Allow using #[pin_project] type with private field types

Previously, given code such as:

```rust

struct Private<T>;

\#[pin_project]
pub struct Public<T> {
    #[pin] private: Private<T>
}
```

we would generate an Unpin impl like this:

```rust
impl Unpin for Public where Private: Unpin {}
```

Unfortunately, since Private is not a public type,
this would cause an E0446 ('private type `Private` in public interface)

When RFC 2145 is implemented (rust-lang/rust#48054),
this will become a lint, rather then a hard error.

In the time being, we need a solution that will work with the current
type privacy rules.

The solution is to generate code like this:

```rust

fn __private_scope() {
    pub struct __UnpinPublic<T> {
        __field0: Private<T>
    }
    impl<T> Unpin for Public<T> where __UnpinPublic<T>: Unpin {}
}
```

That is, we generate a new struct, containing all of the pinned
fields from our #[pin_project] type. This struct is delcared within
a function, which makes it impossible to be named by user code.
This guarnatees that it will use the default auto-trait impl for Unpin -
that is, it will implement Unpin iff all of its fields implement Unpin.
This type can be safely declared as 'public', satisfiying the privacy
checker without actually allowing user code to access it.

This allows users to apply the #[pin_project] attribute to types
regardless of the privacy of the types of their fields.

pin-project-internal/src/pin_project/enums.rs

@@ -79,7 +79,7 @@ fn named(
     for Field { attrs, ident, ty, .. } in fields {
         if let Some(attr) = attrs.find_remove(PIN) {
             let _: Nothing = syn::parse2(attr.tokens)?;
-            cx.push_unpin_bounds(ty);
+            cx.push_unpin_bounds(ty.clone());
             let lifetime = &cx.lifetime;
             proj_body.push(quote!(#ident: ::core::pin::Pin::new_unchecked(#ident)));
             proj_field.push(quote!(#ident: ::core::pin::Pin<&#lifetime mut #ty>));
@@ -108,7 +108,7 @@ fn unnamed(
         let x = format_ident!("_x{}", i);
         if let Some(attr) = attrs.find_remove(PIN) {
             let _: Nothing = syn::parse2(attr.tokens)?;
-            cx.push_unpin_bounds(ty);
+            cx.push_unpin_bounds(ty.clone());
             let lifetime = &cx.lifetime;
             proj_body.push(quote!(::core::pin::Pin::new_unchecked(#x)));
             proj_field.push(quote!(::core::pin::Pin<&#lifetime mut #ty>));

pin-project-internal/src/pin_project/mod.rs

@@ -64,6 +64,8 @@ struct Context {
     /// Where-clause for conditional Unpin implementation.
     impl_unpin: WhereClause,
 
+    pinned_fields: Vec<Type>,
+
     unsafe_unpin: bool,
     pinned_drop: Option<Span>,
 }
@@ -97,6 +99,7 @@ impl Context {
             generics,
             lifetime,
             impl_unpin,
+            pinned_fields: vec![],
             unsafe_unpin: unsafe_unpin.is_some(),
             pinned_drop,
         })
@@ -109,21 +112,83 @@ impl Context {
         generics
     }
 
-    fn push_unpin_bounds(&mut self, ty: &Type) {
-        // We only add bounds for automatically generated impls
-        if !self.unsafe_unpin {
-            self.impl_unpin.predicates.push(syn::parse_quote!(#ty: ::core::marker::Unpin));
-        }
+    fn push_unpin_bounds(&mut self, ty: Type) {
+        self.pinned_fields.push(ty);
     }
 
     /// Makes conditional `Unpin` implementation for original type.
     fn make_unpin_impl(&self) -> TokenStream {
         let orig_ident = &self.orig_ident;
         let (impl_generics, ty_generics, _) = self.generics.split_for_impl();
+        let type_params: Vec<_> = self.generics.type_params().map(|t| t.ident.clone()).collect();
         let where_clause = &self.impl_unpin;
 
-        quote! {
-            impl #impl_generics ::core::marker::Unpin for #orig_ident #ty_generics #where_clause {}
+        if self.unsafe_unpin {
+            quote! {
+                impl #impl_generics ::core::marker::Unpin for #orig_ident #ty_generics #where_clause {}
+            }
+        } else {
+            let struct_ident = format_ident!("__UnpinStruct{}", orig_ident);
+            let fields: Vec<_> = self
+                .pinned_fields
+                .iter()
+                .enumerate()
+                .map(|(i, ty)| {
+                    let field_ident = format_ident!("__field{}", i);
+                    quote! {
+                        #field_ident: #ty
+                    }
+                })
+                .collect();
+
+            let fields_comma = if !fields.is_empty() { quote!(,) } else { quote!() };
+
+            let lifetime_fields: Vec<_> = self
+                .generics
+                .lifetimes()
+                .enumerate()
+                .map(|(i, l)| {
+                    let field_ident = format_ident!("__lifetime{}", i);
+                    quote! {
+                        #field_ident: &#l ()
+                    }
+                })
+                .collect();
+
+            let fn_ident = format_ident!("__unpin_scope_{}", orig_ident);
+
+            let full_generics = &self.generics;
+            let mut full_where_clause = where_clause.clone();
+
+            let unpin_clause: WherePredicate = syn::parse_quote! {
+                #struct_ident #ty_generics: ::core::marker::Unpin
+            };
+
+            full_where_clause.predicates.push(unpin_clause);
+
+            quote! {
+                // By writing our type inside a function scope,
+                // we guarantee that it cannot be named by another
+                // other code
+                #[allow(non_snake_case)]
+                fn #fn_ident() {
+
+                    struct AlwaysUnpin<T: ?Sized> {
+                        val: ::core::marker::PhantomData<T>
+                    }
+
+                    impl<T: ?Sized> ::core::marker::Unpin for AlwaysUnpin<T> {}
+
+                    pub struct #struct_ident #full_generics #where_clause {
+                        __pin_project_use_generics: AlwaysUnpin<(#(#type_params),*)>,
+
+                        #(#fields),* #fields_comma
+                        #(#lifetime_fields),*
+                    }
+
+                    impl #impl_generics ::core::marker::Unpin for #orig_ident #ty_generics #full_where_clause {}
+                }
+            }
         }
     }
 

pin-project-internal/src/pin_project/structs.rs

@@ -53,7 +53,7 @@ fn named(
     for Field { attrs, ident, ty, .. } in fields {
         if let Some(attr) = attrs.find_remove(PIN) {
             let _: Nothing = syn::parse2(attr.tokens)?;
-            cx.push_unpin_bounds(ty);
+            cx.push_unpin_bounds(ty.clone());
             let lifetime = &cx.lifetime;
             proj_fields.push(quote!(#ident: ::core::pin::Pin<&#lifetime mut #ty>));
             proj_init.push(quote!(#ident: ::core::pin::Pin::new_unchecked(&mut this.#ident)));
@@ -79,7 +79,7 @@ fn unnamed(
         let i = Index::from(i);
         if let Some(attr) = attrs.find_remove(PIN) {
             let _: Nothing = syn::parse2(attr.tokens)?;
-            cx.push_unpin_bounds(ty);
+            cx.push_unpin_bounds(ty.clone());
             let lifetime = &cx.lifetime;
             proj_fields.push(quote!(::core::pin::Pin<&#lifetime mut #ty>));
             proj_init.push(quote!(::core::pin::Pin::new_unchecked(&mut this.#i)));

tests/pin_project.rs

@@ -238,3 +238,27 @@ fn combine() {
     #[allow(unsafe_code)]
     unsafe impl<T: Unpin> UnsafeUnpin for Foo<T> {}
 }
+
+#[test]
+// This 'allow' is unrelated to the code
+// generated by pin-project - it's just to
+// allow us to put a private enum in a public enum
+#[allow(private_in_public)]
+fn test_private_type_in_public_type() {
+    #[pin_project]
+    pub struct PublicStruct<T> {
+        #[pin]
+        inner: PrivateStruct<T>
+    }
+
+    struct PrivateStruct<T>(T);
+
+    #[pin_project]
+    pub enum PublicEnum {
+        Variant(#[pin] PrivateEnum)
+    }
+
+    enum PrivateEnum {
+        OtherVariant(u8)
+    }
+}

tests/ui/pin_project/proper_unpin.rs

@@ -0,0 +1,25 @@
+// compile-fail
+
+#![deny(warnings, unsafe_code)]
+
+use pin_project::{pin_project, pinned_drop};
+use std::pin::Pin;
+
+struct Inner<T> {
+    val: T
+}
+
+#[pin_project]
+struct Foo<T, U> {
+    #[pin]
+    inner: Inner<T>,
+    other: U
+}
+
+fn is_unpin<T: Unpin>() {}
+
+fn bar<T, U>() {
+    is_unpin::<Foo<T, U>>(); //~ ERROR E0277
+}
+
+fn main() {}

tests/ui/pin_project/proper_unpin.stderr

@@ -0,0 +1,19 @@
+error[E0277]: the trait bound `T: std::marker::Unpin` is not satisfied in `__unpin_scope_Foo::__UnpinStructFoo<T, U>`
+  --> $DIR/proper_unpin.rs:22:5
+   |
+22 |     is_unpin::<Foo<T, U>>(); //~ ERROR E0277
+   |     ^^^^^^^^^^^^^^^^^^^^^ within `__unpin_scope_Foo::__UnpinStructFoo<T, U>`, the trait `std::marker::Unpin` is not implemented for `T`
+   |
+   = help: consider adding a `where T: std::marker::Unpin` bound
+   = note: required because it appears within the type `Inner<T>`
+   = note: required because it appears within the type `__unpin_scope_Foo::__UnpinStructFoo<T, U>`
+   = note: required because of the requirements on the impl of `std::marker::Unpin` for `Foo<T, U>`
+note: required by `is_unpin`
+  --> $DIR/proper_unpin.rs:19:1
+   |
+19 | fn is_unpin<T: Unpin>() {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.