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Clarify E0381 diagnostics for branch conditions #156603

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66 changes: 53 additions & 13 deletions compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -846,7 +846,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
&& !visitor
.errors
.iter()
.map(|(sp, _)| *sp)
.map(|error| error.span)
.any(|sp| span < sp && !sp.contains(span))
}) {
show_assign_sugg = true;
Expand Down Expand Up @@ -875,8 +875,9 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
err.span_label(span, format!("{path} {used} here but it {isnt_initialized}"));

let mut shown = false;
for (sp, label) in visitor.errors {
if sp < span && !sp.overlaps(span) {
let mut shown_condition_value = false;
for error in visitor.errors {
if error.span < span && !error.span.overlaps(span) {
// When we have a case like `match-cfg-fake-edges.rs`, we don't want to mention
// match arms coming after the primary span because they aren't relevant:
// ```
Expand All @@ -890,7 +891,8 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
// _ => {} // We don't want to point to this.
// };
// ```
err.span_label(sp, label);
shown_condition_value |= error.kind.describes_condition_value();
err.span_label(error.span, error.label);
shown = true;
}
}
Expand All @@ -903,6 +905,12 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
}

err.span_label(decl_span, "binding declared here but left uninitialized");
if shown_condition_value {
err.note(
"when checking initialization, the compiler describes possible control-flow paths \
without evaluating whether branch conditions can actually have the values shown",
);
}
if show_assign_sugg {
struct LetVisitor {
decl_span: Span,
Expand Down Expand Up @@ -4693,7 +4701,31 @@ struct ConditionVisitor<'tcx> {
tcx: TyCtxt<'tcx>,
spans: Vec<Span>,
name: String,
errors: Vec<(Span, String)>,
errors: Vec<ConditionError>,
}

struct ConditionError {
span: Span,
label: String,
kind: ConditionErrorKind,
}

impl ConditionError {
fn new(span: Span, kind: ConditionErrorKind, label: String) -> Self {
Self { span, label, kind }
}
}

#[derive(Clone, Copy)]
enum ConditionErrorKind {
ConditionValue,
Other,
}

impl ConditionErrorKind {
fn describes_condition_value(self) -> bool {
matches!(self, Self::ConditionValue)
}
}

impl<'v, 'tcx> Visitor<'v> for ConditionVisitor<'tcx> {
Expand All @@ -4703,15 +4735,17 @@ impl<'v, 'tcx> Visitor<'v> for ConditionVisitor<'tcx> {
// `if` expressions with no `else` that initialize the binding might be missing an
// `else` arm.
if ReferencedStatementsVisitor(&self.spans).visit_expr(body).is_break() {
self.errors.push((
self.errors.push(ConditionError::new(
cond.span,
ConditionErrorKind::ConditionValue,
format!(
"if this `if` condition is `false`, {} is not initialized",
self.name,
),
));
self.errors.push((
self.errors.push(ConditionError::new(
ex.span.shrink_to_hi(),
ConditionErrorKind::Other,
format!("an `else` arm might be missing here, initializing {}", self.name),
));
}
Expand All @@ -4725,17 +4759,19 @@ impl<'v, 'tcx> Visitor<'v> for ConditionVisitor<'tcx> {
(true, true) | (false, false) => {}
(true, false) => {
if other.span.is_desugaring(DesugaringKind::WhileLoop) {
self.errors.push((
self.errors.push(ConditionError::new(
cond.span,
ConditionErrorKind::ConditionValue,
format!(
"if this condition isn't met and the `while` loop runs 0 \
times, {} is not initialized",
self.name
),
));
} else {
self.errors.push((
self.errors.push(ConditionError::new(
body.span.shrink_to_hi().until(other.span),
ConditionErrorKind::ConditionValue,
format!(
"if the `if` condition is `false` and this `else` arm is \
executed, {} is not initialized",
Expand All @@ -4745,8 +4781,9 @@ impl<'v, 'tcx> Visitor<'v> for ConditionVisitor<'tcx> {
}
}
(false, true) => {
self.errors.push((
self.errors.push(ConditionError::new(
cond.span,
ConditionErrorKind::ConditionValue,
format!(
"if this condition is `true`, {} is not initialized",
self.name
Expand All @@ -4766,8 +4803,9 @@ impl<'v, 'tcx> Visitor<'v> for ConditionVisitor<'tcx> {
for (arm, seen) in arms.iter().zip(results) {
if !seen {
if loop_desugar == hir::MatchSource::ForLoopDesugar {
self.errors.push((
self.errors.push(ConditionError::new(
e.span,
ConditionErrorKind::Other,
format!(
"if the `for` loop runs 0 times, {} is not initialized",
self.name
Expand All @@ -4780,8 +4818,9 @@ impl<'v, 'tcx> Visitor<'v> for ConditionVisitor<'tcx> {
) {
continue;
}
self.errors.push((
self.errors.push(ConditionError::new(
arm.pat.span.to(guard.span),
ConditionErrorKind::ConditionValue,
format!(
Comment thread
eval-exec marked this conversation as resolved.
"if this pattern and condition are matched, {} is not \
initialized",
Expand All @@ -4795,8 +4834,9 @@ impl<'v, 'tcx> Visitor<'v> for ConditionVisitor<'tcx> {
) {
continue;
}
self.errors.push((
self.errors.push(ConditionError::new(
arm.pat.span,
ConditionErrorKind::Other,
format!(
"if this pattern is matched, {} is not initialized",
self.name
Expand Down
2 changes: 2 additions & 0 deletions tests/ui/async-await/no-non-guaranteed-initialization.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -10,6 +10,8 @@ LL | }
| - an `else` arm might be missing here, initializing `y`
LL | y
| ^ `y` used here but it is possibly-uninitialized
|
= note: when checking initialization, the compiler describes possible control-flow paths without evaluating whether branch conditions can actually have the values shown

error: aborting due to 1 previous error

Expand Down
2 changes: 2 additions & 0 deletions tests/ui/borrowck/borrowck-if-no-else.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -8,6 +8,8 @@ LL | let x: isize; if 1 > 2 { x = 10; }
| binding declared here but left uninitialized
LL | foo(x);
| ^ `x` used here but it is possibly-uninitialized
|
= note: when checking initialization, the compiler describes possible control-flow paths without evaluating whether branch conditions can actually have the values shown

error: aborting due to 1 previous error

Expand Down
2 changes: 2 additions & 0 deletions tests/ui/borrowck/borrowck-if-with-else.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -8,6 +8,8 @@ LL | if 1 > 2 {
...
LL | foo(x);
| ^ `x` used here but it is possibly-uninitialized
|
= note: when checking initialization, the compiler describes possible control-flow paths without evaluating whether branch conditions can actually have the values shown

error: aborting due to 1 previous error

Expand Down
2 changes: 2 additions & 0 deletions tests/ui/borrowck/borrowck-while-break.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -8,6 +8,8 @@ LL | while cond {
...
LL | println!("{}", v);
| ^ `v` used here but it is possibly-uninitialized
|
= note: when checking initialization, the compiler describes possible control-flow paths without evaluating whether branch conditions can actually have the values shown

error: aborting due to 1 previous error

Expand Down
2 changes: 2 additions & 0 deletions tests/ui/borrowck/borrowck-while.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,8 @@ LL | while 1 == 1 { x = 10; }
| ------ if this condition isn't met and the `while` loop runs 0 times, `x` is not initialized
LL | return x;
| ^ `x` used here but it is possibly-uninitialized
|
= note: when checking initialization, the compiler describes possible control-flow paths without evaluating whether branch conditions can actually have the values shown

error: aborting due to 1 previous error

Expand Down
55 changes: 55 additions & 0 deletions tests/ui/borrowck/branch-condition-flow-issue-156494.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,55 @@
//@ edition: 2024

fn main() {
let v1: Vec<i32> = vec![1, 2, 3];
let mut iter = v1.iter();

let mut first_encounter = true;
let mut token: i32;
let mut following_token: i32;
loop {
if first_encounter {
if let Some(token) = iter.next() {
match iter.next() {
Some(temp) => {
following_token = *temp;
first_encounter = false;
println!("{} followed by {}", token, following_token);
}
_ => {
println!("{} is last", token);
}
}
} else {
break;
}
} else {
token = following_token; //~ ERROR used binding `following_token` isn't initialized
first_encounter = true;
match iter.next() {
Some(temp) => {
following_token = *temp;
first_encounter = false;
println!("{} followed by {}", token, following_token);
}
_ => {
println!("{} is last", token);
}
}
}
}
}

fn guarded_match(value: i32, condition: bool) {
let y;
match value {
0 => {
y = 1;
}
_ if condition => {}
_ => {
y = 2;
}
}
let _z = y; //~ ERROR used binding `y` is possibly-uninitialized
}
38 changes: 38 additions & 0 deletions tests/ui/borrowck/branch-condition-flow-issue-156494.stderr
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Original file line number Diff line number Diff line change
@@ -0,0 +1,38 @@
error[E0381]: used binding `following_token` isn't initialized
--> $DIR/branch-condition-flow-issue-156494.rs:27:21
|
LL | let mut following_token: i32;
| ------------------- binding declared here but left uninitialized
...
LL | _ => {
| - if this pattern is matched, `following_token` is not initialized
...
LL | } else {
| ------ if the `if` condition is `false` and this `else` arm is executed, `following_token` is not initialized
...
LL | token = following_token;
| ^^^^^^^^^^^^^^^ `following_token` used here but it isn't initialized
|
= note: when checking initialization, the compiler describes possible control-flow paths without evaluating whether branch conditions can actually have the values shown
help: consider assigning a value
|
LL | let mut following_token: i32 = 42;
| ++++

error[E0381]: used binding `y` is possibly-uninitialized
--> $DIR/branch-condition-flow-issue-156494.rs:54:14
|
LL | let y;
| - binding declared here but left uninitialized
...
LL | _ if condition => {}
| -------------- if this pattern and condition are matched, `y` is not initialized
...
LL | let _z = y;
| ^ `y` used here but it is possibly-uninitialized
|
= note: when checking initialization, the compiler describes possible control-flow paths without evaluating whether branch conditions can actually have the values shown

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0381`.
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