[RFC] rustc_parse: improve the error diagnostic for "missing let in let chain" #151493

Unique-Usman · 2026-01-22T15:04:10Z

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rustbot · 2026-01-22T15:04:15Z

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Unique-Usman · 2026-01-22T15:05:51Z

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 · 2026-01-22T15:06:21Z

@estebank

Signed-off-by: Usman Akinyemi <usmanakinyemi202@gmail.com>

Unique-Usman · 2026-01-27T21:35:33Z

r? @estebank

Unique-Usman · 2026-01-27T21:39:57Z

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 · 2026-01-27T21:43:54Z

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`.

estebank

I'm not sure I follow your last comment. The diagnostic you're showing doesn't correspond with the code. I think you meant the following?

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

If so, this is the output I see:

error: expected expression, found `let` statement
 --> foo.rs:4:12
  |
4 |         && let Some(x) = x
  |            ^^^
  |
  = note: -Ztrack-diagnostics: created at /rustc-dev/4fa80a5e733e2202d7ca4c203c2fdfda41cfe7dc/compiler/rustc_parse/src/parser/expr.rs:2775:43
  = 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: -Ztrack-diagnostics: created at /rustc-dev/4fa80a5e733e2202d7ca4c203c2fdfda41cfe7dc/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs:1905:35
  = 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: -Ztrack-diagnostics: created at /rustc-dev/4fa80a5e733e2202d7ca4c203c2fdfda41cfe7dc/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs:1905:35
  = 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 `()`
  |
  = note: -Ztrack-diagnostics: created at /rustc-dev/4fa80a5e733e2202d7ca4c203c2fdfda41cfe7dc/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs:1905:35
help: consider adding `let`
  |
3 |     if let Some(_) = x
  |        +++

And I think it's coming from a different part of the parser, in parse_expr_let, so it would require different specific handling.

View changes since this review

estebank · 2026-01-28T03:31:53Z

tests/ui/expr/if/bad-if-let-suggestion.stderr

 help: a function with a similar name exists
   |
 LL -     if (i + j) = i {}
 LL +     if (a + j) = i {}
   |


Unrelated: Noticing this suggestion and it is quite bad :-/

estebank · 2026-01-28T03:35:32Z

tests/ui/parser/let-chains-assign-add-incorrect.stderr

-error: binary assignment operation `+=` cannot be used in a let chain
-  --> $DIR/let-chains-assign-add-incorrect.rs:8:31
-   |
-LL |     if let _ = 1 && true && y += 2 {};
-   |        ---------------------- ^^ cannot use `+=` in a let chain
-   |        |
-   |        you are add-assigning the right-hand side expression to the result of this let-chain
-   |
-help: you might have meant to compare with `==` instead of assigning with `+=`
-   |
-LL -     if let _ = 1 && true && y += 2 {};
-LL +     if let _ = 1 && true && y == 2 {};
-   |


This change makes me think that we might want to restrict the silencing to cases where we had any suggestions. The need to remove the run-rustfix generally points at us giving less ideal output. That being said, this code as is is not great to begin with, and == would likely be intended... Can you see if we can make it so that we don't cause changes on this output while still silencing the others?

Yeah, that is valid. Looks quite weird to me also, I knew we had to do some restrictions, I will do that.

estebank · 2026-01-28T03:37:45Z

compiler/rustc_parse/src/parser/expr.rs

+                                    missing_let: self.missing_let,
+                                    comparison: self.comparison,
+                                });
+                            self.found_incorrect_let_chain = Some(guar);


I think all you'd have to do is gate this behind missing_let or comparison being Some.

Yeah, thanks. I did gate it earlier, but not around Some parts, so it didn’t work. I’ll gate it around it now.

Unique-Usman · 2026-01-28T04:55:38Z

I'm not sure I follow your last comment. The diagnostic you're showing doesn't correspond with the code. I think you meant the following?

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

If so, this is the output I see:

error: expected expression, found `let` statement
 --> foo.rs:4:12
  |
4 |         && let Some(x) = x
  |            ^^^
  |
  = note: -Ztrack-diagnostics: created at /rustc-dev/4fa80a5e733e2202d7ca4c203c2fdfda41cfe7dc/compiler/rustc_parse/src/parser/expr.rs:2775:43
  = 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: -Ztrack-diagnostics: created at /rustc-dev/4fa80a5e733e2202d7ca4c203c2fdfda41cfe7dc/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs:1905:35
  = 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: -Ztrack-diagnostics: created at /rustc-dev/4fa80a5e733e2202d7ca4c203c2fdfda41cfe7dc/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs:1905:35
  = 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 `()`
  |
  = note: -Ztrack-diagnostics: created at /rustc-dev/4fa80a5e733e2202d7ca4c203c2fdfda41cfe7dc/compiler/rustc_trait_selection/src/error_reYeahporting/infer/mod.rs:1905:35
help: consider adding `let`
  |
3 |     if let Some(_) = x
  |        +++

And I think it's coming from a different part of the parser, in parse_expr_let, so it would require different specific handling.

View changes since this review

Yeah, I meant this. Yeah, it needs to be handle in different part of the code.

rustbot assigned chenyukang Jan 22, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jan 22, 2026

Unique-Usman marked this pull request as draft January 22, 2026 15:06

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jan 22, 2026

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

f1467c2

Signed-off-by: Usman Akinyemi <usmanakinyemi202@gmail.com>

Unique-Usman force-pushed the ua/missinglet branch from 6b5f379 to f1467c2 Compare January 27, 2026 21:33

Unique-Usman marked this pull request as ready for review January 27, 2026 21:35

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jan 27, 2026

rustbot assigned estebank and unassigned chenyukang Jan 27, 2026

estebank reviewed Jan 28, 2026

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[RFC] rustc_parse: improve the error diagnostic for "missing let in let chain" #151493

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[RFC] rustc_parse: improve the error diagnostic for "missing let in let chain" #151493

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