Rollup merge of #137289 - compiler-errors:coerce-unsized-errors, r=oli-obk

Consolidate and improve error messaging for `CoerceUnsized` and `DispatchFromDyn`

Firstly, this PR consolidates and reworks the error diagnostics for `CoercePointee` and `DispatchFromDyn`. There was a ton of duplication for no reason -- this reworks both the errors and also the error codes, since they can be shared between both traits since they report the same thing.

Secondly, when encountering a struct with multiple fields that must be coerced, point out the field spans, rather than mentioning the fields by name. This makes the error message clearer, but also means that we don't mention the `__S` dummy parameter for `derive(CoercePointee)`.

Thirdly, emit a custom error message when we encounter a trait error that comes from the recursive field `CoerceUnsized`/`DispatchFromDyn` trait check. **Note:** This is the only one I'm not too satisfied with -- I think it could use some more refinement, but ideally it explains that the field must be an unsize-able pointer... Feedback welcome.

Finally, don't emit `DispatchFromDyn` validity errors if we detect `CoerceUnsized` validity errors from an impl of the same ADT.

This is best reviewed per commit.

r? `@oli-obk` perhaps?

cc `@dingxiangfei2009` -- sorry for making my own attempt at this PR, but I wanted to see if I could implement a fix for #136796 in a less complicated way, since communicating over github review comments can be a bit slow. I'll leave comments inline to explain my thinking about the diagnostics changes.

4 files changed, 44 insertions, 117 deletions

diff --git a/compiler/rustc_error_codes/src/error_codes/E0374.md b/compiler/rustc_error_codes/src/error_codes/E0374.md
index 6d7dc88823c..63c243b54ff 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0374.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0374.md
@@ -1,5 +1,5 @@
-`CoerceUnsized` was implemented on a struct which does not contain a field with
-an unsized type.
+`CoerceUnsized` or `DispatchFromDyn` was implemented on a struct which does not
+contain a field that is being unsized.
 
 Example of erroneous code:
 
@@ -11,47 +11,20 @@ struct Foo<T: ?Sized> {
     a: i32,
 }
 
-// error: Struct `Foo` has no unsized fields that need `CoerceUnsized`.
+// error: Struct `Foo` has no unsized fields that need to be coerced.
 impl<T, U> CoerceUnsized<Foo<U>> for Foo<T>
     where T: CoerceUnsized<U> {}
 ```
 
-An [unsized type][1] is any type where the compiler does not know the length or
-alignment of at compile time. Any struct containing an unsized type is also
-unsized.
+`CoerceUnsized` is used to coerce structs that have a field that can be unsized,
+like a custom `MyBox<T>` being unsized to `MyBox<dyn Trait>`. `DispatchFromDyn`
+is used to dispatch from `MyBox<dyn Trait>` to `MyBox<Self>` in a dyn-compatible
+trait.
 
-[1]: https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait
+If the struct doesn't have any fields of unsized types then there is no
+meaningful way to implement `CoerceUnsized` or `DispatchFromDyn`, since
+there is no coercion taking place.
 
-`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.
+Note that `CoerceUnsized` and `DispatchFromDyn` 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.
diff --git a/compiler/rustc_error_codes/src/error_codes/E0375.md b/compiler/rustc_error_codes/src/error_codes/E0375.md
index 71e53057165..7abb3b6afd0 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0375.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0375.md
@@ -1,5 +1,5 @@
-`CoerceUnsized` was implemented on a struct which contains more than one field
-with an unsized type.
+`CoerceUnsized` or `DispatchFromDyn` was implemented on a struct which contains
+more than one field that is being unsized.
 
 Erroneous code example:
 
@@ -17,39 +17,14 @@ struct Foo<T: ?Sized, U: ?Sized> {
 impl<T, U> CoerceUnsized<Foo<U, T>> for Foo<T, U> {}
 ```
 
-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][1] 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.
+`CoerceUnsized` is used to coerce structs that have a field that can be unsized,
+like a custom `MyBox<T>` being unsized to `MyBox<dyn Trait>`. `DispatchFromDyn`
+is used to dispatch from `MyBox<dyn Trait>` to `MyBox<Self>` in a dyn-compatible
+trait.
 
-`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.
+If the struct has multiple fields that must be unsized, then the compiler has no
+way to generate a valid implementation of `CoerceUnsized` or `DispatchFromDyn`.
 
-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
-}
-```
-
-[1]: https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait
+Note that `CoerceUnsized` and `DispatchFromDyn` 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.
diff --git a/compiler/rustc_error_codes/src/error_codes/E0376.md b/compiler/rustc_error_codes/src/error_codes/E0376.md
index 50de15bd30f..5b564ec22fc 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0376.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0376.md
@@ -1,8 +1,11 @@
-`CoerceUnsized` was implemented on something that isn't a struct.
+#### Note: this error code is no longer emitted by the compiler.
+
+`CoerceUnsized` or `DispatchFromDyn` was implemented between two types that
+are not structs.
 
 Erroneous code example:
 
-```compile_fail,E0376
+```compile_fail,E0377
 #![feature(coerce_unsized)]
 use std::ops::CoerceUnsized;
 
@@ -14,33 +17,4 @@ struct Foo<T: ?Sized> {
 impl<T, U> CoerceUnsized<U> for Foo<T> {}
 ```
 
-`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][1]
-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.
-
-[1]: https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait
-
-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.
+`CoerceUnsized` or `DispatchFromDyn` can only be implemented between structs.
diff --git a/compiler/rustc_error_codes/src/error_codes/E0377.md b/compiler/rustc_error_codes/src/error_codes/E0377.md
index b1d36406332..cd2b26260a8 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0377.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0377.md
@@ -1,5 +1,5 @@
-The trait `CoerceUnsized` may only be implemented for a coercion between
-structures with the same definition.
+`CoerceUnsized` or `DispatchFromDyn` may only be implemented between structs
+of the same type.
 
 Example of erroneous code:
 
@@ -20,10 +20,15 @@ pub struct Bar<T: ?Sized> {
 impl<T, U> CoerceUnsized<Bar<U>> for Foo<T> where T: CoerceUnsized<U> {}
 ```
 
-When attempting to implement `CoerceUnsized`, the `impl` signature must look
-like: `impl CoerceUnsized<Type<U>> for Type<T> where T: CoerceUnsized<U>`;
-the *implementer* and *`CoerceUnsized` type parameter* must be the same
-type. In this example, `Bar` and `Foo` (even though structurally identical)
-are *not* the same type and are rejected. Learn more about the `CoerceUnsized`
-trait and DST coercion in
-[the `CoerceUnsized` docs](../std/ops/trait.CoerceUnsized.html).
+`CoerceUnsized` is used to coerce structs that have a field that can be unsized,
+like a custom `MyBox<T>` being unsized to `MyBox<dyn Trait>`. `DispatchFromDyn`
+is used to dispatch from `MyBox<dyn Trait>` to `MyBox<Self>` in a dyn-compatible
+trait.
+
+The compiler cannot support coercions between structs of different types, so
+a valid implementation of `CoerceUnsized` or `DispatchFromDyn` should be
+implemented between the same struct with different generic parameters.
+
+Note that `CoerceUnsized` and `DispatchFromDyn` 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.