Note expr being cast when encounter NonScalar cast error #140787

xizheyin · 2025-05-08T08:31:44Z

I added note for expr so that it doesn't treat &x as T as &(x as T) but (&x) as T. But I'm not sure if I want to add note for all NonScalar, maybe for specific expr_ty?

r? compiler

nnethercote

Most of the time this results in a longer message but no extra information, e.g.:

LL | fn main() { let u = 0u32 as (); }
   |                     ^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
   |
   = note: casting expr `0u32` as `()`

It's obvious that 0u32 is cast as (), it's right there above. Is it possible to only add the note when the type being cast to is partially/fully unspecified?

Also, in the issue-73886.rs we see some cases where the type is partially unspecified but the error message doesn't give any extra information:

   = note: casting expr `&&[0]` as `&[_]`
   = note: casting expr `7u32` as `Option<_>`

It would be nice if the _ were actually expanded here.

BTW, two thumbs up for adding the new test in the first commit and then doing the rest of the changes in the second commit. That makes reviewing easier.

compiler/rustc_hir_typeck/src/cast.rs

xizheyin · 2025-05-09T06:40:55Z

Thanks for review ：）

It's obvious that 0u32 is cast as (), it's right there above. Is it possible to only add the note when the type being cast to is partially/fully unspecified?

Indeed, it doesn't seem necessary to add duplicate notes for some simple types, and we can add notes when expressions are composite types such as references, for example, are added. However, the type may not be what this PR needs to address, but rather should indicate which expression is being converted.

error[E0605]: non-primitive cast: &&[i32; 1] as &[_]

In issue-73886.rs, the original message is here. I think this is a very good expansion to know what _ is and needs a separate PR. this PR is specific to #140491 and we may just need to hint at the expression.

BTW, two thumbs up for adding the new test in the first commit and then doing the rest of the changes in the second commit. That makes reviewing easier.

Yes, this was handed to me the last time you reviewed my PR, and I've been doing this ever since.

xizheyin

@rustbot ready

xizheyin · 2025-05-09T07:31:58Z

compiler/rustc_hir_typeck/src/cast.rs

+
+                if let Ok(snippet) = fcx.tcx.sess.source_map().span_to_snippet(self.expr_span)
+                    && matches!(self.expr.kind, ExprKind::AddrOf(..))
+                {
+                    err.note(format!("casting reference expression `{}`", snippet));
+                }
+


Now, it will emit note only when the expression is &....

tests/ui/coercion/issue-73886.stderr

Signed-off-by: xizheyin <xizheyin@smail.nju.edu.cn>

xizheyin · 2025-05-24T08:35:07Z

Never mind.

Is it worth explaining that & binds tighter than as?

I think it is a useful hint!

It would be even better if it could suggest &(&[0] as &[_]) as a fix

I'm concerned that simply suggesting parentheses may be inconsistent with the original intent of the code writer. There are a variety of situations that trigger such hints, such as &[0] as &&[_], which may require further categorization to refine the suggestions.

The current use of NOTE seems to be sufficient without being unnecessarily intrusive. Specific suggestion information can be left to be enhanced later. :)

@rustbot ready

nnethercote · 2025-05-25T09:23:16Z

tests/ui/coercion/issue-73886.stderr

@@ -3,6 +3,8 @@ error[E0605]: non-primitive cast: `&&[i32; 1]` as `&[_]`
   |
 LL |     let _ = &&[0] as &[_];
   |             ^^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
+   |
+   = note: casting reference expression `&&[0]`, where `&` binds tighter than `as`


I think because works better than where. Otherwise, this looks good.

Good suggestion. I will revise it later.

Signed-off-by: xizheyin <xizheyin@smail.nju.edu.cn>

xizheyin

@rustbot ready

xizheyin · 2025-05-31T04:20:45Z

tests/ui/cast/func-pointer-issue-140491.stderr

+LL | ..._>) = &my_fn as _;
+   |          ^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
+   |
+   = note: casting reference expression `&my_fn` because `&` binds tighter than `as`


because is indeed better.

nnethercote · 2025-05-31T05:19:11Z

@bors r+ rollup

bors · 2025-05-31T05:19:14Z

📌 Commit 17352e6 has been approved by nnethercote

It is now in the queue for this repository.

Rollup of 8 pull requests Successful merges: - #140787 (Note expr being cast when encounter NonScalar cast error) - #141112 (std: note that `std::str::from_utf8*` functions are aliases to `<str>::from_utf8*` methods) - #141646 (Document what `distcheck` is intended to exercise) - #141740 (Hir item kind field order) - #141793 (`tests/ui`: A New Order [1/N]) - #141805 (Update `compiler-builtins` to 0.1.160) - #141815 (Enable non-leaf Frame Pointers for mingw-w64 Arm64 Windows) - #141819 (Fixes for building windows-gnullvm hosts) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #140787 - xizheyin:issue-140491, r=nnethercote Note expr being cast when encounter NonScalar cast error Fixes #140491 I added note for `expr` so that it doesn't treat `&x as T` as `&(x as T)` but `(&x) as T`. But I'm not sure if I want to add note for all NonScalar, maybe for specific `expr_ty`? r? compiler

…iaskrgr Rollup of 8 pull requests Successful merges: - rust-lang#140787 (Note expr being cast when encounter NonScalar cast error) - rust-lang#141112 (std: note that `std::str::from_utf8*` functions are aliases to `<str>::from_utf8*` methods) - rust-lang#141646 (Document what `distcheck` is intended to exercise) - rust-lang#141740 (Hir item kind field order) - rust-lang#141793 (`tests/ui`: A New Order [1/N]) - rust-lang#141805 (Update `compiler-builtins` to 0.1.160) - rust-lang#141815 (Enable non-leaf Frame Pointers for mingw-w64 Arm64 Windows) - rust-lang#141819 (Fixes for building windows-gnullvm hosts) r? `@ghost` `@rustbot` modify labels: rollup

rustbot assigned nnethercote May 8, 2025

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Add ui test func-pointer-issue-140491

31ee840

Signed-off-by: xizheyin <xizheyin@smail.nju.edu.cn>

xizheyin force-pushed the issue-140491 branch from 67af15b to 88609fd Compare May 24, 2025 08:34

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Note ref expr being cast when encounter NonScalar cast error

17352e6

Signed-off-by: xizheyin <xizheyin@smail.nju.edu.cn>

xizheyin force-pushed the issue-140491 branch from 88609fd to 17352e6 Compare May 31, 2025 04:20

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Rollup of 8 pull requests #141824

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bors merged commit 05debb0 into rust-lang:master May 31, 2025
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