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, 65 insertions, 0 deletions

diff --git a/rust/syn/whitespace.rs b/rust/syn/whitespace.rs
new file mode 100644
index 000000000000..a50b5069a68b
--- /dev/null
+++ b/rust/syn/whitespace.rs
@@ -0,0 +1,65 @@
+pub(crate) fn skip(mut s: &str) -> &str {
+    'skip: while !s.is_empty() {
+        let byte = s.as_bytes()[0];
+        if byte == b'/' {
+            if s.starts_with("//")
+                && (!s.starts_with("///") || s.starts_with("////"))
+                && !s.starts_with("//!")
+            {
+                if let Some(i) = s.find('\n') {
+                    s = &s[i + 1..];
+                    continue;
+                } else {
+                    return "";
+                }
+            } else if s.starts_with("/**/") {
+                s = &s[4..];
+                continue;
+            } else if s.starts_with("/*")
+                && (!s.starts_with("/**") || s.starts_with("/***"))
+                && !s.starts_with("/*!")
+            {
+                let mut depth = 0;
+                let bytes = s.as_bytes();
+                let mut i = 0;
+                let upper = bytes.len() - 1;
+                while i < upper {
+                    if bytes[i] == b'/' && bytes[i + 1] == b'*' {
+                        depth += 1;
+                        i += 1; // eat '*'
+                    } else if bytes[i] == b'*' && bytes[i + 1] == b'/' {
+                        depth -= 1;
+                        if depth == 0 {
+                            s = &s[i + 2..];
+                            continue 'skip;
+                        }
+                        i += 1; // eat '/'
+                    }
+                    i += 1;
+                }
+                return s;
+            }
+        }
+        match byte {
+            b' ' | 0x09..=0x0D => {
+                s = &s[1..];
+                continue;
+            }
+            b if b <= 0x7F => {}
+            _ => {
+                let ch = s.chars().next().unwrap();
+                if is_whitespace(ch) {
+                    s = &s[ch.len_utf8()..];
+                    continue;
+                }
+            }
+        }
+        return s;
+    }
+    s
+}
+
+fn is_whitespace(ch: char) -> bool {
+    // Rust treats left-to-right mark and right-to-left mark as whitespace
+    ch.is_whitespace() || ch == '\u{200e}' || ch == '\u{200f}'
+}