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Suggest inline const blocks for array initialization #126899

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Jun 24, 2024
Merged

Suggest inline const blocks for array initialization #126899

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40 changes: 7 additions & 33 deletions compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2915,54 +2915,28 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
}
ObligationCauseCode::RepeatElementCopy {
is_constable,
elt_type,
elt_type: _,
elt_span,
elt_stmt_span,
elt_stmt_span: _,
} => {
err.note(
"the `Copy` trait is required because this value will be copied for each element of the array",
);
let value_kind = match is_constable {
IsConstable::Fn => Some("the result of the function call"),
IsConstable::Ctor => Some("the result of the constructor"),
_ => None,
};
let sm = tcx.sess.source_map();
if let Some(value_kind) = value_kind
if matches!(is_constable, IsConstable::Fn | IsConstable::Ctor)
&& let Ok(snip) = sm.span_to_snippet(elt_span)
{
let help_msg = format!(
"consider creating a new `const` item and initializing it with {value_kind} \
to be used in the repeat position"
);
let indentation = sm.indentation_before(elt_stmt_span).unwrap_or_default();
err.multipart_suggestion(
help_msg,
vec![
(
elt_stmt_span.shrink_to_lo(),
format!(
"const ARRAY_REPEAT_VALUE: {elt_type} = {snip};\n{indentation}"
),
),
(elt_span, "ARRAY_REPEAT_VALUE".to_string()),
],
err.span_suggestion(
elt_span,
"create an inline `const` block",
format!("const {{ {snip} }}"),
Applicability::MachineApplicable,
);
} else {
// FIXME: we may suggest array::repeat instead
err.help("consider using `core::array::from_fn` to initialize the array");
err.help("see https://doc.rust-lang.org/stable/std/array/fn.from_fn.html for more information");
}

if tcx.sess.is_nightly_build()
&& matches!(is_constable, IsConstable::Fn | IsConstable::Ctor)
{
err.help(
"create an inline `const` block, see RFC #2920 \
<https://github.com/rust-lang/rfcs/pull/2920> for more information",
);
}
}
ObligationCauseCode::VariableType(hir_id) => {
if let Some(typeck_results) = &self.typeck_results
Expand Down
8 changes: 3 additions & 5 deletions tests/ui/consts/const-blocks/fn-call-in-non-const.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -6,17 +6,15 @@ LL | let _: [Option<Bar>; 2] = [no_copy(); 2];
|
= note: required for `Option<Bar>` to implement `Copy`
= note: the `Copy` trait is required because this value will be copied for each element of the array
= help: create an inline `const` block, see RFC #2920 <https://github.com/rust-lang/rfcs/pull/2920> for more information
help: consider annotating `Bar` with `#[derive(Copy)]`
|
LL + #[derive(Copy)]
LL | struct Bar;
|
help: consider creating a new `const` item and initializing it with the result of the function call to be used in the repeat position
|
LL ~ const ARRAY_REPEAT_VALUE: Option<Bar> = no_copy();
LL ~ let _: [Option<Bar>; 2] = [ARRAY_REPEAT_VALUE; 2];
help: create an inline `const` block
|
LL | let _: [Option<Bar>; 2] = [const { no_copy() }; 2];
| ~~~~~~~~~~~~~~~~~~~

error: aborting due to 1 previous error

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11 changes: 4 additions & 7 deletions tests/ui/consts/const-blocks/trait-error.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -2,21 +2,18 @@ error[E0277]: the trait bound `String: Copy` is not satisfied
--> $DIR/trait-error.rs:5:6
|
LL | [Foo(String::new()); 4];
| ^^^^^^^^^^^^^^^^^^ the trait `Copy` is not implemented for `String`, which is required by `Foo<String>: Copy`
| ^^^^^^^^^^^^^^^^^^
| |
| the trait `Copy` is not implemented for `String`, which is required by `Foo<String>: Copy`
| help: create an inline `const` block: `const { Foo(String::new()) }`
|
note: required for `Foo<String>` to implement `Copy`
--> $DIR/trait-error.rs:1:10
|
LL | #[derive(Copy, Clone)]
| ^^^^ unsatisfied trait bound introduced in this `derive` macro
= note: the `Copy` trait is required because this value will be copied for each element of the array
= help: create an inline `const` block, see RFC #2920 <https://github.com/rust-lang/rfcs/pull/2920> for more information
= note: this error originates in the derive macro `Copy` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider creating a new `const` item and initializing it with the result of the function call to be used in the repeat position
|
LL ~ const ARRAY_REPEAT_VALUE: Foo<String> = Foo(String::new());
LL ~ [ARRAY_REPEAT_VALUE; 4];
|

error: aborting due to 1 previous error

Expand Down
33 changes: 12 additions & 21 deletions tests/ui/consts/const-fn-in-vec.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -2,45 +2,36 @@ error[E0277]: the trait bound `String: Copy` is not satisfied
--> $DIR/const-fn-in-vec.rs:1:47
|
LL | static _MAYBE_STRINGS: [Option<String>; 5] = [None; 5];
| ^^^^ the trait `Copy` is not implemented for `String`, which is required by `Option<String>: Copy`
| ^^^^
| |
| the trait `Copy` is not implemented for `String`, which is required by `Option<String>: Copy`
| help: create an inline `const` block: `const { None }`
|
= note: required for `Option<String>` to implement `Copy`
= note: the `Copy` trait is required because this value will be copied for each element of the array
= help: create an inline `const` block, see RFC #2920 <https://github.com/rust-lang/rfcs/pull/2920> for more information
help: consider creating a new `const` item and initializing it with the result of the constructor to be used in the repeat position
|
LL + const ARRAY_REPEAT_VALUE: Option<String> = None;
LL ~ static _MAYBE_STRINGS: [Option<String>; 5] = [ARRAY_REPEAT_VALUE; 5];
|

error[E0277]: the trait bound `String: Copy` is not satisfied
--> $DIR/const-fn-in-vec.rs:7:34
|
LL | let _strings: [String; 5] = [String::new(); 5];
| ^^^^^^^^^^^^^ the trait `Copy` is not implemented for `String`
| ^^^^^^^^^^^^^
| |
| the trait `Copy` is not implemented for `String`
| help: create an inline `const` block: `const { String::new() }`
|
= note: the `Copy` trait is required because this value will be copied for each element of the array
= help: create an inline `const` block, see RFC #2920 <https://github.com/rust-lang/rfcs/pull/2920> for more information
help: consider creating a new `const` item and initializing it with the result of the function call to be used in the repeat position
|
LL ~ const ARRAY_REPEAT_VALUE: String = String::new();
LL ~ let _strings: [String; 5] = [ARRAY_REPEAT_VALUE; 5];
|

error[E0277]: the trait bound `String: Copy` is not satisfied
--> $DIR/const-fn-in-vec.rs:9:48
|
LL | let _maybe_strings: [Option<String>; 5] = [None; 5];
| ^^^^ the trait `Copy` is not implemented for `String`, which is required by `Option<String>: Copy`
| ^^^^
| |
| the trait `Copy` is not implemented for `String`, which is required by `Option<String>: Copy`
| help: create an inline `const` block: `const { None }`
|
= note: required for `Option<String>` to implement `Copy`
= note: the `Copy` trait is required because this value will be copied for each element of the array
= help: create an inline `const` block, see RFC #2920 <https://github.com/rust-lang/rfcs/pull/2920> for more information
help: consider creating a new `const` item and initializing it with the result of the constructor to be used in the repeat position
|
LL ~ const ARRAY_REPEAT_VALUE: Option<String> = None;
LL ~ let _maybe_strings: [Option<String>; 5] = [ARRAY_REPEAT_VALUE; 5];
|

error: aborting due to 3 previous errors

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