[PATCH v4 04/15] rust: pin-init: internal: add utility API for syn error handling

[Benno Lossin <lossin@kernel.org>]

The API is similar to diagnostics handling in rustc and uses a
`ErrorGuaranteed` value to signify that an error has been emitted. It
supports both fatal errors (which abort the macro expansion immediately
by returning `Err(ErrorGuaranteed)`) and non-fatal ones at at generation
time. These errors are appended to the token stream after generation has
finished normally. This allows giving good errors while still expanding
most of the code as expected to avoid the user encountering additional
errors (for example missing definitions).

Suggested-by: Gary Guo <gary@garyguo.net>
Tested-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Benno Lossin <lossin@kernel.org>
---
Changes in v4: none
Changes in v3: switch to Gary's suggestion
Changes in v2: added this patch
---
 rust/pin-init/internal/src/diagnostics.rs | 32 +++++++++++++++++++++++
 rust/pin-init/internal/src/lib.rs         |  1 +
 2 files changed, 33 insertions(+)
 create mode 100644 rust/pin-init/internal/src/diagnostics.rs

diff --git a/rust/pin-init/internal/src/diagnostics.rs b/rust/pin-init/internal/src/diagnostics.rs
new file mode 100644
index 000000000000..555876c01bab
--- /dev/null
+++ b/rust/pin-init/internal/src/diagnostics.rs
@@ -0,0 +1,32 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+use std::fmt::Display;
+
+use proc_macro2::TokenStream;
+use syn::{spanned::Spanned, Error};
+
+pub(crate) struct DiagCtxt(TokenStream);
+pub(crate) struct ErrorGuaranteed(());
+
+impl DiagCtxt {
+    #[expect(dead_code)]
+    pub(crate) fn error(&mut self, span: impl Spanned, msg: impl Display) -> ErrorGuaranteed {
+        let error = Error::new(span.span(), msg);
+        self.0.extend(error.into_compile_error());
+        ErrorGuaranteed(())
+    }
+
+    #[expect(dead_code)]
+    pub(crate) fn with(
+        fun: impl FnOnce(&mut DiagCtxt) -> Result<TokenStream, ErrorGuaranteed>,
+    ) -> TokenStream {
+        let mut dcx = Self(TokenStream::new());
+        match fun(&mut dcx) {
+            Ok(mut stream) => {
+                stream.extend(dcx.0);
+                stream
+            }
+            Err(ErrorGuaranteed(())) => dcx.0,
+        }
+    }
+}
diff --git a/rust/pin-init/internal/src/lib.rs b/rust/pin-init/internal/src/lib.rs
index 4c4dc639ce82..0e1a4724549d 100644
--- a/rust/pin-init/internal/src/lib.rs
+++ b/rust/pin-init/internal/src/lib.rs
@@ -12,6 +12,7 @@
 
 use proc_macro::TokenStream;
 
+mod diagnostics;
 mod helpers;
 mod pin_data;
 mod pinned_drop;
-- 
2.52.0