Gracefully handle python-esque sequence slicing

[hkmatsumoto]

Code

fn main() {
    let a = &[1, 2, 3][1:2];
}

Current output

Compiling playground v0.0.1 (/playground)
error: expected type, found `2`
 --> src/main.rs:2:26
  |
2 |     let a = &[1, 2, 3][1:2];
  |                         -^ expected type
  |                         |
  |                         tried to parse a type due to this type ascription
  |
  = note: `#![feature(type_ascription)]` lets you annotate an expression with a type: `<expr>: <type>`
  = note: see issue #23416 <https://github.com/rust-lang/rust/issues/23416> for more information

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

Desired output

help: did you mean [1..2]?

Rationale and extra context

Having written Python for several hours, I made this mistake and couldn't realize what was wrong for a while.

Other cases

No response

Anything else?

@rustbot claim

[Noratrieb]

I'm not sure whether it's worth to fix this right now. #101728 will significantly change this code, basically requiring a rewrite of this diagnostic.

[hkmatsumoto]

Agreed, it's reasonable to wait for type ascription removal.
Thanks for the info!

[chenyukang]

@hkmatsumoto
Now we should fix this error message:

error: expected one of `.`, `?`, `]`, or an operator, found `:`
 --> src/main.rs:2:25
  |
2 |     let a = &[1, 2, 3][1:2];
  |                         ^ expected one of `.`, `?`, `]`, or an operator
  |
  = note: type ascription syntax has been removed, [see issue #101728 <https://github.com/rust-lang/rust/issues/101728>](https://github.com/rust-lang/rust/issues/101728)
help: maybe write a path separator here
  |
2 |     let a = &[1, 2, 3][1::2];

help: maybe write a path separator here is not OK, we'd better suggest to use [1..2].
I think you need to add proper logic for this part of code:

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

Line 563 in cad92b4

"maybe write a path separator here",