Auto merge of #149535 - Jamesbarford:chore/refactor-struct-placeholder, r=lcnr

Move `struct Placeholder<T>`

r? ghost

Couple of issues I've encountered;
- `compiler/rustc_infer/src/infer/region_constraints/mod.rs` `GenericKind` I can't call `write!(f, "{p}")` due to error 1. Which looks like I may need to implement `Lift` for `Placeholder`?
- Using the `define_print_and_forward_display!` for `ty::PlaceholderType` caused error 2, as I've moved the struct it no longer exists in the crate. I suspect because I'm not using that macro it causes the error for `GenericKind`

<details>
<summary>Error 1</summary>

```
error: lifetime may not live long enough
   --> compiler/rustc_infer/src/infer/region_constraints/mod.rs:672:38
    |
668 | impl<'tcx> fmt::Display for GenericKind<'tcx> {
    |      ---- lifetime `'tcx` defined here
...
672 |             GenericKind::Placeholder(ref p) => write!(f, "{p}"),
    |                                      ^^^^^ assignment requires that `'tcx` must outlive `'static`
    |
    = note: requirement occurs because of the type `rustc_middle::ty::TyCtxt<'_>`, which makes the generic argument `'_` invariant
    = note: the struct `rustc_middle::ty::TyCtxt<'tcx>` is invariant over the parameter `'tcx`
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance

error: implementation of `Lift` is not general enough
   --> compiler/rustc_infer/src/infer/region_constraints/mod.rs:672:38
    |
672 |             GenericKind::Placeholder(ref p) => write!(f, "{p}"),
    |                                      ^^^^^ implementation of `Lift` is not general enough
    |
    = note: `Lift<rustc_middle::ty::TyCtxt<'0>>` would have to be implemented for the type `rustc_type_ir::Placeholder<rustc_middle::ty::TyCtxt<'_>, BoundTy>`, for any lifetime `'0`...
    = note: ...but `Lift<rustc_middle::ty::TyCtxt<'1>>` is actually implemented for the type `rustc_type_ir::Placeholder<rustc_middle::ty::TyCtxt<'1>, BoundTy>`, for some specific lifetime `'1`

error: implementation of `Print` is not general enough
   --> compiler/rustc_infer/src/infer/region_constraints/mod.rs:672:38
    |
672 |             GenericKind::Placeholder(ref p) => write!(f, "{p}"),
    |                                      ^^^^^ implementation of `Print` is not general enough
```

</details>

<details>
<summary>Error 2</summary>

```
error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
    --> compiler/rustc_middle/src/ty/print/pretty.rs:3060:38
     |
3057 | / macro_rules! forward_display_to_print {
3058 | |     ($($ty:ty),+) => {
3059 | |         // Some of the $ty arguments may not actually use 'tcx
3060 | |         $(#[allow(unused_lifetimes)] impl<'tcx> fmt::Display for $ty {
     | |                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
...    |
3072 | |     };
3073 | | }
     | |_- in this expansion of `forward_display_to_print!`
...
3093 | / forward_display_to_print! {
3094 | |     ty::Region<'tcx>,
3095 | |     Ty<'tcx>,
3096 | |     &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
3097 | |     ty::Const<'tcx>,
3098 | |     &'tcx ty::PlaceholderType<'tcx>
     | |     ------------------------------- `rustc_type_ir::Placeholder` is not defined in the current crate
3099 | | }
     | |_- in this macro invocation
     |
     = note: impl doesn't have any local type before any uncovered type parameters
     = note: for more information see https://doc.rust-lang.org/reference/items/implementations.html#orphan-rules
     = note: define and implement a trait or new type instead

error: implementation of `Lift` is not general enough
    --> compiler/rustc_middle/src/ty/print/pretty.rs:3060:38
     |
3057 | / macro_rules! forward_display_to_print {
3058 | |     ($($ty:ty),+) => {
3059 | |         // Some of the $ty arguments may not actually use 'tcx
3060 | |         $(#[allow(unused_lifetimes)] impl<'tcx> fmt::Display for $ty {
     | |                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Lift` is not general enough
...    |
3072 | |     };
3073 | | }
     | |_- in this expansion of `forward_display_to_print!`
...
3093 | / forward_display_to_print! {
3094 | |     ty::Region<'tcx>,
3095 | |     Ty<'tcx>,
3096 | |     &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
3097 | |     ty::Const<'tcx>,
3098 | |     &'tcx ty::PlaceholderType<'tcx>
3099 | | }
     | |_- in this macro invocation
     |
     = note: `Lift<context::TyCtxt<'0>>` would have to be implemented for the type `rustc_type_ir::Placeholder<context::TyCtxt<'tcx>, BoundTy>`, for any lifetime `'0`...
     = note: ...but `Lift<context::TyCtxt<'1>>` is actually implemented for the type `rustc_type_ir::Placeholder<context::TyCtxt<'1>, BoundTy>`, for some specific lifetime `'1`

error: implementation of `print::Print` is not general enough
    --> compiler/rustc_middle/src/ty/print/pretty.rs:3060:38
     |
3057 | / macro_rules! forward_display_to_print {
3058 | |     ($($ty:ty),+) => {
3059 | |         // Some of the $ty arguments may not actually use 'tcx
3060 | |         $(#[allow(unused_lifetimes)] impl<'tcx> fmt::Display for $ty {
     | |                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `print::Print` is not general enough
...    |
3072 | |     };
3073 | | }
     | |_- in this expansion of `forward_display_to_print!`
...
3093 | / forward_display_to_print! {
3094 | |     ty::Region<'tcx>,
3095 | |     Ty<'tcx>,
3096 | |     &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
3097 | |     ty::Const<'tcx>,
3098 | |     &'tcx ty::PlaceholderType<'tcx>
3099 | | }
     | |_- in this macro invocation
     |
     = note: `print::Print<'0, print::pretty::FmtPrinter<'a, '0>>` would have to be implemented for the type `rustc_type_ir::Placeholder<context::TyCtxt<'tcx>, BoundTy>`, for any lifetime `'0`...
     = note: ...but `print::Print<'1, print::pretty::FmtPrinter<'a, 'tcx>>` is actually implemented for the type `rustc_type_ir::Placeholder<context::TyCtxt<'1>, BoundTy>`, for some specific lifetime `'1`

error: specializing impl repeats parameter `'tcx`
    --> compiler/rustc_middle/src/ty/print/pretty.rs:3060:38
     |
3057 | / macro_rules! forward_display_to_print {
3058 | |     ($($ty:ty),+) => {
3059 | |         // Some of the $ty arguments may not actually use 'tcx
3060 | |         $(#[allow(unused_lifetimes)] impl<'tcx> fmt::Display for $ty {
     | |                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
...    |
3072 | |     };
3073 | | }
     | |_- in this expansion of `forward_display_to_print!`
...
3093 | / forward_display_to_print! {
3094 | |     ty::Region<'tcx>,
3095 | |     Ty<'tcx>,
3096 | |     &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
3097 | |     ty::Const<'tcx>,
3098 | |     &'tcx ty::PlaceholderType<'tcx>
3099 | | }
     | |_- in this macro invocation

error[E0277]: the trait bound `&Placeholder<TyCtxt<'tcx>, BoundTy>: Lift<...>` is not satisfied
    --> compiler/rustc_middle/src/ty/print/pretty.rs:3064:30
     |
3057 | / macro_rules! forward_display_to_print {
3058 | |     ($($ty:ty),+) => {
3059 | |         // Some of the $ty arguments may not actually use 'tcx
3060 | |         $(#[allow(unused_lifetimes)] impl<'tcx> fmt::Display for $ty {
...    |
3064 | |                     tcx.lift(*self)
     | |                         ---- ^^^^^ unsatisfied trait bound
     | |                         |
     | |                         required by a bound introduced by this call
...    |
3072 | |     };
3073 | | }
     | |_- in this expansion of `forward_display_to_print!`
...
3093 | / forward_display_to_print! {
3094 | |     ty::Region<'tcx>,
3095 | |     Ty<'tcx>,
3096 | |     &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
3097 | |     ty::Const<'tcx>,
3098 | |     &'tcx ty::PlaceholderType<'tcx>
3099 | | }
     | |_- in this macro invocation
     |
     = help: the trait `Lift<context::TyCtxt<'_>>` is not implemented for `&rustc_type_ir::Placeholder<context::TyCtxt<'tcx>, BoundTy>`
note: required by a bound in `context::TyCtxt::<'tcx>::lift`
    --> compiler/rustc_middle/src/ty/context.rs:1807:20
     |
1807 |     pub fn lift<T: Lift<TyCtxt<'tcx>>>(self, value: T) -> Option<T::Lifted> {
     |                    ^^^^^^^^^^^^^^^^^^ required by this bound in `TyCtxt::<'tcx>::lift`
     = note: the full name for the type has been written to '/home/jambar02/Documents/arm/rust/build/x86_64-unknown-linux-gnu/stage1-rustc/x86_64-unknown-linux-gnu/release/deps/rustc_middle-16e5c44041028d8b.long-type-3843651570422266958.txt'
     = note: consider using `--verbose` to print the full type name to the console
help: consider dereferencing here
     |
3064 |                     tcx.lift(**self)
     |                              +

error[E0282]: type annotations needed
    --> compiler/rustc_middle/src/ty/print/pretty.rs:3064:21
     |
3057 | /  macro_rules! forward_display_to_print {
3058 | |      ($($ty:ty),+) => {
3059 | |          // Some of the $ty arguments may not actually use 'tcx
3060 | |          $(#[allow(unused_lifetimes)] impl<'tcx> fmt::Display for $ty {
...    |
3064 | |/                     tcx.lift(*self)
3065 | ||                         .expect("could not lift for printing")
     | ||______________________________________________________________^ cannot infer type
...    |
3072 | |      };
3073 | |  }
     | |__- in this expansion of `forward_display_to_print!`
...
3093 |  / forward_display_to_print! {
3094 |  |     ty::Region<'tcx>,
3095 |  |     Ty<'tcx>,
3096 |  |     &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
3097 |  |     ty::Const<'tcx>,
3098 |  |     &'tcx ty::PlaceholderType<'tcx>
3099 |  | }
     |  |_- in this macro invocation
```

</details>