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Add error explanations for E0374, E0375, E0376 on issue #32777 #33415

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165 changes: 158 additions & 7 deletions src/librustc_typeck/diagnostics.rs
Original file line number Diff line number Diff line change
Expand Up @@ -3284,6 +3284,164 @@ impl Baz for Bar { } // Note: This is OK
```
"##,

E0374: r##"
A struct without a field containing an unsized type cannot implement
`CoerceUnsized`. An
[unsized type](https://doc.rust-lang.org/book/unsized-types.html)
is any type that the compiler doesn't know the length or alignment of at
compile time. Any struct containing an unsized type is also unsized.

Example of erroneous code:

```compile_fail
#![feature(coerce_unsized)]
use std::ops::CoerceUnsized;

struct Foo<T: ?Sized> {
a: i32,
}

// error: Struct `Foo` has no unsized fields that need `CoerceUnsized`.
impl<T, U> CoerceUnsized<Foo<U>> for Foo<T>
where T: CoerceUnsized<U> {}
```

`CoerceUnsized` is used to coerce one struct containing an unsized type
into another struct containing a different unsized type. If the struct
doesn't have any fields of unsized types then you don't need explicit
coercion to get the types you want. To fix this you can either
not try to implement `CoerceUnsized` or you can add a field that is
unsized to the struct.

Example:

```
#![feature(coerce_unsized)]
use std::ops::CoerceUnsized;

// We don't need to impl `CoerceUnsized` here.
struct Foo {
a: i32,
}

// We add the unsized type field to the struct.
struct Bar<T: ?Sized> {
a: i32,
b: T,
}

// The struct has an unsized field so we can implement
// `CoerceUnsized` for it.
impl<T, U> CoerceUnsized<Bar<U>> for Bar<T>
where T: CoerceUnsized<U> {}
```

Note that `CoerceUnsized` is mainly used by smart pointers like `Box`, `Rc`
and `Arc` to be able to mark that they can coerce unsized types that they
are pointing at.
"##,

E0375: r##"
A struct with more than one field containing an unsized type cannot implement
`CoerceUnsized`. This only occurs when you are trying to coerce one of the
types in your struct to another type in the struct. In this case we try to
impl `CoerceUnsized` from `T` to `U` which are both types that the struct
takes. An [unsized type](https://doc.rust-lang.org/book/unsized-types.html)
is any type that the compiler doesn't know the length or alignment of at
compile time. Any struct containing an unsized type is also unsized.

Example of erroneous code:

```compile_fail
#![feature(coerce_unsized)]
use std::ops::CoerceUnsized;

struct Foo<T: ?Sized, U: ?Sized> {
a: i32,
b: T,
c: U,
}

// error: Struct `Foo` has more than one unsized field.
impl<T, U> CoerceUnsized<Foo<U, T>> for Foo<T, U> {}
```

`CoerceUnsized` only allows for coercion from a structure with a single
unsized type field to another struct with a single unsized type field.
In fact Rust only allows for a struct to have one unsized type in a struct
and that unsized type must be the last field in the struct. So having two
unsized types in a single struct is not allowed by the compiler. To fix this
use only one field containing an unsized type in the struct and then use
multiple structs to manage each unsized type field you need.

Example:

```
#![feature(coerce_unsized)]
use std::ops::CoerceUnsized;

struct Foo<T: ?Sized> {
a: i32,
b: T,
}

impl <T, U> CoerceUnsized<Foo<U>> for Foo<T>
where T: CoerceUnsized<U> {}

fn coerce_foo<T: CoerceUnsized<U>, U>(t: T) -> Foo<U> {
Foo { a: 12i32, b: t } // we use coercion to get the `Foo<U>` type we need
}
```

"##,

E0376: r##"
The type you are trying to impl `CoerceUnsized` for is not a struct.
`CoerceUnsized` can only be implemented for a struct. Unsized types are
already able to be coerced without an implementation of `CoerceUnsized`
whereas a struct containing an unsized type needs to know the unsized type
field it's containing is able to be coerced. An
[unsized type](https://doc.rust-lang.org/book/unsized-types.html)
is any type that the compiler doesn't know the length or alignment of at
compile time. Any struct containing an unsized type is also unsized.

Example of erroneous code:

```compile_fail
#![feature(coerce_unsized)]
use std::ops::CoerceUnsized;

struct Foo<T: ?Sized> {
a: T,
}

// error: The type `U` is not a struct
impl<T, U> CoerceUnsized<U> for Foo<T> {}
```

The `CoerceUnsized` trait takes a struct type. Make sure the type you are
providing to `CoerceUnsized` is a struct with only the last field containing an
unsized type.

Example:

```
#![feature(coerce_unsized)]
use std::ops::CoerceUnsized;

struct Foo<T> {
a: T,
}

// The `Foo<U>` is a struct so `CoerceUnsized` can be implemented
impl<T, U> CoerceUnsized<Foo<U>> for Foo<T> where T: CoerceUnsized<U> {}
```

Note that in Rust, structs can only contain an unsized type if the field
containing the unsized type is the last and only unsized type field in the
struct.
"##,

E0379: r##"
Trait methods cannot be declared `const` by design. For more information, see
[RFC 911].
Expand Down Expand Up @@ -3777,13 +3935,6 @@ register_diagnostics! {
E0320, // recursive overflow during dropck
E0328, // cannot implement Unsize explicitly
// E0372, // coherence not object safe
E0374, // the trait `CoerceUnsized` may only be implemented for a coercion
// between structures with one field being coerced, none found
E0375, // the trait `CoerceUnsized` may only be implemented for a coercion
// between structures with one field being coerced, but multiple
// fields need coercions
E0376, // the trait `CoerceUnsized` may only be implemented for a coercion
// between structures
E0377, // the trait `CoerceUnsized` may only be implemented for a coercion
// between structures with the same definition
E0399, // trait items need to be implemented because the associated
Expand Down