rust: syn: import crate

This is a subset of the Rust `syn` crate, version 2.0.106 (released
2025-08-16), licensed under "Apache-2.0 OR MIT", from:

    https://github.com/dtolnay/syn/raw/2.0.106/src

The files are copied as-is, with no modifications whatsoever (not even
adding the SPDX identifiers).

For copyright details, please see:

    https://github.com/dtolnay/syn/blob/2.0.106/README.md#license
    https://github.com/dtolnay/syn/blob/2.0.106/LICENSE-APACHE
    https://github.com/dtolnay/syn/blob/2.0.106/LICENSE-MIT

The next two patches modify these files as needed for use within the
kernel. This patch split allows reviewers to double-check the import
and to clearly see the differences introduced.

The following script may be used to verify the contents:

    for path in $(cd rust/syn/ && find . -type f -name '*.rs'); do
        curl --silent --show-error --location \
            https://github.com/dtolnay/syn/raw/2.0.106/src/$path \
            | diff --unified rust/syn/$path - && echo $path: OK
    done

Reviewed-by: Gary Guo <gary@garyguo.net>
Tested-by: Gary Guo <gary@garyguo.net>
Tested-by: Jesung Yang <y.j3ms.n@gmail.com>
Link: https://patch.msgid.link/20251124151837.2184382-16-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 108 insertions, 0 deletions

diff --git a/rust/syn/ident.rs b/rust/syn/ident.rs
new file mode 100644
index 000000000000..8a8e8a50d9b0
--- /dev/null
+++ b/rust/syn/ident.rs
@@ -0,0 +1,108 @@
+#[cfg(feature = "parsing")]
+use crate::lookahead;
+
+pub use proc_macro2::Ident;
+
+#[cfg(feature = "parsing")]
+pub_if_not_doc! {
+    #[doc(hidden)]
+    #[allow(non_snake_case)]
+    pub fn Ident(marker: lookahead::TokenMarker) -> Ident {
+        match marker {}
+    }
+}
+
+macro_rules! ident_from_token {
+    ($token:ident) => {
+        impl From<Token![$token]> for Ident {
+            fn from(token: Token![$token]) -> Ident {
+                Ident::new(stringify!($token), token.span)
+            }
+        }
+    };
+}
+
+ident_from_token!(self);
+ident_from_token!(Self);
+ident_from_token!(super);
+ident_from_token!(crate);
+ident_from_token!(extern);
+
+impl From<Token![_]> for Ident {
+    fn from(token: Token![_]) -> Ident {
+        Ident::new("_", token.span)
+    }
+}
+
+pub(crate) fn xid_ok(symbol: &str) -> bool {
+    let mut chars = symbol.chars();
+    let first = chars.next().unwrap();
+    if !(first == '_' || unicode_ident::is_xid_start(first)) {
+        return false;
+    }
+    for ch in chars {
+        if !unicode_ident::is_xid_continue(ch) {
+            return false;
+        }
+    }
+    true
+}
+
+#[cfg(feature = "parsing")]
+mod parsing {
+    use crate::buffer::Cursor;
+    use crate::error::Result;
+    use crate::parse::{Parse, ParseStream};
+    use crate::token::Token;
+    use proc_macro2::Ident;
+
+    fn accept_as_ident(ident: &Ident) -> bool {
+        match ident.to_string().as_str() {
+            "_" |
+            // Based on https://doc.rust-lang.org/1.65.0/reference/keywords.html
+            "abstract" | "as" | "async" | "await" | "become" | "box" | "break" |
+            "const" | "continue" | "crate" | "do" | "dyn" | "else" | "enum" |
+            "extern" | "false" | "final" | "fn" | "for" | "if" | "impl" | "in" |
+            "let" | "loop" | "macro" | "match" | "mod" | "move" | "mut" |
+            "override" | "priv" | "pub" | "ref" | "return" | "Self" | "self" |
+            "static" | "struct" | "super" | "trait" | "true" | "try" | "type" |
+            "typeof" | "unsafe" | "unsized" | "use" | "virtual" | "where" |
+            "while" | "yield" => false,
+            _ => true,
+        }
+    }
+
+    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
+    impl Parse for Ident {
+        fn parse(input: ParseStream) -> Result<Self> {
+            input.step(|cursor| {
+                if let Some((ident, rest)) = cursor.ident() {
+                    if accept_as_ident(&ident) {
+                        Ok((ident, rest))
+                    } else {
+                        Err(cursor.error(format_args!(
+                            "expected identifier, found keyword `{}`",
+                            ident,
+                        )))
+                    }
+                } else {
+                    Err(cursor.error("expected identifier"))
+                }
+            })
+        }
+    }
+
+    impl Token for Ident {
+        fn peek(cursor: Cursor) -> bool {
+            if let Some((ident, _rest)) = cursor.ident() {
+                accept_as_ident(&ident)
+            } else {
+                false
+            }
+        }
+
+        fn display() -> &'static str {
+            "identifier"
+        }
+    }
+}