Suggest fn if fun, func, function or def is used instead

[Rageking8]

Given the following code:

func foo() -> i32 {
    123
}

function foo() -> i32 {
    123
}

The current output is:

   Compiling playground v0.0.1 (/playground)
error: expected one of `!` or `::`, found `foo`
 --> src/main.rs:1:6
  |
1 | func foo() -> i32 {
  |      ^^^ expected one of `!` or `::`

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

Ideally the error message should suggest replacing func or function with fn.

[EagleTw]

@rustbot claim

[EagleTw]

New to rustc community and wants to contribute code.

1. Are there any options to debug rustc which line of which file the error abort happens?
2. How can I dump the hierarchy tree for debugging use?
3. In C++ we usually use env for compiler debugging. How do rustc often use?

Very newbie questions, thank you in advance.

[gimbling-away]

@rustbot label +E-easy +E-mentor

@ypprog Welcome to Rust! ✨

This might be some help --
This issue is very similar to #99903, so take a peek at the diff there. IMO the following tokens are enough to check against

• function (Julia/JavaScript/etc)
• func (Go)
• fun (Kotlin)
• def (Python)

Have fun! <3

[gimbling-away]

Instructions --

• Add a check if prev_token is either function/func/fun/etc here
• Emit a message, something along the lines of "use fn instead of <prev_token> to make a function" using err.span_suggestion_short, there are a few usages of this in the same file so those might be good examples.
• Add tests for this in src/test/ui/parser/ -- Ideally each suggestion would have a test.

Should be helpful to take #99903 as a reference

[Rageking8]

We can include def if necessary.

[adam-tokarski]

Hi guys, another rust newbie looking for something easy to get started here. @ypprog are you still on this or could go with my attempt here?

[gimbling-away]

@adam-tokarski ypprog does seem to be active and it hasn't been that long, if you'd like you can pick another issue. =)

Try going through the E-Easy and/or E-Mentor category for easy issues. However, to clarify, if an issue which is claimed doesn't seem to have any work done for a long period of time (A month or so is a good amount of time) -- Feel free to ping the assignee to confirm and handle it accordingly.

[bindsdev]

With these changes, I also think it is important to implement improved error messages for when a user puts the return type at the start of the function instead of the end.

Given this code:

u32 foo() {
    100
}

The error output is:

error: expected one of `!` or `::`, found `foo`
 --> src/main.rs:1:5
  |
1 | u32 foo() {
  |     ^^^ expected one of `!` or `::`

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

[gimbling-away]

I feel like adding auto might also be helpful for C++ developers :)

[bindsdev]

Is this still being worked on? @ypprog

[EagleTw]

@rustbot release-assignment

[bindsdev]

@rustbot claim