rust: pin-init: internal: turn PhantomPinned error into warnings

The `PhantomPinned` detection is just a lint, and is emitted as an error
because there is no `compile_warning!()` macro, and
`proc-macro-diagnostics` is not stable.

Use of `#[deprecated = ""]` attribute to approximate custom proc-macro
warnings. A new line is added before message for visual clarity.

An example warning with this trick looks like this:

    warning: use of deprecated function `_::warn`:
             The field `pin` of type `PhantomPinned` only has an effect if it has the `#[pin]` attribute
     --> test.rs:9:5
      |
    9 |     pin: marker::PhantomPinned,
      |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^

Suggested-by: Benno Lossin <lossin@kernel.org>
Link: https://github.com/Rust-for-Linux/pin-init/issues/51
Link: https://patch.msgid.link/20260428-pin-init-sync-v1-10-07f9bd3859fb@garyguo.net
Signed-off-by: Gary Guo <gary@garyguo.net>

rust/pin-init/internal/src/diagnostics.rs

@@ -3,6 +3,7 @@
 use std::fmt::Display;
 
 use proc_macro2::TokenStream;
+use quote::quote_spanned;
 use syn::{spanned::Spanned, Error};
 
 pub(crate) struct DiagCtxt(TokenStream);
@@ -15,6 +16,19 @@ pub(crate) fn error(&mut self, span: impl Spanned, msg: impl Display) -> ErrorGu
         ErrorGuaranteed(())
     }
 
+    pub(crate) fn warn(&mut self, span: impl Spanned, msg: impl Display) {
+        // Have the message start on a new line for visual clarity.
+        let msg = format!("\n{}", msg);
+        self.0.extend(quote_spanned!(span.span() =>
+            // Approximate using deprecated warning while `proc_macro_diagnostic` is unstable.
+            const _: () = {
+                #[deprecated = #msg]
+                const fn warn() {}
+                warn();
+            };
+        ));
+    }
+
     pub(crate) fn with(
         fun: impl FnOnce(&mut DiagCtxt) -> Result<TokenStream, ErrorGuaranteed>,
     ) -> TokenStream {

rust/pin-init/internal/src/pin_data.rs

@@ -85,7 +85,7 @@ pub(crate) fn pin_data(
 
     for (pinned, field) in &fields {
         if !pinned && is_phantom_pinned(&field.ty) {
-            dcx.error(
+            dcx.warn(
                 field,
                 format!(
                     "The field `{}` of type `PhantomPinned` only has an effect \