fix: more descriptive error message for enum to integer by Jaidenmagnan · Pull Request #151122 · rust-lang/rust

Jaidenmagnan · 2026-01-14T15:58:42Z

Fixes #151116
A more descriptive error message when casting an enum to an Integer. Please review issue linked above.

rustbot · 2026-01-14T15:58:48Z

rustbot has assigned @chenyukang.
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reddevilmidzy · 2026-01-14T16:21:50Z

+
+fn main() {
+    let priority = &Priority::Normal;
+    let priority = priority as u8; //~ ERROR casting `&Priority` as `u8` is invalid


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

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

Ty! will add this.

chenyukang · 2026-01-17T16:58:48Z

you can use Fixes #151116 in PR description to link origin issue, so that if the PR is merged the issue will be closed automatically.

there is no need to paste a screenshot of CI testing, since if any tests in CI failed, we will get it in Github action .

Jaidenmagnan · 2026-01-17T17:10:26Z

you can use Fixes #151116 in PR description to link origin issue, so that if the PR is merged the issue will be closed automatically.

there is no need to paste a screenshot of CI testing, since if any tests in CI failed, we will get it in Github action .

ty, just fixed this.

chenyukang · 2026-01-17T17:21:11Z

+                            self.expr_span.shrink_to_lo(),
+                            "dereference the expression",
+                            "*",
+                            Applicability::MachineApplicable,


prefer to use MaybeIncorrect?

since if we apply the suggestion there maybe still some errors:

error[E0507]: cannot move out of `*priority` which is behind a shared reference --> tests/ui/cast/cast-enum-to-int-issue-151116.rs:10:20 | 10 | let priority = *priority as u8; //~ ERROR casting `&Priority` as `u8` is invalid | ^^^^^^^^^ move occurs because `*priority` has type `Priority`, which does not implement the `Copy` trait | note: if `Priority` implemented `Clone`, you could clone the value --> tests/ui/cast/cast-enum-to-int-issue-151116.rs:2:1 | 2 | enum Priority { | ^^^^^^^^^^^^^ consider implementing `Clone` for this type ... 10 | let priority = *priority as u8; //~ ERROR casting `&Priority` as `u8` is invalid | --------- you could clone this value

chenyukang · 2026-02-10T01:17:32Z

@rustbot author

rustbot · 2026-02-10T01:17:36Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

Jaidenmagnan · 2026-02-10T04:22:39Z

I will fix this up next week, thanks! Didn't see this.

rustbot · 2026-04-26T18:48:43Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

Jaidenmagnan · 2026-04-26T20:37:17Z

@rustbot ready

chenyukang · 2026-04-27T00:39:03Z

+                        err.span_suggestion_verbose(
+                            self.expr_span.shrink_to_lo(),
+                            "dereference the expression",
+                            "*",
+                            Applicability::MaybeIncorrect,
+                        );


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

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, ); }

chenyukang · 2026-04-27T00:39:52Z

+#[repr(u8)]
+enum Priority {
+    High = 255,
+    Normal = 127,


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

chenyukang · 2026-05-05T08:08:10Z

@rustbot author

Jaidenmagnan · 2026-05-05T20:38:31Z

@rustbot ready

chenyukang · 2026-05-05T23:25:57Z

Thanks!
@bors r=chenyukang

rust-bors · 2026-05-05T23:26:00Z

📌 Commit 0f5d89f has been approved by chenyukang

It is now in the queue for this repository.

fix: more descriptive error message for enum to integer Fixes rust-lang#151116 A more descriptive error message when casting an enum to an Integer. Please review issue linked above.

Rollup of 15 pull requests Successful merges: - #151122 (fix: more descriptive error message for enum to integer) - #155341 (generic_const_args: allow paths to non type consts) - #156062 (Added command-line argument support for `wasm32-wali-linux-musl`) - #156159 ([AIX] add -bdbg:namedsects:ss link arg) - #156174 (Wasm: remove implicit `__heap_base`/`__data_end` exports) - #156186 (fix: remap ci-llvm debug paths via `-ffile-prefix-map`) - #156193 (port `rustc_ast*` crates from `box_` to `deref_patterns`) - #156201 (Don't run ui-fulldeps tests twice in stage 1) - #155808 (Always use `ConstFn` context for `const` closures) - #156105 (interpret: correctly deal with repr(transparent) enums) - #156148 (Use `all_impls` instead of handrolling it) - #156156 (Adjust getMCSubtargetInfo signature for LLVM 23+) - #156170 (add known-bug test for coroutine 'static-yields-non-'static unsoundness (#144442)) - #156195 (Move tests codegen) - #156205 (move generalization test)

rustbot assigned chenyukang Jan 14, 2026

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reddevilmidzy reviewed Jan 14, 2026

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chenyukang reviewed Jan 17, 2026

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Comment thread tests/ui/cast/cast-enum-to-int-issue-151116.rs

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Jaidenmagnan added 3 commits April 26, 2026 15:05

fix: more descriptive error message for enum to integer

da13353

fix: add the HELP message

53f0ce8

feat: fixing PR feedback

9f43e6d

Jaidenmagnan force-pushed the main branch from 9d8ccb1 to 9f43e6d Compare April 26, 2026 19:07

rustbot removed has-merge-commits PR has merge commits, merge with caution. S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Apr 26, 2026

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Apr 26, 2026

chenyukang reviewed Apr 27, 2026

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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 May 5, 2026

fix: adding more verbose error message

61743bf

Jaidenmagnan requested a review from chenyukang May 5, 2026 20:34

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 May 5, 2026

fix: CopyPriority addition

0f5d89f

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels May 5, 2026

jhpratt mentioned this pull request May 6, 2026

Rollup of 15 pull requests #156217

Open

	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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