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Help is not helpful for error #37338

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est31 opened this issue Oct 22, 2016 · 2 comments
Closed

Help is not helpful for error #37338

est31 opened this issue Oct 22, 2016 · 2 comments
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A-diagnostics Area: Messages for errors, warnings, and lints

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@est31
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est31 commented Oct 22, 2016

If you try to compile the code (play link):

vec![0.0].iter().map(|s| s as i16).collect::<Vec<i16>>();

You get the error

rustc 1.14.0-nightly (6e8f92f11 2016-10-07)
error: casting `&f64` as `i16` is invalid
 --> <anon>:2:30
  |
2 |     vec![0.0].iter().map(|s| s as i16).collect();
  |                              ^^^^^^^^
  |
  = help: cast through a raw pointer first

error: aborting due to previous error

But casting through a raw pointer (s as *const f64 as *const i16 as i16) is not the idiomatic way IMO. Instead it should be fixed by simply dereferencing:

vec![0.0].iter().map(|s| *s as i16).collect();
@bluss bluss added the A-diagnostics Area: Messages for errors, warnings, and lints label Oct 22, 2016
@fhartwig
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But casting through a raw pointer (s as *const f64 as *const i16 as i16) is not the idiomatic way IMO.

It's not just unidiomatic, it actually does something completely different.

@GuillaumeGomez
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Funny thing: casting *const f32 as i16 doesn't throw errors. I replace this error with another one which fit better.

@GuillaumeGomez GuillaumeGomez mentioned this issue Oct 24, 2016
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@estebank estebank mentioned this issue Oct 28, 2016
Merged
bors added a commit that referenced this issue Nov 17, 2016
@bors
Auto merge of #37375 - GuillaumeGomez:cast_message, r=arielb1
f22fdb0 
Improve reference cast help message

Fixes #37338.
GuillaumeGomez added a commit to GuillaumeGomez/rust that referenced this issue Nov 23, 2016
@GuillaumeGomez
Rollup merge of rust-lang#37442 - estebank:cast-deref-hint, r=jonatha…
464cce9 
…ndturner

Provide hint when cast needs a dereference

For a given code:

``` rust
vec![0.0].iter().map(|s| s as i16).collect::<Vec<i16>>();
```

display:

``` nocode
error: casting `&f64` as `i16` is invalid
 --> file3.rs:2:35
  |
2 |     vec![0.0].iter().map(|s| s as i16).collect::<Vec<i16>>();
  |                              -    ^^^
  |                              |
  |                              did you mean `*s`?
```

instead of:

``` nocode
error: casting `&f64` as `i16` is invalid
 --> <anon>:2:30
  |
2 |     vec![0.0].iter().map(|s| s as i16).collect();
  |                              ^^^^^^^^
  |
  = help: cast through a raw pointer first
```

Fixes rust-lang#37338.
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@fhartwig @GuillaumeGomez @bluss @est31