improve const infer error
cc #72328
reduces it from
```
error[E0282]: type annotations needed
--> src/main.rs:17:5
|
17 | Foo.bar().bar().bar().bar().baz();
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: unable to infer the value of a const parameter
```
to
```
error[E0282]: type annotations needed
--> $DIR/method-chain.rs:21:33
|
LL | Foo.bar().bar().bar().bar().baz();
| ^^^
|
= note: cannot infer the value of the const parameter `N`
```
r? @varkor
fix syntax error in suggesting generic constraint in trait parameter
suggest `where T: Foo` for the first bound on a trait, then suggest
`, T: Foo` when the suggested bound would add to an existing set of
`where` clauses. `where T: Foo` may be the first bound if `T` has a
default, because we'd rather suggest
```
trait A<T=()> where T: Copy
```
than
```
trait A<T: Copy=()>
```
for legibility reasons.
the test case i added here is derived from [this reproduction](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=0bf3ace9f2a183d0bdbd748c6b8e3971):
```
struct B<T: Copy> {
t: T
}
trait A<T = ()> {
fn returns_constrained_type(&self, t: T) -> B<T> {
B { t }
}
}
```
where the suggested fix,
```
trait A<T = ()>, T: Copy { ... }
```
is in fact invalid syntax!
i also found an error in the existing suggestion for `trait Base<T = String>: Super<T>` where rustc would suggest `trait Base<T = String>: Super<T>, T: Copy`, but `T: Copy` is the first of the trait's `where` clauses and should be `where T: Copy` as well. the test for that suggestion expects invalid syntax, and has been revised to a compiler-pleasing `trait Base<T = String>: Super<T> where T: Copy`.
judging by https://github.com/rust-lang/rust/pull/70009 i'll.. cc @estebank ?
Note when a a move/borrow error is caused by a deref coercion
Fixes#73268
When a deref coercion occurs, we may end up with a move error if the
base value has been partially moved out of. However, we do not indicate
anywhere that a deref coercion is occuring, resulting in an error
message with a confusing span.
This PR adds an explicit note to move errors when a deref coercion is
involved. We mention the name of the type that the deref-coercion
resolved to, as well as the `Deref::Target` associated type being used.
More structured suggestions for boxed trait objects instead of impl Trait on non-coerceable tail expressions
When encountering a `match` or `if` as a tail expression where the
different arms do not have the same type *and* the return type of that
`fn` is an `impl Trait`, check whether those arms can implement `Trait`
and if so, suggest using boxed trait objects.
Use structured suggestion for `impl T` to `Box<dyn T>`.
Fix https://github.com/rust-lang/rust/issues/69107
This commit improves the diagnostic when a type parameter is expected
and a closure is found, noting that each closure has a distinct type and
therefore could not always match the caller-chosen type of the
parameter.
Signed-off-by: David Wood <david@davidtw.co>
suggest `where T: Foo` for the first bound on a trait, then suggest
`, T: Foo` when the suggested bound would add to an existing set of
`where` clauses. `where T: Foo` may be the first bound if `T` has a
default, because we'd rather suggest
```
trait A<T=()> where T: Copy
```
than
```
trait A<T: Copy=()>
```
for legibility reasons.
When encountering a `match` or `if` as a tail expression where the
different arms do not have the same type *and* the return type of that
`fn` is an `impl Trait`, check whether those arms can implement `Trait`
and if so, suggest using boxed trait objects.
Fixes#73268
When a deref coercion occurs, we may end up with a move error if the
base value has been partially moved out of. However, we do not indicate
anywhere that a deref coercion is occuring, resulting in an error
message with a confusing span.
This PR adds an explicit note to move errors when a deref coercion is
involved. We mention the name of the type that the deref-coercion
resolved to, as well as the `Deref::Target` associated type being used.
Add CONST_ITEM_MUTATION lint
Fixes#74053Fixes#55721
This PR adds a new lint `CONST_ITEM_MUTATION`.
Given an item `const FOO: SomeType = ..`, this lint fires on:
* Attempting to write directly to a field (`FOO.field = some_val`) or
array entry (`FOO.array_field[0] = val`)
* Taking a mutable reference to the `const` item (`&mut FOO`), including
through an autoderef `FOO.some_mut_self_method()`
The lint message explains that since each use of a constant creates a
new temporary, the original `const` item will not be modified.
make `ConstEvaluatable` more strict
relevant zulip discussion: https://rust-lang.zulipchat.com/#narrow/stream/146212-t-compiler.2Fconst-eval/topic/.60ConstEvaluatable.60.20generic.20functions/near/204125452
Let's see how much this impacts. Depending on how this goes this should probably be a future compat warning.
Short explanation: we currently forbid anonymous constants which depend on generic types, e.g. `[0; std::mem::size_of::<T>]` currently errors.
We previously checked this by evaluating the constant and returned an error if that failed. This however allows things like
```rust
const fn foo<T>() -> usize {
if std::mem::size_of::<*mut T>() < 8 { // size of *mut T does not depend on T
std::mem::size_of::<T>()
} else {
8
}
}
fn test<T>() {
let _ = [0; foo::<T>()];
}
```
which is a backwards compatibility hazard. This also has worrying interactions with mir optimizations (https://github.com/rust-lang/rust/pull/74491#issuecomment-661890421) and intrinsics (#74538).
r? `@oli-obk` `@eddyb`
Fixes#74053Fixes#55721
This PR adds a new lint `CONST_ITEM_MUTATION`.
Given an item `const FOO: SomeType = ..`, this lint fires on:
* Attempting to write directly to a field (`FOO.field = some_val`) or
array entry (`FOO.array_field[0] = val`)
* Taking a mutable reference to the `const` item (`&mut FOO`), including
through an autoderef `FOO.some_mut_self_method()`
The lint message explains that since each use of a constant creates a
new temporary, the original `const` item will not be modified.
