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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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53 changes: 40 additions & 13 deletions compiler/rustc_parse/src/parser/expr.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2760,9 +2760,13 @@ impl<'a> Parser<'a> {
let (mut cond, _) =
self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL | Restrictions::ALLOW_LET, attrs)?;

CondChecker::new(self, let_chains_policy).visit_expr(&mut cond);

Ok(cond)
let mut checker = CondChecker::new(self, let_chains_policy);
checker.visit_expr(&mut cond);
Ok(if let Some(guar) = checker.found_incorrect_let_chain {
self.mk_expr_err(cond.span, guar)
} else {
cond
})
}

/// Parses a `let $pat = $expr` pseudo-expression.
Expand Down Expand Up @@ -3484,13 +3488,19 @@ impl<'a> Parser<'a> {
let if_span = self.prev_token.span;
let mut cond = self.parse_match_guard_condition()?;

CondChecker::new(self, LetChainsPolicy::AlwaysAllowed).visit_expr(&mut cond);
let mut checker = CondChecker::new(self, LetChainsPolicy::AlwaysAllowed);
checker.visit_expr(&mut cond);

if has_let_expr(&cond) {
let span = if_span.to(cond.span);
self.psess.gated_spans.gate(sym::if_let_guard, span);
}
Ok(Some(cond))

Ok(Some(if let Some(guar) = checker.found_incorrect_let_chain {
self.mk_expr_err(cond.span, guar)
} else {
cond
}))
}

fn parse_match_arm_pat_and_guard(&mut self) -> PResult<'a, (Pat, Option<Box<Expr>>)> {
Expand All @@ -3511,13 +3521,23 @@ impl<'a> Parser<'a> {
let ast::PatKind::Paren(subpat) = pat.kind else { unreachable!() };
let ast::PatKind::Guard(_, mut cond) = subpat.kind else { unreachable!() };
self.psess.gated_spans.ungate_last(sym::guard_patterns, cond.span);
CondChecker::new(self, LetChainsPolicy::AlwaysAllowed).visit_expr(&mut cond);
let mut checker = CondChecker::new(self, LetChainsPolicy::AlwaysAllowed);
checker.visit_expr(&mut cond);

let right = self.prev_token.span;
self.dcx().emit_err(errors::ParenthesesInMatchPat {
span: vec![left, right],
sugg: errors::ParenthesesInMatchPatSugg { left, right },
});
Ok((self.mk_pat(span, ast::PatKind::Wild), Some(cond)))

Ok((
self.mk_pat(span, ast::PatKind::Wild),
(if let Some(guar) = checker.found_incorrect_let_chain {
Some(self.mk_expr_err(cond.span, guar))
} else {
Some(cond)
}),
))
} else {
Ok((pat, self.parse_match_arm_guard()?))
}
Expand Down Expand Up @@ -4208,6 +4228,7 @@ struct CondChecker<'a> {
forbid_let_reason: Option<ForbiddenLetReason>,
missing_let: Option<errors::MaybeMissingLet>,
comparison: Option<errors::MaybeComparison>,
found_incorrect_let_chain: Option<ErrorGuaranteed>,
}

impl<'a> CondChecker<'a> {
Expand All @@ -4218,6 +4239,7 @@ impl<'a> CondChecker<'a> {
missing_let: None,
comparison: None,
let_chains_policy,
found_incorrect_let_chain: None,
depth: 0,
}
}
Expand All @@ -4236,12 +4258,17 @@ impl MutVisitor for CondChecker<'_> {
NotSupportedOr(or_span) => {
self.parser.dcx().emit_err(errors::OrInLetChain { span: or_span })
}
_ => self.parser.dcx().emit_err(errors::ExpectedExpressionFoundLet {
span,
reason,
missing_let: self.missing_let,
comparison: self.comparison,
}),
_ => {
let guar =
self.parser.dcx().emit_err(errors::ExpectedExpressionFoundLet {
span,
reason,
missing_let: self.missing_let,
comparison: self.comparison,
});
self.found_incorrect_let_chain = Some(guar);
guar
}
};
*recovered = Recovered::Yes(error);
} else if self.depth > 1 {
Expand Down
4 changes: 1 addition & 3 deletions tests/ui/expr/if/bad-if-let-suggestion.rs
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Original file line number Diff line number Diff line change
@@ -1,8 +1,6 @@
fn a() {
if let x = 1 && i = 2 {}
//~^ ERROR cannot find value `i` in this scope
//~| ERROR mismatched types
//~| ERROR expected expression, found `let` statement
//~^ ERROR expected expression, found `let` statement
}

fn b() {
Expand Down
30 changes: 6 additions & 24 deletions tests/ui/expr/if/bad-if-let-suggestion.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -15,13 +15,7 @@ LL | if let x = 1 && i == 2 {}
| +

error[E0425]: cannot find value `i` in this scope
--> $DIR/bad-if-let-suggestion.rs:2:21
|
LL | if let x = 1 && i = 2 {}
| ^ not found in this scope

error[E0425]: cannot find value `i` in this scope
--> $DIR/bad-if-let-suggestion.rs:9:9
--> $DIR/bad-if-let-suggestion.rs:7:9
|
LL | fn a() {
| ------ similarly named function `a` defined here
Expand All @@ -36,7 +30,7 @@ LL + if (a + j) = i {}
|

error[E0425]: cannot find value `j` in this scope
--> $DIR/bad-if-let-suggestion.rs:9:13
--> $DIR/bad-if-let-suggestion.rs:7:13
|
LL | fn a() {
| ------ similarly named function `a` defined here
Expand All @@ -51,7 +45,7 @@ LL + if (i + a) = i {}
|

error[E0425]: cannot find value `i` in this scope
--> $DIR/bad-if-let-suggestion.rs:9:18
--> $DIR/bad-if-let-suggestion.rs:7:18
|
LL | fn a() {
| ------ similarly named function `a` defined here
Expand All @@ -66,7 +60,7 @@ LL + if (i + j) = a {}
|

error[E0425]: cannot find value `x` in this scope
--> $DIR/bad-if-let-suggestion.rs:16:8
--> $DIR/bad-if-let-suggestion.rs:14:8
|
LL | fn a() {
| ------ similarly named function `a` defined here
Expand All @@ -80,18 +74,6 @@ LL - if x[0] = 1 {}
LL + if a[0] = 1 {}
|

error[E0308]: mismatched types
--> $DIR/bad-if-let-suggestion.rs:2:8
|
LL | if let x = 1 && i = 2 {}
| ^^^^^^^^^^^^^^^^^^ expected `bool`, found `()`
|
help: you might have meant to compare for equality
|
LL | if let x = 1 && i == 2 {}
| +

error: aborting due to 7 previous errors
error: aborting due to 5 previous errors

Some errors have detailed explanations: E0308, E0425.
For more information about an error, try `rustc --explain E0308`.
For more information about this error, try `rustc --explain E0425`.
36 changes: 0 additions & 36 deletions tests/ui/parser/let-chains-assign-add-incorrect.fixed
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This file was deleted.

14 changes: 0 additions & 14 deletions tests/ui/parser/let-chains-assign-add-incorrect.rs
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Original file line number Diff line number Diff line change
@@ -1,5 +1,4 @@
//@ edition:2024
//@ run-rustfix

#![allow(irrefutable_let_patterns)]

Expand All @@ -8,26 +7,13 @@ fn test_where_left_is_not_let() {
if let _ = 1 && true && y += 2 {};
//~^ ERROR expected expression, found `let` statement
//~| NOTE only supported directly in conditions of `if` and `while` expressions
//~| ERROR mismatched types
//~| NOTE expected `bool`, found integer
//~| NOTE you are add-assigning the right-hand side expression to the result of this let-chain
//~| NOTE expected because this is `bool`
//~| ERROR binary assignment operation `+=` cannot be used in a let chain
//~| NOTE cannot use `+=` in a let chain
//~| HELP you might have meant to compare with `==` instead of assigning with `+=`
}

fn test_where_left_is_let() {
let y = 2;
if let _ = 1 && y += 2 {};
//~^ ERROR expected expression, found `let` statement
//~| NOTE only supported directly in conditions of `if` and `while` expressions
//~| ERROR mismatched types
//~| NOTE expected `bool`, found integer
//~| NOTE you are add-assigning the right-hand side expression to the result of this let-chain
//~| ERROR binary assignment operation `+=` cannot be used in a let chain
//~| NOTE cannot use `+=` in a let chain
//~| HELP you might have meant to compare with `==` instead of assigning with `+=`
}

fn main() {
Expand Down
49 changes: 3 additions & 46 deletions tests/ui/parser/let-chains-assign-add-incorrect.stderr
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Original file line number Diff line number Diff line change
@@ -1,61 +1,18 @@
error: expected expression, found `let` statement
--> $DIR/let-chains-assign-add-incorrect.rs:8:8
--> $DIR/let-chains-assign-add-incorrect.rs:7:8
|
LL | if let _ = 1 && true && y += 2 {};
| ^^^^^^^^^
|
= note: only supported directly in conditions of `if` and `while` expressions

error: expected expression, found `let` statement
--> $DIR/let-chains-assign-add-incorrect.rs:22:8
--> $DIR/let-chains-assign-add-incorrect.rs:14:8
|
LL | if let _ = 1 && y += 2 {};
| ^^^^^^^^^
|
= note: only supported directly in conditions of `if` and `while` expressions

error[E0308]: mismatched types
--> $DIR/let-chains-assign-add-incorrect.rs:8:29
|
LL | if let _ = 1 && true && y += 2 {};
| ----------------- ^ expected `bool`, found integer
| |
| expected because this is `bool`

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 {};
|

error[E0308]: mismatched types
--> $DIR/let-chains-assign-add-incorrect.rs:22:21
|
LL | if let _ = 1 && y += 2 {};
| ^ expected `bool`, found integer

error: binary assignment operation `+=` cannot be used in a let chain
--> $DIR/let-chains-assign-add-incorrect.rs:22:23
|
LL | if let _ = 1 && 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 && y += 2 {};
LL + if let _ = 1 && y == 2 {};
|

error: aborting due to 6 previous errors
error: aborting due to 2 previous errors

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