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, 210 insertions, 0 deletions

diff --git a/rust/syn/precedence.rs b/rust/syn/precedence.rs
new file mode 100644
index 000000000000..1891bfc202fd
--- /dev/null
+++ b/rust/syn/precedence.rs
@@ -0,0 +1,210 @@
+#[cfg(all(feature = "printing", feature = "full"))]
+use crate::attr::{AttrStyle, Attribute};
+#[cfg(feature = "printing")]
+use crate::expr::Expr;
+#[cfg(all(feature = "printing", feature = "full"))]
+use crate::expr::{
+    ExprArray, ExprAsync, ExprAwait, ExprBlock, ExprBreak, ExprCall, ExprConst, ExprContinue,
+    ExprField, ExprForLoop, ExprGroup, ExprIf, ExprIndex, ExprInfer, ExprLit, ExprLoop, ExprMacro,
+    ExprMatch, ExprMethodCall, ExprParen, ExprPath, ExprRepeat, ExprReturn, ExprStruct, ExprTry,
+    ExprTryBlock, ExprTuple, ExprUnsafe, ExprWhile, ExprYield,
+};
+use crate::op::BinOp;
+#[cfg(all(feature = "printing", feature = "full"))]
+use crate::ty::ReturnType;
+use std::cmp::Ordering;
+
+// Reference: https://doc.rust-lang.org/reference/expressions.html#expression-precedence
+pub(crate) enum Precedence {
+    // return, break, closures
+    Jump,
+    // = += -= *= /= %= &= |= ^= <<= >>=
+    Assign,
+    // .. ..=
+    Range,
+    // ||
+    Or,
+    // &&
+    And,
+    // let
+    #[cfg(feature = "printing")]
+    Let,
+    // == != < > <= >=
+    Compare,
+    // |
+    BitOr,
+    // ^
+    BitXor,
+    // &
+    BitAnd,
+    // << >>
+    Shift,
+    // + -
+    Sum,
+    // * / %
+    Product,
+    // as
+    Cast,
+    // unary - * ! & &mut
+    #[cfg(feature = "printing")]
+    Prefix,
+    // paths, loops, function calls, array indexing, field expressions, method calls
+    #[cfg(feature = "printing")]
+    Unambiguous,
+}
+
+impl Precedence {
+    pub(crate) const MIN: Self = Precedence::Jump;
+
+    pub(crate) fn of_binop(op: &BinOp) -> Self {
+        match op {
+            BinOp::Add(_) | BinOp::Sub(_) => Precedence::Sum,
+            BinOp::Mul(_) | BinOp::Div(_) | BinOp::Rem(_) => Precedence::Product,
+            BinOp::And(_) => Precedence::And,
+            BinOp::Or(_) => Precedence::Or,
+            BinOp::BitXor(_) => Precedence::BitXor,
+            BinOp::BitAnd(_) => Precedence::BitAnd,
+            BinOp::BitOr(_) => Precedence::BitOr,
+            BinOp::Shl(_) | BinOp::Shr(_) => Precedence::Shift,
+
+            BinOp::Eq(_)
+            | BinOp::Lt(_)
+            | BinOp::Le(_)
+            | BinOp::Ne(_)
+            | BinOp::Ge(_)
+            | BinOp::Gt(_) => Precedence::Compare,
+
+            BinOp::AddAssign(_)
+            | BinOp::SubAssign(_)
+            | BinOp::MulAssign(_)
+            | BinOp::DivAssign(_)
+            | BinOp::RemAssign(_)
+            | BinOp::BitXorAssign(_)
+            | BinOp::BitAndAssign(_)
+            | BinOp::BitOrAssign(_)
+            | BinOp::ShlAssign(_)
+            | BinOp::ShrAssign(_) => Precedence::Assign,
+        }
+    }
+
+    #[cfg(feature = "printing")]
+    pub(crate) fn of(e: &Expr) -> Self {
+        #[cfg(feature = "full")]
+        fn prefix_attrs(attrs: &[Attribute]) -> Precedence {
+            for attr in attrs {
+                if let AttrStyle::Outer = attr.style {
+                    return Precedence::Prefix;
+                }
+            }
+            Precedence::Unambiguous
+        }
+
+        match e {
+            #[cfg(feature = "full")]
+            Expr::Closure(e) => match e.output {
+                ReturnType::Default => Precedence::Jump,
+                ReturnType::Type(..) => prefix_attrs(&e.attrs),
+            },
+
+            #[cfg(feature = "full")]
+            Expr::Break(ExprBreak { expr, .. })
+            | Expr::Return(ExprReturn { expr, .. })
+            | Expr::Yield(ExprYield { expr, .. }) => match expr {
+                Some(_) => Precedence::Jump,
+                None => Precedence::Unambiguous,
+            },
+
+            Expr::Assign(_) => Precedence::Assign,
+            Expr::Range(_) => Precedence::Range,
+            Expr::Binary(e) => Precedence::of_binop(&e.op),
+            Expr::Let(_) => Precedence::Let,
+            Expr::Cast(_) => Precedence::Cast,
+            Expr::RawAddr(_) | Expr::Reference(_) | Expr::Unary(_) => Precedence::Prefix,
+
+            #[cfg(feature = "full")]
+            Expr::Array(ExprArray { attrs, .. })
+            | Expr::Async(ExprAsync { attrs, .. })
+            | Expr::Await(ExprAwait { attrs, .. })
+            | Expr::Block(ExprBlock { attrs, .. })
+            | Expr::Call(ExprCall { attrs, .. })
+            | Expr::Const(ExprConst { attrs, .. })
+            | Expr::Continue(ExprContinue { attrs, .. })
+            | Expr::Field(ExprField { attrs, .. })
+            | Expr::ForLoop(ExprForLoop { attrs, .. })
+            | Expr::Group(ExprGroup { attrs, .. })
+            | Expr::If(ExprIf { attrs, .. })
+            | Expr::Index(ExprIndex { attrs, .. })
+            | Expr::Infer(ExprInfer { attrs, .. })
+            | Expr::Lit(ExprLit { attrs, .. })
+            | Expr::Loop(ExprLoop { attrs, .. })
+            | Expr::Macro(ExprMacro { attrs, .. })
+            | Expr::Match(ExprMatch { attrs, .. })
+            | Expr::MethodCall(ExprMethodCall { attrs, .. })
+            | Expr::Paren(ExprParen { attrs, .. })
+            | Expr::Path(ExprPath { attrs, .. })
+            | Expr::Repeat(ExprRepeat { attrs, .. })
+            | Expr::Struct(ExprStruct { attrs, .. })
+            | Expr::Try(ExprTry { attrs, .. })
+            | Expr::TryBlock(ExprTryBlock { attrs, .. })
+            | Expr::Tuple(ExprTuple { attrs, .. })
+            | Expr::Unsafe(ExprUnsafe { attrs, .. })
+            | Expr::While(ExprWhile { attrs, .. }) => prefix_attrs(attrs),
+
+            #[cfg(not(feature = "full"))]
+            Expr::Array(_)
+            | Expr::Async(_)
+            | Expr::Await(_)
+            | Expr::Block(_)
+            | Expr::Call(_)
+            | Expr::Const(_)
+            | Expr::Continue(_)
+            | Expr::Field(_)
+            | Expr::ForLoop(_)
+            | Expr::Group(_)
+            | Expr::If(_)
+            | Expr::Index(_)
+            | Expr::Infer(_)
+            | Expr::Lit(_)
+            | Expr::Loop(_)
+            | Expr::Macro(_)
+            | Expr::Match(_)
+            | Expr::MethodCall(_)
+            | Expr::Paren(_)
+            | Expr::Path(_)
+            | Expr::Repeat(_)
+            | Expr::Struct(_)
+            | Expr::Try(_)
+            | Expr::TryBlock(_)
+            | Expr::Tuple(_)
+            | Expr::Unsafe(_)
+            | Expr::While(_) => Precedence::Unambiguous,
+
+            Expr::Verbatim(_) => Precedence::Unambiguous,
+
+            #[cfg(not(feature = "full"))]
+            Expr::Break(_) | Expr::Closure(_) | Expr::Return(_) | Expr::Yield(_) => unreachable!(),
+        }
+    }
+}
+
+impl Copy for Precedence {}
+
+impl Clone for Precedence {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+impl PartialEq for Precedence {
+    fn eq(&self, other: &Self) -> bool {
+        *self as u8 == *other as u8
+    }
+}
+
+impl PartialOrd for Precedence {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        let this = *self as u8;
+        let other = *other as u8;
+        Some(this.cmp(&other))
+    }
+}