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, 311 insertions, 0 deletions

diff --git a/rust/syn/classify.rs b/rust/syn/classify.rs
new file mode 100644
index 000000000000..8eab19dbc37c
--- /dev/null
+++ b/rust/syn/classify.rs
@@ -0,0 +1,311 @@
+#[cfg(feature = "full")]
+use crate::expr::Expr;
+#[cfg(any(feature = "printing", feature = "full"))]
+use crate::generics::TypeParamBound;
+#[cfg(any(feature = "printing", feature = "full"))]
+use crate::path::{Path, PathArguments};
+#[cfg(any(feature = "printing", feature = "full"))]
+use crate::punctuated::Punctuated;
+#[cfg(any(feature = "printing", feature = "full"))]
+use crate::ty::{ReturnType, Type};
+#[cfg(feature = "full")]
+use proc_macro2::{Delimiter, TokenStream, TokenTree};
+#[cfg(any(feature = "printing", feature = "full"))]
+use std::ops::ControlFlow;
+
+#[cfg(feature = "full")]
+pub(crate) fn requires_semi_to_be_stmt(expr: &Expr) -> bool {
+    match expr {
+        Expr::Macro(expr) => !expr.mac.delimiter.is_brace(),
+        _ => requires_comma_to_be_match_arm(expr),
+    }
+}
+
+#[cfg(feature = "full")]
+pub(crate) fn requires_comma_to_be_match_arm(expr: &Expr) -> bool {
+    match expr {
+        Expr::If(_)
+        | Expr::Match(_)
+        | Expr::Block(_) | Expr::Unsafe(_) // both under ExprKind::Block in rustc
+        | Expr::While(_)
+        | Expr::Loop(_)
+        | Expr::ForLoop(_)
+        | Expr::TryBlock(_)
+        | Expr::Const(_) => false,
+
+        Expr::Array(_)
+        | Expr::Assign(_)
+        | Expr::Async(_)
+        | Expr::Await(_)
+        | Expr::Binary(_)
+        | Expr::Break(_)
+        | Expr::Call(_)
+        | Expr::Cast(_)
+        | Expr::Closure(_)
+        | Expr::Continue(_)
+        | Expr::Field(_)
+        | Expr::Group(_)
+        | Expr::Index(_)
+        | Expr::Infer(_)
+        | Expr::Let(_)
+        | Expr::Lit(_)
+        | Expr::Macro(_)
+        | Expr::MethodCall(_)
+        | Expr::Paren(_)
+        | Expr::Path(_)
+        | Expr::Range(_)
+        | Expr::RawAddr(_)
+        | Expr::Reference(_)
+        | Expr::Repeat(_)
+        | Expr::Return(_)
+        | Expr::Struct(_)
+        | Expr::Try(_)
+        | Expr::Tuple(_)
+        | Expr::Unary(_)
+        | Expr::Yield(_)
+        | Expr::Verbatim(_) => true,
+    }
+}
+
+#[cfg(feature = "printing")]
+pub(crate) fn trailing_unparameterized_path(mut ty: &Type) -> bool {
+    loop {
+        match ty {
+            Type::BareFn(t) => match &t.output {
+                ReturnType::Default => return false,
+                ReturnType::Type(_, ret) => ty = ret,
+            },
+            Type::ImplTrait(t) => match last_type_in_bounds(&t.bounds) {
+                ControlFlow::Break(trailing_path) => return trailing_path,
+                ControlFlow::Continue(t) => ty = t,
+            },
+            Type::Path(t) => match last_type_in_path(&t.path) {
+                ControlFlow::Break(trailing_path) => return trailing_path,
+                ControlFlow::Continue(t) => ty = t,
+            },
+            Type::Ptr(t) => ty = &t.elem,
+            Type::Reference(t) => ty = &t.elem,
+            Type::TraitObject(t) => match last_type_in_bounds(&t.bounds) {
+                ControlFlow::Break(trailing_path) => return trailing_path,
+                ControlFlow::Continue(t) => ty = t,
+            },
+
+            Type::Array(_)
+            | Type::Group(_)
+            | Type::Infer(_)
+            | Type::Macro(_)
+            | Type::Never(_)
+            | Type::Paren(_)
+            | Type::Slice(_)
+            | Type::Tuple(_)
+            | Type::Verbatim(_) => return false,
+        }
+    }
+
+    fn last_type_in_path(path: &Path) -> ControlFlow<bool, &Type> {
+        match &path.segments.last().unwrap().arguments {
+            PathArguments::None => ControlFlow::Break(true),
+            PathArguments::AngleBracketed(_) => ControlFlow::Break(false),
+            PathArguments::Parenthesized(arg) => match &arg.output {
+                ReturnType::Default => ControlFlow::Break(false),
+                ReturnType::Type(_, ret) => ControlFlow::Continue(ret),
+            },
+        }
+    }
+
+    fn last_type_in_bounds(
+        bounds: &Punctuated<TypeParamBound, Token![+]>,
+    ) -> ControlFlow<bool, &Type> {
+        match bounds.last().unwrap() {
+            TypeParamBound::Trait(t) => last_type_in_path(&t.path),
+            TypeParamBound::Lifetime(_)
+            | TypeParamBound::PreciseCapture(_)
+            | TypeParamBound::Verbatim(_) => ControlFlow::Break(false),
+        }
+    }
+}
+
+/// Whether the expression's first token is the label of a loop/block.
+#[cfg(all(feature = "printing", feature = "full"))]
+pub(crate) fn expr_leading_label(mut expr: &Expr) -> bool {
+    loop {
+        match expr {
+            Expr::Block(e) => return e.label.is_some(),
+            Expr::ForLoop(e) => return e.label.is_some(),
+            Expr::Loop(e) => return e.label.is_some(),
+            Expr::While(e) => return e.label.is_some(),
+
+            Expr::Assign(e) => expr = &e.left,
+            Expr::Await(e) => expr = &e.base,
+            Expr::Binary(e) => expr = &e.left,
+            Expr::Call(e) => expr = &e.func,
+            Expr::Cast(e) => expr = &e.expr,
+            Expr::Field(e) => expr = &e.base,
+            Expr::Index(e) => expr = &e.expr,
+            Expr::MethodCall(e) => expr = &e.receiver,
+            Expr::Range(e) => match &e.start {
+                Some(start) => expr = start,
+                None => return false,
+            },
+            Expr::Try(e) => expr = &e.expr,
+
+            Expr::Array(_)
+            | Expr::Async(_)
+            | Expr::Break(_)
+            | Expr::Closure(_)
+            | Expr::Const(_)
+            | Expr::Continue(_)
+            | Expr::Group(_)
+            | Expr::If(_)
+            | Expr::Infer(_)
+            | Expr::Let(_)
+            | Expr::Lit(_)
+            | Expr::Macro(_)
+            | Expr::Match(_)
+            | Expr::Paren(_)
+            | Expr::Path(_)
+            | Expr::RawAddr(_)
+            | Expr::Reference(_)
+            | Expr::Repeat(_)
+            | Expr::Return(_)
+            | Expr::Struct(_)
+            | Expr::TryBlock(_)
+            | Expr::Tuple(_)
+            | Expr::Unary(_)
+            | Expr::Unsafe(_)
+            | Expr::Verbatim(_)
+            | Expr::Yield(_) => return false,
+        }
+    }
+}
+
+/// Whether the expression's last token is `}`.
+#[cfg(feature = "full")]
+pub(crate) fn expr_trailing_brace(mut expr: &Expr) -> bool {
+    loop {
+        match expr {
+            Expr::Async(_)
+            | Expr::Block(_)
+            | Expr::Const(_)
+            | Expr::ForLoop(_)
+            | Expr::If(_)
+            | Expr::Loop(_)
+            | Expr::Match(_)
+            | Expr::Struct(_)
+            | Expr::TryBlock(_)
+            | Expr::Unsafe(_)
+            | Expr::While(_) => return true,
+
+            Expr::Assign(e) => expr = &e.right,
+            Expr::Binary(e) => expr = &e.right,
+            Expr::Break(e) => match &e.expr {
+                Some(e) => expr = e,
+                None => return false,
+            },
+            Expr::Cast(e) => return type_trailing_brace(&e.ty),
+            Expr::Closure(e) => expr = &e.body,
+            Expr::Let(e) => expr = &e.expr,
+            Expr::Macro(e) => return e.mac.delimiter.is_brace(),
+            Expr::Range(e) => match &e.end {
+                Some(end) => expr = end,
+                None => return false,
+            },
+            Expr::RawAddr(e) => expr = &e.expr,
+            Expr::Reference(e) => expr = &e.expr,
+            Expr::Return(e) => match &e.expr {
+                Some(e) => expr = e,
+                None => return false,
+            },
+            Expr::Unary(e) => expr = &e.expr,
+            Expr::Verbatim(e) => return tokens_trailing_brace(e),
+            Expr::Yield(e) => match &e.expr {
+                Some(e) => expr = e,
+                None => return false,
+            },
+
+            Expr::Array(_)
+            | Expr::Await(_)
+            | Expr::Call(_)
+            | Expr::Continue(_)
+            | Expr::Field(_)
+            | Expr::Group(_)
+            | Expr::Index(_)
+            | Expr::Infer(_)
+            | Expr::Lit(_)
+            | Expr::MethodCall(_)
+            | Expr::Paren(_)
+            | Expr::Path(_)
+            | Expr::Repeat(_)
+            | Expr::Try(_)
+            | Expr::Tuple(_) => return false,
+        }
+    }
+
+    fn type_trailing_brace(mut ty: &Type) -> bool {
+        loop {
+            match ty {
+                Type::BareFn(t) => match &t.output {
+                    ReturnType::Default => return false,
+                    ReturnType::Type(_, ret) => ty = ret,
+                },
+                Type::ImplTrait(t) => match last_type_in_bounds(&t.bounds) {
+                    ControlFlow::Break(trailing_brace) => return trailing_brace,
+                    ControlFlow::Continue(t) => ty = t,
+                },
+                Type::Macro(t) => return t.mac.delimiter.is_brace(),
+                Type::Path(t) => match last_type_in_path(&t.path) {
+                    Some(t) => ty = t,
+                    None => return false,
+                },
+                Type::Ptr(t) => ty = &t.elem,
+                Type::Reference(t) => ty = &t.elem,
+                Type::TraitObject(t) => match last_type_in_bounds(&t.bounds) {
+                    ControlFlow::Break(trailing_brace) => return trailing_brace,
+                    ControlFlow::Continue(t) => ty = t,
+                },
+                Type::Verbatim(t) => return tokens_trailing_brace(t),
+
+                Type::Array(_)
+                | Type::Group(_)
+                | Type::Infer(_)
+                | Type::Never(_)
+                | Type::Paren(_)
+                | Type::Slice(_)
+                | Type::Tuple(_) => return false,
+            }
+        }
+    }
+
+    fn last_type_in_path(path: &Path) -> Option<&Type> {
+        match &path.segments.last().unwrap().arguments {
+            PathArguments::None | PathArguments::AngleBracketed(_) => None,
+            PathArguments::Parenthesized(arg) => match &arg.output {
+                ReturnType::Default => None,
+                ReturnType::Type(_, ret) => Some(ret),
+            },
+        }
+    }
+
+    fn last_type_in_bounds(
+        bounds: &Punctuated<TypeParamBound, Token![+]>,
+    ) -> ControlFlow<bool, &Type> {
+        match bounds.last().unwrap() {
+            TypeParamBound::Trait(t) => match last_type_in_path(&t.path) {
+                Some(t) => ControlFlow::Continue(t),
+                None => ControlFlow::Break(false),
+            },
+            TypeParamBound::Lifetime(_) | TypeParamBound::PreciseCapture(_) => {
+                ControlFlow::Break(false)
+            }
+            TypeParamBound::Verbatim(t) => ControlFlow::Break(tokens_trailing_brace(t)),
+        }
+    }
+
+    fn tokens_trailing_brace(tokens: &TokenStream) -> bool {
+        if let Some(TokenTree::Group(last)) = tokens.clone().into_iter().last() {
+            last.delimiter() == Delimiter::Brace
+        } else {
+            false
+        }
+    }
+}