Suggest using equality comparison instead of pattern matching on non-structural constant in pattern
When encountering a pattern containing a non-structural constant (not marked as `#[derive(PartialEq)]` to make it suitable for pattern matching, `C` in the examples below), we would previously not provide additional guidance. With this PR, the `help` in the following examples are added:
```
error: constant of non-structural type `partial_eq::S` in a pattern
--> $DIR/suggest_equality_comparison_instead_of_pattern_matching.rs:16:18
|
LL | struct S;
| -------- `partial_eq::S` must be annotated with `#[derive(PartialEq)]` to be usable in patterns
...
LL | const C: S = S;
| ---------- constant defined here
...
LL | Some(C) => {}
| ^ constant of non-structural type
|
note: the `PartialEq` trait must be derived, manual `impl`s are not sufficient; see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details
--> $DIR/suggest_equality_comparison_instead_of_pattern_matching.rs:5:5
|
LL | impl PartialEq<S> for S {
| ^^^^^^^^^^^^^^^^^^^^^^^
help: add a condition to the match arm checking for equality
|
LL - Some(C) => {}
LL + Some(binding) if binding == C => {}
|
```
```
error: constant of non-structural type `partial_eq::S` in a pattern
--> $DIR/suggest_equality_comparison_instead_of_pattern_matching.rs:22:18
|
LL | struct S;
| -------- `partial_eq::S` must be annotated with `#[derive(PartialEq)]` to be usable in patterns
...
LL | const C: S = S;
| ---------- constant defined here
...
LL | let Some(C) = Some(S) else { return; };
| ^ constant of non-structural type
|
note: the `PartialEq` trait must be derived, manual `impl`s are not sufficient; see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details
--> $DIR/suggest_equality_comparison_instead_of_pattern_matching.rs:5:5
|
LL | impl PartialEq<S> for S {
| ^^^^^^^^^^^^^^^^^^^^^^^
help: check for equality instead of pattern matching
|
LL - let Some(C) = Some(S) else { return; };
LL + if Some(C) == Some(S) { return; };
|
```
The suggestion accounts for a few conditions:
- if the type is not from the local crate and has no `PartialEq` impl, the user can't make it structural, so we don't provide the suggestion
- regardless of whether the type is local or remote, if it has a manual `PartialEq`, explain that with a derived `PartialEq` you could use equality
- if the type is local and has no impl, suggest adding a derived `PartialEq` and use equality check instead of pattern matching
- when suggesting equality, account for `if-let` to suggest chaining (edition dependent), `match` arm with a present `if` check, `match` arm without an existing `if` check
- when encountering `let-else`, we suggest turning it into an `if` expression instead (this doesn't check for additional bindings beyond the constant, which would suggest incorrect code in some more complex cases).
Fixrust-lang/rust#42753.
```
error: constant of non-structural type `partial_eq::S` in a pattern
--> $DIR/suggest_equality_comparison_instead_of_pattern_matching.rs:22:18
|
LL | struct S;
| -------- `partial_eq::S` must be annotated with `#[derive(PartialEq)]` to be usable in patterns
...
LL | const C: S = S;
| ---------- constant defined here
...
LL | let Some(C) = Some(S) else { return; };
| ^ constant of non-structural type
|
note: the `PartialEq` trait must be derived, manual `impl`s are not sufficient; see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details
--> $DIR/suggest_equality_comparison_instead_of_pattern_matching.rs:5:5
|
LL | impl PartialEq<S> for S {
| ^^^^^^^^^^^^^^^^^^^^^^^
help: check for equality instead of pattern matching
|
LL - let Some(C) = Some(S) else { return; };
LL + if Some(C) == Some(S) { return; };
|
```
```
error: constant of non-structural type `partial_eq::S` in a pattern
--> $DIR/suggest_equality_comparison_instead_of_pattern_matching.rs:16:18
|
LL | struct S;
| -------- `partial_eq::S` must be annotated with `#[derive(PartialEq)]` to be usable in patterns
...
LL | const C: S = S;
| ---------- constant defined here
...
LL | Some(C) => {}
| ^ constant of non-structural type
|
note: the `PartialEq` trait must be derived, manual `impl`s are not sufficient; see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details
--> $DIR/suggest_equality_comparison_instead_of_pattern_matching.rs:5:5
|
LL | impl PartialEq<S> for S {
| ^^^^^^^^^^^^^^^^^^^^^^^
help: add a condition to the match arm checking for equality
|
LL - Some(C) => {}
LL + Some(binding) if binding == C => {}
|
```
Consts and unit structs in patterns can be confusing if they are mistaken for new bindings. We already provide some context for unit structs and consts that come from the current crate, we now also point at those from foreign crates, and we properly skip cases where the pattern has type parameters which can't be confused with a new binding. Make new binding suggestion verbose.
When we encounter a constant in a pattern, we check if it is non-structural. If so, we check if the type implements `PartialEq`, but for types with escaping bound vars the check would be incorrect as is, so we break early. This is ok because these types would be filtered anyways.
Fix#134764.
Point at types that need to be marked with `#[derive(PartialEq)]`.
We use a visitor to look at a type that isn't structural, looking for all ADTs that don't derive `PartialEq`. These can either be manual `impl PartialEq`s or no `impl` at all, so we differentiate between those two cases to provide more context to the user. We also only point at types and impls from the local crate, otherwise show only a note.
```
error: constant of non-structural type `&[B]` in a pattern
--> $DIR/issue-61188-match-slice-forbidden-without-eq.rs:15:9
|
LL | struct B(i32);
| -------- must be annotated with `#[derive(PartialEq)]` to be usable in patterns
LL |
LL | const A: &[B] = &[];
| ------------- constant defined here
...
LL | A => (),
| ^ constant of non-structural type
|
= note: see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details
```
- Point at type that should derive `PartialEq` to be structural.
- Point at manual `impl PartialEq`, explaining that it is not sufficient to be structural.
```
error: constant of non-structural type `MyType` in a pattern
--> $DIR/const-partial_eq-fallback-ice.rs:14:12
|
LL | struct MyType;
| ------------- `MyType` must be annotated with `#[derive(PartialEq)]` to be usable in patterns
...
LL | const CONSTANT: &&MyType = &&MyType;
| ------------------------ constant defined here
...
LL | if let CONSTANT = &&MyType {
| ^^^^^^^^ constant of non-structural type
|
note: the `PartialEq` trait must be derived, manual `impl`s are not sufficient; see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details
--> $DIR/const-partial_eq-fallback-ice.rs:5:1
|
LL | impl PartialEq<usize> for MyType {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```
- Add primary span labels.
- Point at const generic parameter used as pattern.
- Point at statics used as pattern.
- Point at let bindings used in const pattern.
Centralize emitting an error in `const_to_pat` so that all errors from that evaluating a `const` in a pattern can add addditional information. With this, now point at the `const` item's definition:
```
error[E0158]: constant pattern depends on a generic parameter
--> $DIR/associated-const-type-parameter-pattern.rs:20:9
|
LL | pub trait Foo {
| -------------
LL | const X: EFoo;
| ------------- constant defined here
...
LL | A::X => println!("A::X"),
| ^^^^
```
Setting `RUSTC_LOG_COLOR=always` is sometimes useful if tools that one
pipes `RUSTC_LOG` into support coloured output, but it makes this test
fail.
Signed-off-by: David Wood <david@davidtw.co>
bors