replace shadowed return token by unsafe-to-create token

We use a unit struct `__InitOk` in the closure generated by the
initializer macros as the return value. We shadow it by creating a
struct with the same name again inside of the closure, preventing early
returns of `Ok` in the initializer (before all fields have been
initialized).

In the face of Type Alias Impl Trait (TAIT) and the next trait solver,
this solution no longer works [1]. The shadowed struct can be named
through type inference. In addition, there is an RFC proposing to add
the feature of path inference to Rust, which would similarly allow [2]

Thus remove the shadowed token and replace it with an `unsafe` to create
token.

The reason we initially used the shadowing solution was because an
alternative solution used a builder pattern. Gary writes [3]:

    In the early builder-pattern based InitOk, having a single InitOk
    type for token is unsound because one can launder an InitOk token
    used for one place to another initializer. I used a branded lifetime
    solution, and then you figured out that using a shadowed type would
    work better because nobody could construct it at all.

The laundering issue does not apply to the approach we ended up with
today.

With this change, the example by Tim Chirananthavat in [1] no longer
compiles and results in this error:

    error: cannot construct `pin_init::__internal::InitOk` with struct literal syntax due to private fields
      --> src/main.rs:26:17
       |
    26 |                 InferredType {}
       |                 ^^^^^^^^^^^^
       |
       = note: private field `0` that was not provided
    help: you might have meant to use the `new` associated function
       |
    26 -                 InferredType {}
    26 +                 InferredType::new()
       |

Applying the suggestion of using the `::new()` function, results in
another expected error:

    error[E0133]: call to unsafe function `pin_init::__internal::InitOk::new` is unsafe and requires unsafe block
      --> src/main.rs:26:17
       |
    26 |                 InferredType::new()
       |                 ^^^^^^^^^^^^^^^^^^^ call to unsafe function
       |
       = note: consult the function's documentation for information on how to avoid undefined behavior

Reported-by: Tim Chirananthavat <theemathas@gmail.com>
Link: rust-lang/rust#153535 [1]
Link: rust-lang/rfcs#3444 (comment) [2]
Link: rust-lang/rust#153535 (comment) [3]
Signed-off-by: Benno Lossin <lossin@kernel.org>

Author: BennoLossin

internal/src/init.rs

@@ -148,11 +148,6 @@ pub(crate) fn expand(
     let init_fields = init_fields(&fields, pinned, &data, &slot);
     let field_check = make_field_check(&fields, init_kind, &path);
     Ok(quote! {{
-        // We do not want to allow arbitrary returns, so we declare this type as the `Ok` return
-        // type and shadow it later when we insert the arbitrary user code. That way there will be
-        // no possibility of returning without `unsafe`.
-        struct __InitOk;
-
         // Get the data about fields from the supplied type.
         // SAFETY: TODO
         let #data = unsafe {
@@ -162,18 +157,15 @@ pub(crate) fn expand(
             #path::#get_data()
         };
         // Ensure that `#data` really is of type `#data` and help with type inference:
-        let init = ::pin_init::__internal::#data_trait::make_closure::<_, __InitOk, #error>(
+        let init = ::pin_init::__internal::#data_trait::make_closure::<_, #error>(
             #data,
             move |slot| {
-                {
-                    // Shadow the structure so it cannot be used to return early.
-                    struct __InitOk;
-                    #zeroable_check
-                    #this
-                    #init_fields
-                    #field_check
-                }
-                Ok(__InitOk)
+                #zeroable_check
+                #this
+                #init_fields
+                #field_check
+                // SAFETY: we are the `init!` macro that is allowed to call this.
+                Ok(unsafe { ::pin_init::__internal::InitOk::new() })
             }
         );
         let init = move |slot| -> ::core::result::Result<(), #error> {

src/__internal.rs

@@ -46,6 +46,24 @@ where
     }
 }
 
+/// Token type to signify successful initialization.
+///
+/// Can only be constructed via the unsafe [`Self::new`] function. The initializer macros use this
+/// token type to prevent returning `Ok` from an initializer without initializing all fields.
+pub struct InitOk(());
+
+impl InitOk {
+    /// Creates a new token.
+    ///
+    /// # Safety
+    ///
+    /// This function may only be called from the `init!` macro in `../internal/src/init.rs`.
+    #[inline(always)]
+    pub unsafe fn new() -> Self {
+        Self(())
+    }
+}
+
 /// This trait is only implemented via the `#[pin_data]` proc-macro. It is used to facilitate
 /// the pin projections within the initializers.
 ///
@@ -68,9 +86,10 @@ pub unsafe trait PinData: Copy {
     type Datee: ?Sized + HasPinData;
 
     /// Type inference helper function.
-    fn make_closure<F, O, E>(self, f: F) -> F
+    #[inline(always)]
+    fn make_closure<F, E>(self, f: F) -> F
     where
-        F: FnOnce(*mut Self::Datee) -> Result<O, E>,
+        F: FnOnce(*mut Self::Datee) -> Result<InitOk, E>,
     {
         f
     }
@@ -98,9 +117,10 @@ pub unsafe trait InitData: Copy {
     type Datee: ?Sized + HasInitData;
 
     /// Type inference helper function.
-    fn make_closure<F, O, E>(self, f: F) -> F
+    #[inline(always)]
+    fn make_closure<F, E>(self, f: F) -> F
     where
-        F: FnOnce(*mut Self::Datee) -> Result<O, E>,
+        F: FnOnce(*mut Self::Datee) -> Result<InitOk, E>,
     {
         f
     }

tests/ui/compile-fail/init/early_return.rs

@@ -0,0 +1,14 @@
+use pin_init::*;
+
+struct Foo {
+    a: usize,
+}
+
+fn main() {
+    let _ = init!(Foo {
+        _: {
+            return Ok(());
+        },
+        a: 42,
+    });
+}

tests/ui/compile-fail/init/early_return.stderr

@@ -0,0 +1,36 @@
+error[E0308]: mismatched types
+  --> tests/ui/compile-fail/init/early_return.rs:10:23
+   |
+10 |             return Ok(());
+   |                    -- ^^ expected `InitOk`, found `()`
+   |                    |
+   |                    arguments to this enum variant are incorrect
+   |
+help: the type constructed contains `()` due to the type of the argument passed
+  --> tests/ui/compile-fail/init/early_return.rs:10:20
+   |
+10 |             return Ok(());
+   |                    ^^^--^
+   |                       |
+   |                       this argument influences the type of `Ok`
+note: tuple variant defined here
+  --> $RUST/core/src/result.rs
+   |
+   |     Ok(#[stable(feature = "rust1", since = "1.0.0")] T),
+   |     ^^
+
+warning: unreachable statement
+  --> tests/ui/compile-fail/init/early_return.rs:8:13
+   |
+ 8 |       let _ = init!(Foo {
+   |  _____________^
+ 9 | |         _: {
+10 | |             return Ok(());
+   | |             ------------- any code following this expression is unreachable
+11 | |         },
+12 | |         a: 42,
+13 | |     });
+   | |______^ unreachable statement
+   |
+   = note: `#[warn(unreachable_code)]` (part of `#[warn(unused)]`) on by default
+   = note: this warning originates in the macro `init` (in Nightly builds, run with -Z macro-backtrace for more info)

tests/ui/expand/simple-init.expanded.rs

@@ -2,24 +2,19 @@ use pin_init::*;
 struct Foo {}
 fn main() {
     let _ = {
-        struct __InitOk;
         let __data = unsafe {
             use ::pin_init::__internal::HasInitData;
             Foo::__init_data()
         };
         let init = ::pin_init::__internal::InitData::make_closure::<
             _,
-            __InitOk,
             ::core::convert::Infallible,
         >(
             __data,
             move |slot| {
-                {
-                    struct __InitOk;
-                    #[allow(unreachable_code, clippy::diverging_sub_expression)]
-                    let _ = || unsafe { ::core::ptr::write(slot, Foo {}) };
-                }
-                Ok(__InitOk)
+                #[allow(unreachable_code, clippy::diverging_sub_expression)]
+                let _ = || unsafe { ::core::ptr::write(slot, Foo {}) };
+                Ok(unsafe { ::pin_init::__internal::InitOk::new() })
             },
         );
         let init = move |