ["enum", "struct", "fn", "extern", "trait"];
if can_be_public.contains(&self.token.as_str()) {
    // ...
}

The block

rust/compiler/rustc_parse/src/parser/diagnostics.rs

Lines 605 to 614 in affe0d3

	if let TokenKind::Ident(symbol, _) = &self.prev_token.kind {
	if symbol.as_str() == "public" {
	err.span_suggestion_short(
	self.prev_token.span,
	"write `pub` instead of `public` to make the item public",
	"pub",
	appl,
	);
	}
	}

[TheAwiteb]

@rustbot release-assignment

[chenyukang]

@rustbot claim

[chenyukang]

Trying to add a method in token:

    /// Returns `true` if the token the start of a item 
    pub fn can_begin_item(&self) -> bool {
        self.is_keyword(kw::Use)
        || self.is_keyword(kw::Fn)
        || self.is_keyword(kw::Extern)
        || self.is_keyword(kw::Crate)
        || self.is_keyword(kw::Mod)
        || self.is_keyword(kw::Const)
        || self.is_keyword(kw::Static)
        || self.is_keyword(kw::Trait)
        || self.is_keyword(kw::Impl)
        || self.is_keyword(kw::Mod)
        || self.is_keyword(kw::Type)
        || self.is_keyword(kw::Enum)
        || self.is_keyword(kw::Struct)
        || self.is_keyword(kw::Union)
    }

And add a check:

 if self.prev_token.is_ident_named(sym::public) && self.token.can_begin_item() {
            err.span_suggestion_short(
                self.prev_token.span,
                "write `pub` instead of `public` to make the item public",
                "pub",
                appl,
            );
        }

Any other better idea? @gimbles

1. This could be the macro keyword, as well.
2. That looks alright, ideally the repeated block could be prevented using a macro? I'm afraid that might slightly overengineer it, though. 😂