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Rollup merge of #81503 - henryboisdequin:fix-const-fn-arr-err-msg, r=estebank

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Suggest to create a new `const` item if the `fn` in the array is a `const fn`

Fixes #73734. If the `fn` in the array repeat expression is a `const fn`, suggest creating a new `const` item. On nightly, suggest creating an inline `const` block. This PR also removes the `suggest_const_in_array_repeat_expressions` as it is no longer necessary.

Example:

```rust
fn main() {
    // Should not compile but hint to create a new const item (stable) or an inline const block (nightly)
    let strings: [String; 5] = [String::new(); 5];
    println!("{:?}", strings);
}

```

Gives this error:

```
error[E0277]: the trait bound `std::string::String: std::marker::Copy` is not satisfied
 --> $DIR/const-fn-in-vec.rs:3:32
  |
2 |     let strings: [String; 5] = [String::new(); 5];
  |                             ^^^^^^^^^^^^^^^^^^ the trait `std::marker::Copy` is not implemented for `String`
  |
  = note: the `Copy` trait is required because the repeated element will be copied
```

With this change, this is the error message:

```
error[E0277]: the trait bound `String: Copy` is not satisfied
  --> $DIR/const-fn-in-vec.rs:3:32
   |
LL |     let strings: [String; 5] = [String::new(); 5];
   |                                ^^^^^^^^^^^^^^^^^^ the trait `Copy` is not implemented for `String`
   |
   = help: moving the function call to a new `const` item will resolve the error
```
This commit is contained in:
Jonas Schievink 2021-02-15 16:06:47 +01:00 • committed by GitHub
commit f02f7b05b2
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8 changed files with 90 additions and 4 deletions
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18
compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
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@ -1881,10 +1881,26 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
ObligationCauseCode::Coercion { source: _, target } => {
err.note(&format!("required by cast to type `{}`", self.ty_to_string(target)));
}
ObligationCauseCode::RepeatVec => {
ObligationCauseCode::RepeatVec(is_const_fn) => {
err.note(
"the `Copy` trait is required because the repeated element will be copied",
);
if is_const_fn {
err.help(
"consider creating a new `const` item and initializing with the result \
of the function call to be used in the repeat position, like \
`const VAL: Type = const_fn();` and `let x = [VAL; 42];`",
);
}
if self.tcx.sess.is_nightly_build() && is_const_fn {
err.help(
"create an inline `const` block, see PR \
#2920 <https://github.com/rust-lang/rfcs/pull/2920> \
for more information",
);
}
}
ObligationCauseCode::VariableType(hir_id) => {
let parent_node = self.tcx.hir().get_parent_node(hir_id);