Tweak self arg not as first argument of a method diagnostic

[estebank]

Mention that self is only valid on "associated functions"

error: unexpected `self` argument in function
--> $DIR/self-in-function-arg.rs:1:15
|
LL | fn foo(x:i32, self: i32) -> i32 { self }
| ^^^^ not valid as function argument
|
= note: `self` is only valid as the first argument of an associated function

When it is a method, mention it must be first

error: unexpected `self` argument in function
--> $DIR/trait-fn.rs:4:20
|
LL | fn c(foo: u32, self) {}
| ^^^^ must be the first associated function argument

Move a bunch of error recovery methods to diagnostics.rs away from parser.rs.

Fix#51547. CC #60015.

[rust-highfive]

r? @pnkfelix

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[estebank]

r? @Centril parser code shuffle

[Centril]

I'll do a more thorough review later.

@estebank Can you split the "move stuff" and "change diagnostics output" into separate commits and then a third commit for addressing review comments?

[Centril]

Refactor out b.push_str(connective)?

[estebank]

@Centril addressed all review comments but one, which I think should be part of a separate PR cleaning up all the times this one-or-more string handling has been done in the parser.

[Centril]

r=me with rebase and possibly a fix to the comment below

[Centril]

Why not set the TyKind to what it would have been had it been in the right position?

[estebank]

Because we don't know wether this is any of the following:

• a function with an arbitrary argument called self
• a method with self not at the start
• a method with self at the start and a second self

I thought it'd be safer to just mark it as TyKind::Err and avoid potential incorrect diagnostics elsewhere. I prefer to be silent on actual problems (given other errors are emitted) over being too verbose with possibly unnecessary errors.

[Centril]

Good insight; on the other hand, I'd say that self not at the start is probabilistically more correct than the others... but, in any case, let's land this as-is. :)

[estebank]

@bors r=Centril

[bors]

📌 Commit 4e68ddc has been approved by Centril