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fix: more descriptive error message for enum to integer #151122

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rust-bors merged 5 commits into from
May 6, 2026
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fix: more descriptive error message for enum to integer #151122

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da13353
fix: more descriptive error message for enum to integer
Jaidenmagnan Jan 14, 2026
53f0ce8
fix: add the HELP message
Jaidenmagnan Jan 14, 2026
9f43e6d
feat: fixing PR feedback
Jaidenmagnan Apr 26, 2026
61743bf
fix: adding more verbose error message
Jaidenmagnan May 5, 2026
0f5d89f
fix: CopyPriority addition
Jaidenmagnan May 5, 2026
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38 changes: 33 additions & 5 deletions compiler/rustc_hir_typeck/src/cast.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -291,12 +291,40 @@ impl<'a, 'tcx> CastCheck<'tcx> {
CastError::NeedViaThinPtr | CastError::NeedViaPtr => {
let mut err =
make_invalid_casting_error(self.span, self.expr_ty, self.cast_ty, fcx);

if self.cast_ty.is_integral() {
err.help(format!("cast through {} first", match e {
CastError::NeedViaPtr => "a raw pointer",
CastError::NeedViaThinPtr => "a thin pointer",
e => unreachable!("control flow means we should never encounter a {e:?}"),
}));
if !matches!(self.expr.kind, ExprKind::AddrOf(..))
&& let ty::Ref(_, inner_ty, _) = *self.expr_ty.kind()
&& let ty::Adt(adt_def, _) = *inner_ty.kind()
&& adt_def.is_enum()
&& adt_def.is_payloadfree()
{
err.span_suggestion_verbose(
self.expr_span.shrink_to_lo(),
"try dereferencing before the cast",
"*",
Applicability::MaybeIncorrect,
);
Comment on lines +302 to +307
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@chenyukang chenyukang Apr 27, 2026 •
edited by rustbot
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The current version does not imply the suggestion is not correct.
maybe this is more verbose and also give tip for type without Copy derive:

Suggested change
err.span_suggestion_verbose(
self.expr_span.shrink_to_lo(),
"dereference the expression",
"*",
Applicability::MaybeIncorrect,
);
let help = format!(
"try dereferencing before the cast{}",
if !fcx.type_is_copy_modulo_regions(fcx.param_env, inner_ty) {
format!(
", you also need to add `#[derive(Copy, Clone)]` to the enum definition",
)
} else {
String::new()
}
);
err.span_suggestion_verbose(
self.expr_span.shrink_to_lo(),
help,
"*",
Applicability::MaybeIncorrect,
);

View changes since the review

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@chenyukang chenyukang Apr 27, 2026 •
edited
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or maybe this is better:

err.span_suggestion_verbose(
    self.expr_span.shrink_to_lo(),
    "try dereferencing before the cast",
    "*",
    Applicability::MaybeIncorrect,
);
if !fcx.type_is_copy_modulo_regions(fcx.param_env, inner_ty) {
    err.span_suggestion_verbose(
        fcx.tcx.def_span(adt_def.did()).shrink_to_lo(),
        "add `#[derive(Copy, Clone)]` to the enum definition",
        "#[derive(Copy, Clone)]\n",
        Applicability::MaybeIncorrect,
    );
}

if !fcx.type_is_copy_modulo_regions(fcx.param_env, inner_ty) {
err.span_suggestion_verbose(
fcx.tcx.def_span(adt_def.did()).shrink_to_lo(),
"add `#[derive(Copy, Clone)]` to the enum definition",
"#[derive(Copy, Clone)]\n",
Applicability::MaybeIncorrect,
);
}
} else {
err.help(format!(
"cast through {} first",
match e {
CastError::NeedViaPtr => "a raw pointer",
CastError::NeedViaThinPtr => "a thin pointer",
e => unreachable!(
"control flow means we should never encounter a {e:?}"
),
}
));
}
}

self.try_suggest_collection_to_bool(fcx, &mut err);
Expand Down
27 changes: 27 additions & 0 deletions tests/ui/cast/cast-enum-to-int-issue-151116.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,27 @@
#[repr(u8)]
enum Priority { //~ HELP: add `#[derive(Copy, Clone)]` to the enum definition
High = 255,
Normal = 127,
Low = 1,
}

#[repr(u8)]
#[derive(Copy, Clone)]
enum CopyPriority {
High = 255,
Normal = 127,
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@chenyukang chenyukang Apr 27, 2026 •
edited by rustbot
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and we add a new type:

#[derive(Copy, Clone)]
#[repr(u8)]
enum CopyPriority {
    High = 255,
    Normal = 127,
    Low = 1,
}

for https://github.com/rust-lang/rust/pull/151122/changes#r3144432881

View changes since the review

Low = 1,
}

fn main() {
let priority = &Priority::Normal;
let priority = priority as u8; //~ ERROR casting `&Priority` as `u8` is invalid
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@reddevilmidzy reddevilmidzy Jan 14, 2026

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It might be a good idea to check the HELP message as well.
like

Suggested change
let priority = priority as u8; //~ ERROR casting `&Priority` as `u8` is invalid
let priority = priority as u8; //~ ERROR casting `&Priority` as `u8` is invalid
//~| HELP: dereference the expression

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@Jaidenmagnan Jaidenmagnan Jan 14, 2026

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It might be a good idea to check the HELP message as well. like

Ty! will add this.

Comment thread
Jaidenmagnan marked this conversation as resolved.
//~| HELP: try dereferencing before the cast

let priority = &Priority::Normal as u8; //~ ERROR casting `&Priority` as `u8` is invalid
//~| HELP: cast through a raw pointer first

let priority = &CopyPriority::Normal;
let priority = priority as u8; //~ ERROR casting `&CopyPriority` as `u8` is invalid
//~| HELP: try dereferencing before the cast
}
38 changes: 38 additions & 0 deletions tests/ui/cast/cast-enum-to-int-issue-151116.stderr
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Original file line number Diff line number Diff line change
@@ -0,0 +1,38 @@
error[E0606]: casting `&Priority` as `u8` is invalid
--> $DIR/cast-enum-to-int-issue-151116.rs:18:20
|
LL | let priority = priority as u8;
| ^^^^^^^^^^^^^^
|
help: try dereferencing before the cast
|
LL | let priority = *priority as u8;
| +
help: add `#[derive(Copy, Clone)]` to the enum definition
|
LL + #[derive(Copy, Clone)]
LL | enum Priority {
|

error[E0606]: casting `&Priority` as `u8` is invalid
--> $DIR/cast-enum-to-int-issue-151116.rs:21:20
|
LL | let priority = &Priority::Normal as u8;
| ^^^^^^^^^^^^^^^^^^^^^^^
|
= help: cast through a raw pointer first

error[E0606]: casting `&CopyPriority` as `u8` is invalid
--> $DIR/cast-enum-to-int-issue-151116.rs:25:20
|
LL | let priority = priority as u8;
| ^^^^^^^^^^^^^^
|
help: try dereferencing before the cast
|
LL | let priority = *priority as u8;
| +

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0606`.
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