[RFC] rustc_parse: improve the error diagnostic for "missing let in let chain"

[Unique-Usman]

No description provided.

[rustbot]

r? @chenyukang

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[Unique-Usman]

My approach seems not to be perfect and I have been stucked on this for a while. This is the issue this is supposed to fix - >#117977

[Unique-Usman]

@estebank

[Unique-Usman]

r? @estebank

[Unique-Usman]

This is an improvement for

fn main() {
    let x = Some(42);
    if let Some(_) = x
        && Some(x) = x
    {}
}

initially it gives

error: expected expression, found `let` statement
 --> foo.rs:3:8
  |
3 |     if let Some(_) = x
  |        ^^^^^^^^^^^^^^^
  |
  = note: only supported directly in conditions of `if` and `while` expressions
help: you might have meant to continue the let-chain
  |
4 |         && let Some(x) = x
  |            +++
help: you might have meant to compare for equality
  |
4 |         && Some(x) == x
  |                     +

error[E0308]: mismatched types
 --> foo.rs:4:12
  |
4 |         && Some(x) = x
  |            ^^^^^^^ expected `bool`, found `Option<Option<{integer}>>`
  |
  = note: expected type `bool`
             found enum `Option<Option<{integer}>>`
help: use `Option::is_some` to test if the `Option` has a value
  |
4 |         && Some(x).is_some() = x
  |                   ++++++++++

error[E0308]: mismatched types
 --> foo.rs:3:8
  |
3 |       if let Some(_) = x
  |  ________^
4 | |         && Some(x) = x
  | |______________________^ expected `bool`, found `()`

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0308`.

but now it gives

error: expected expression, found `let` statement
 --> foo.rs:3:8
  |
3 |     if let Some(_) = x
  |        ^^^^^^^^^^^^^^^
  |
  = note: -Ztrack-diagnostics: created at compiler/rustc_parse/src/parser/expr.rs:4263:51
  = note: only supported directly in conditions of `if` and `while` expressions
help: you might have meant to continue the let-chain
  |
4 |         && let Some(x) = x
  |            +++
help: you might have meant to compare for equality
  |
4 |         && Some(x) == x
  |                     +

error: aborting due to 1 previous error

[Unique-Usman]

Interestingly,

fn main() {
    let x = Some(42);
    if let Some(_) = x
        && Some(x) = x
    {}
}

still give the same result.

error: expected expression, found `let` statement
 --> foo.rs:4:12
  |
4 |         && let Some(x) = x
  |            ^^^
  |
  = note: only supported directly in conditions of `if` and `while` expressions

error[E0308]: mismatched types
 --> foo.rs:3:18
  |
3 |     if Some(_) = x
  |                  ^ expected `bool`, found `Option<{integer}>`
  |
  = note: expected type `bool`
             found enum `Option<{integer}>`

error[E0308]: mismatched types
 --> foo.rs:3:8
  |
3 |       if Some(_) = x
  |  ________^^^^^^^___-
  | |        |
  | |        expected `bool`, found `Option<_>`
4 | |         && let Some(x) = x
  | |__________________________- this expression has type `bool`
  |
  = note: expected type `bool`
             found enum `Option<_>`

error[E0308]: mismatched types
 --> foo.rs:3:8
  |
3 |       if Some(_) = x
  |  ________^
4 | |         && let Some(x) = x
  | |__________________________^ expected `bool`, found `()`
  |
help: consider adding `let`
  |
3 |     if let Some(_) = x
  |        +++

error: aborting due to 4 previous errors

For more information about this error, try `rustc --explain E0308`.