CTFE error shown thrice #52922

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1. RalfJung commented on Jul 31, 2018

   

   RalfJung

   on Jul 31, 2018

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   It gets even better when having warn(const_err): Then we get if 4 times.

   warning: this constant cannot be used
    --> src/main.rs:8:1
     |
   8 | const BAD: usize = unsafe { Foo { a: &1 }.b * 2};
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^-------------------^^
     |                             |
     |                             "pointer arithmetic or comparison" needs an rfc before being allowed inside constants
     |
   note: lint level defined here
    --> src/main.rs:1:9
     |
   1 | #![warn(const_err)]
     |         ^^^^^^^^^
   
   error[E0080]: referenced constant has errors
     --> src/main.rs:10:13
      |
   8  | const BAD: usize = unsafe { Foo { a: &1 }.b * 2};
      |                             ------------------- "pointer arithmetic or comparison" needs an rfc before being allowed inside constants
   9  | fn main() {
   10 |     let f = BAD;
      |             ^^^
   
   error[E0080]: could not evaluate constant
     --> src/main.rs:10:13
      |
   10 |     let f = BAD;
      |             ^^^ referenced constant has errors
   
   error[E0080]: constant evaluation error
    --> src/main.rs:8:1
     |
   8 | const BAD: usize = unsafe { Foo { a: &1 }.b * 2};
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^-------------------^^
     |                             |
     |                             "pointer arithmetic or comparison" needs an rfc before being allowed inside constants
   
2. RalfJung commented on Aug 1, 2018

   

   RalfJung

   on Aug 1, 2018

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   From conversation with @oli-obk:

   The proper way to fix this is to move the errors/warnings into the query, instead of having to do this at all places where the query is made. This is both the right fix for #52849 and avoids duplicate errors.

   So the const_eval query would be renamed to const_eval_raw and only used internally. There would be three new queries eval_const, eval_static, eval_promoted with different behavior wrt. showing an error or a warning, and whether to do validation of the final value.

   The lint would just trigger all these queries. Thanks to caching, a warning triggered by a query is only ever shown once.
3. RalfJung commented on Nov 9, 2018

   

   RalfJung

   on Nov 9, 2018

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   Thanks to @oli-obk's const-query-error PR, this is now fixed.

   @oli-obk should this get a dedicated regression test? My impression is that we already have tons of tests like this.
4. oli-obk commented on Nov 9, 2018

   

   oli-obk

   on Nov 9, 2018

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   Yea, there's enough things testing this. The only concern I see now is that if you allow(const_err) you have no idea anymore what's going on with the constant, rustc just tells you

   error[E0080]: could not evaluate constant
     --> src/main.rs:10:13
      |
   10 |     let f = BAD;
      |             ^^^ referenced constant has errors
   

   without any further info.
5. estebank

   added

   A-diagnosticsArea: Messages for errors, warnings, and lintsArea: Messages for errors, warnings, and lints

   on Jan 8, 2019
6. estebank commented on Jan 8, 2019

   

   estebank

   on Jan 8, 2019

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   Closing as per #53821:

   error: any use of this value will cause an error
    --> src/main.rs:8:1
     |
   8 | const BAD: usize = unsafe { Foo { a: &1 }.b * 2};
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^-------------------^^
     |                             |
     |                             "pointer arithmetic or comparison" needs an rfc before being allowed inside constants
     |
     = note: #[deny(const_err)] on by default
   
7. estebank

   closed this as completedon Jan 8, 2019