[E0532] Error when using functions in patterns should give an explanation of patterns

[porglezomp]

Given the following code: (Playground)

fn main() {
    let (x, drop(y)) = (1, 2);
}

The current output is:

error[E0532]: expected tuple struct or tuple variant, found function `drop`
 --> src/main.rs:2:13
  |
2 |     let (x, drop(y)) = (1, 2);
  |             ^^^^ not a tuple struct or tuple variant

The error message should give/link to an explanation of what kinds of things are expected in patterns in general, maybe in a note.

(Filed at request of @estebank)

[Milo123459]

What should be linked? Something in Rust by Example or the Rust docs (or something else?)

[Milo123459]

@rustbot claim

[porglezomp]

@estebank would know better what the precedent for existing errors is, but I'd expect it to link to the reference page for patterns, https://doc.rust-lang.org/reference/patterns.html

I'm slightly unsure about this because of the intimidating grammar block at the top if it's gonna end up pointing beginners to that, but it's probably the best reference for understanding all the details of patterns…

If not that, then I'd expect the Book chapter on patterns https://doc.rust-lang.org/book/ch18-00-patterns.html

[Milo123459]

I'll wait for him to respond before starting.

[compiler-errors]

@Milo123459, there are a few existing error notes that point to book chapters in the compiler, search for references to doc.rust-lang.org/book under the compiler/ subdirectory.

I can review a PR attaches such a note to some cases. You might need to be careful about only suggesting it for items that aren't valid in patterns but not* those used incorrectly. For example, E0532 fires if you try to use a unit enum variant as a tuple enum variant. I think we only want to suggest it when we're trying to use, e.g., a function or method name as a pattern variant.

[Milo123459]

For example, E0532 fires if you try to use a unit enum variant as a tuple enum variant. I think we only want to suggest it when we're trying to use, e.g., a function or method name as a pattern variant.

Hmm, how do we control that / know when it is only invalid?

[compiler-errors]

You should be able to check the res of the item?

You might need to do some experimentation -- if you can't, then feel free to just fall back to suggesting it always, but leave a FIXME.

[estebank]

I'd expect it to link to the reference page for patterns

I'm slightly unsure about this because of the intimidating grammar block at the top if it's gonna end up pointing beginners to that, but it's probably the best reference for understanding all the details of patterns…

If not that, then I'd expect the Book chapter on patterns https://doc.rust-lang.org/book/ch18-00-patterns.html

I would likely link at https://doc.rust-lang.org/book/ch18-00-patterns.html, precisely because it is more conversational than the reference, but we could link to both, if that seemed necessary.

You should be able to check the res of the item?

The Res contains gets populated by resolve and associates to the underlying type of the thing being evaluated, including whether they are a struct or enum or enum variant, for example. From it you can get the DefId of a user writable type (some things like i8 don't have DefIds, don't worry too much about them). Once you have a DefId you can get the item itself or the Span that lets you point at them.

For the wording changes, I would borrow heavily from the reference and book, and at least add a help with something along the lines of "a pattern can only be composed of literals (like 42 or "hi"), arrays (like [a, b, c]), enums, structs, tuples, variable names (like foo), wildcards (like ..), and placeholders (like _)". I would likely change the main label (in this case) to say "function calls are not allowed in patterns".

[Milo123459]

@rustbot release-assignment