missing `fn` before `main()` -- error could be improved

[ultrasaurus]

As a newcomer to Rust, sometimes I forget fn before main() and the error message doesn't make sense in this instance:

Example file:

main() {
}

Error message

error: expected one of `!` or `::`, found `(`
 --> examples/tuple.rs:1:5
  |
1 | main() {
  |     ^ expected one of `!` or `::` here

suggested change:

"^ expected prior fn for a function declaration or suffix ! for a macro"

I'm not familiar with a case where :: would make sense here without other changes too.

[Centril]

cc @estebank @petrochenkov -- While this would be good to fix, it is probably difficult and I suspect it requires some thought to avoid erroring on e.g. main(); (a function call -- consider function defs as "statements"). Also, I would like to merge the function grammars (free, foreign, trait, impl) before adding a diagnostic here to ensure that they can still be merged.

[estebank]

Note that we already do this for trait items. In those cases we already do some bounded lookahead to look for { or ; to make sure the suggestion is correct. In this case it would only look for {.

[MatsPalmgren]

Fwiw, I made this typo today:

268 |     pub const initial_value() -> Self {
    |                            ^ expected `:`

A better error message would be:

268 |     pub const initial_value() -> Self {
    |              ^ did you forget `fn` ?

[dbofmmbt]

I don't know if it fits here, but it would be good to improve the error messages for users using function, func, fun etc too. The message which appears for this kind of mistake is the same as the error message on this issue. IMO the current message may look very confusing for beginners and even for more experienced folks.

Example:

fun main() {}

error: expected one of `!` or `::`, found `main`
 --> src/lib.rs:1:5
  |
1 | fun main() {}
  |     ^^^^ expected one of `!` or `::`

error: aborting due to previous error

error: could not compile `playground`

To learn more, run the command again with --verbose.

[estebank]

For the bare C-style main() {}, we still don't handle it, but for fun main() {} we do:

error: expected one of `!` or `::`, found `main`
 --> src/lib.rs:1:5
  |
1 | fun main() {}
  | --- ^^^^ expected one of `!` or `::`
  | |
  | help: write `fn` instead of `fun` to declare a function

[gurry]

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