rewrite the initializer macros using syn

Rewrite the initializer macros `[pin_]init!` using `syn`. No functional
changes intended aside from improved error messages on syntactic and
semantical errors. For example if one forgets to use `<-` with an
initializer (and instead uses `:`):

    impl Bar {
        fn new() -> impl PinInit<Self> { ... }
    }

    impl Foo {
        fn new() -> impl PinInit<Self> {
            pin_init!(Self { bar: Bar::new() })
        }
    }

Then the declarative macro would report:

    error[E0308]: mismatched types
      --> tests/ui/compile-fail/init/colon_instead_of_arrow.rs:21:9
       |
    14 |     fn new() -> impl PinInit<Self> {
       |                 ------------------ the found opaque type
    ...
    21 |         pin_init!(Self { bar: Bar::new() })
       |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
       |         |
       |         expected `Bar`, found opaque type
       |         arguments to this function are incorrect
       |
       = note:   expected struct `Bar`
               found opaque type `impl pin_init::PinInit<Bar>`
    note: function defined here
      --> $RUST/core/src/ptr/mod.rs
       |
       | pub const unsafe fn write<T>(dst: *mut T, src: T) {
       |                     ^^^^^
       = note: this error originates in the macro `$crate::__init_internal` which comes from the expansion of the macro `pin_init` (in Nightly builds, run with -Z macro-backtrace for more info)

And the new error is:

    error[E0308]: mismatched types
      --> tests/ui/compile-fail/init/colon_instead_of_arrow.rs:21:31
       |
    14 |     fn new() -> impl PinInit<Self> {
       |                 ------------------ the found opaque type
    ...
    21 |         pin_init!(Self { bar: Bar::new() })
       |                          ---  ^^^^^^^^^^ expected `Bar`, found opaque type
       |                          |
       |                          arguments to this function are incorrect
       |
       = note:   expected struct `Bar`
               found opaque type `impl pin_init::PinInit<Bar>`
    note: function defined here
      --> $RUST/core/src/ptr/mod.rs
       |
       | pub const unsafe fn write<T>(dst: *mut T, src: T) {
       |                     ^^^^^

Importantly, this error gives much more accurate span locations,
pointing to the offending field, rather than the entire macro
invocation.

Signed-off-by: Benno Lossin <[email protected]>

Author: BennoLossin

internal/src/init.rs

@@ -13,6 +13,7 @@
 use proc_macro::TokenStream;
 use syn::parse_macro_input;
 
+mod init;
 mod pin_data;
 mod pinned_drop;
 mod zeroable;
@@ -44,3 +45,23 @@ pub fn derive_zeroable(input: TokenStream) -> TokenStream {
 pub fn maybe_derive_zeroable(input: TokenStream) -> TokenStream {
     zeroable::maybe_derive(parse_macro_input!(input as _)).into()
 }
+
+#[proc_macro]
+pub fn init(input: TokenStream) -> TokenStream {
+    init::expand(
+        parse_macro_input!(input as _),
+        Some("::core::convert::Infallible"),
+        false,
+    )
+    .into()
+}
+
+#[proc_macro]
+pub fn pin_init(input: TokenStream) -> TokenStream {
+    init::expand(
+        parse_macro_input!(input as _),
+        Some("::core::convert::Infallible"),
+        true,
+    )
+    .into()
+}

internal/src/lib.rs

@@ -315,6 +315,7 @@ fn generate_projections(
         });
     quote! {
         #[allow(dead_code)]
+        #[doc(hidden)]
         #vis struct #projection #generics {
             #(#fields_decl)*
             ___pin_phantom_data: ::core::marker::PhantomData<&'__pin mut ()>,
@@ -330,6 +331,7 @@ fn generate_projections(
             ///
             /// These fields are **not** structurally pinned:
             #(#not_structurally_pinned_fields_docs)*
+            #[inline]
             #vis fn project<'__pin>(
                 self: ::core::pin::Pin<&'__pin mut Self>,
             ) -> #projection #ty_gen {
@@ -394,7 +396,7 @@ fn generate_the_pin_data(
                 quote!(__init),
                 quote!(&'__slot mut #ty),
                 quote!(slot),
-                quote!(#[doc = ""]),
+                quote!(),
             )
         };
         let slot_safety = format!(

internal/src/pin_data.rs

@@ -297,8 +297,6 @@ extern crate self as pin_init;
 
 #[doc(hidden)]
 pub mod __internal;
-#[doc(hidden)]
-pub mod macros;
 
 #[cfg(any(feature = "std", feature = "alloc"))]
 mod alloc;
@@ -781,32 +779,7 @@ macro_rules! stack_try_pin_init {
 /// ```
 ///
 /// [`NonNull<Self>`]: core::ptr::NonNull
-// For a detailed example of how this macro works, see the module documentation of the hidden
-// module `macros` inside of `macros.rs`.
-#[macro_export]
-macro_rules! pin_init {
-    ($(&$this:ident in)? $t:ident $(::<$($generics:ty),* $(,)?>)? {
-        $($fields:tt)*
-    }) => {
-        $crate::pin_init!($(&$this in)? $t $(::<$($generics),*>)? {
-            $($fields)*
-        }? ::core::convert::Infallible)
-    };
-    ($(&$this:ident in)? $t:ident $(::<$($generics:ty),* $(,)?>)? {
-        $($fields:tt)*
-    }? $err:ty) => {
-        $crate::__init_internal!(
-            @this($($this)?),
-            @typ($t $(::<$($generics),*>)? ),
-            @fields($($fields)*),
-            @error($err),
-            @data(PinData, use_data),
-            @has_data(HasPinData, __pin_data),
-            @construct_closure(pin_init_from_closure),
-            @munch_fields($($fields)*),
-        )
-    }
-}
+pub use pin_init_internal::pin_init;
 
 /// Construct an in-place, fallible initializer for `struct`s.
 ///
@@ -844,32 +817,7 @@ macro_rules! pin_init {
 /// }
 /// # let _ = Box::init(BigBuf::new());
 /// ```
-// For a detailed example of how this macro works, see the module documentation of the hidden
-// module `macros` inside of `macros.rs`.
-#[macro_export]
-macro_rules! init {
-    ($(&$this:ident in)? $t:ident $(::<$($generics:ty),* $(,)?>)? {
-        $($fields:tt)*
-    }) => {
-        $crate::init!($(&$this in)? $t $(::<$($generics),*>)? {
-            $($fields)*
-        }? ::core::convert::Infallible)
-    };
-    ($(&$this:ident in)? $t:ident $(::<$($generics:ty),* $(,)?>)? {
-        $($fields:tt)*
-    }? $err:ty) => {
-        $crate::__init_internal!(
-            @this($($this)?),
-            @typ($t $(::<$($generics),*>)?),
-            @fields($($fields)*),
-            @error($err),
-            @data(InitData, /*no use_data*/),
-            @has_data(HasInitData, __init_data),
-            @construct_closure(init_from_closure),
-            @munch_fields($($fields)*),
-        )
-    }
-}
+pub use pin_init_internal::init;
 
 /// Asserts that a field on a struct using `#[pin_data]` is marked with `#[pin]` ie. that it is
 /// structurally pinned.