Separately check equality of the scalar types and compound types in the order of declaration.

Author: Sol-Ell

compiler/rustc_ast/src/ast.rs

@@ -2195,6 +2195,34 @@ impl FloatTy {
     }
 }
 
+impl<'a> TryFrom<&'a str> for FloatTy {
+    type Error = ();
+
+    fn try_from(value: &'a str) -> Result<Self, Self::Error> {
+        Ok(match value {
+            "f16" => Self::F16,
+            "f32" => Self::F32,
+            "f64" => Self::F64,
+            "f128" => Self::F128,
+            _ => return Err(()),
+        })
+    }
+}
+
+impl TryFrom<Symbol> for FloatTy {
+    type Error = ();
+
+    fn try_from(value: Symbol) -> Result<Self, Self::Error> {
+        Ok(match value {
+            sym::f16 => Self::F16,
+            sym::f32 => Self::F32,
+            sym::f64 => Self::F64,
+            sym::f128 => Self::F128,
+            _ => return Err(()),
+        })
+    }
+}
+
 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
 #[derive(Encodable, Decodable, HashStable_Generic)]
 pub enum IntTy {
@@ -2230,6 +2258,38 @@ impl IntTy {
     }
 }
 
+impl<'a> TryFrom<&'a str> for IntTy {
+    type Error = ();
+
+    fn try_from(value: &'a str) -> Result<Self, Self::Error> {
+        Ok(match value {
+            "isize" => Self::Isize,
+            "i8" => Self::I8,
+            "i16" => Self::I16,
+            "i32" => Self::I32,
+            "i64" => Self::I64,
+            "i128" => Self::I128,
+            _ => return Err(()),
+        })
+    }
+}
+
+impl TryFrom<Symbol> for IntTy {
+    type Error = ();
+
+    fn try_from(value: Symbol) -> Result<Self, Self::Error> {
+        Ok(match value {
+            sym::isize => Self::Isize,
+            sym::i8 => Self::I8,
+            sym::i16 => Self::I16,
+            sym::i32 => Self::I32,
+            sym::i64 => Self::I64,
+            sym::i128 => Self::I128,
+            _ => return Err(()),
+        })
+    }
+}
+
 #[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Copy, Debug)]
 #[derive(Encodable, Decodable, HashStable_Generic)]
 pub enum UintTy {
@@ -2265,6 +2325,38 @@ impl UintTy {
     }
 }
 
+impl<'a> TryFrom<&'a str> for UintTy {
+    type Error = ();
+
+    fn try_from(value: &'a str) -> Result<Self, Self::Error> {
+        Ok(match value {
+            "usize" => Self::Usize,
+            "u8" => Self::U8,
+            "u16" => Self::U16,
+            "u32" => Self::U32,
+            "u64" => Self::U64,
+            "u128" => Self::U128,
+            _ => return Err(()),
+        })
+    }
+}
+
+impl TryFrom<Symbol> for UintTy {
+    type Error = ();
+
+    fn try_from(value: Symbol) -> Result<Self, Self::Error> {
+        Ok(match value {
+            sym::usize => Self::Usize,
+            sym::u8 => Self::U8,
+            sym::u16 => Self::U16,
+            sym::u32 => Self::U32,
+            sym::u64 => Self::U64,
+            sym::u128 => Self::U128,
+            _ => return Err(()),
+        })
+    }
+}
+
 /// A constraint on an associated item.
 ///
 /// ### Examples
@@ -2452,6 +2544,21 @@ impl TyKind {
             None
         }
     }
+
+    pub fn is_scalar(&self) -> bool {
+        let Some(ty_kind) = self.is_simple_path() else {
+            match self {
+                TyKind::Tup(tys) => return tys.is_empty(), // unit type
+                _ => return false,
+            }
+        };
+
+        ty_kind == sym::bool
+            || ty_kind == sym::char
+            || IntTy::try_from(ty_kind).is_ok()
+            || UintTy::try_from(ty_kind).is_ok()
+            || FloatTy::try_from(ty_kind).is_ok()
+    }
 }
 
 /// A pattern type pattern.

compiler/rustc_builtin_macros/src/deriving/cmp/partial_eq.rs

@@ -8,6 +8,7 @@ use crate::deriving::generic::ty::*;
 use crate::deriving::generic::*;
 use crate::deriving::{path_local, path_std};
 
+/// Expands a `#[derive(PartialEq)]` attribute into an implementation for the target item.
 pub(crate) fn expand_deriving_partial_eq(
     cx: &ExtCtxt<'_>,
     span: Span,
@@ -16,62 +17,6 @@ pub(crate) fn expand_deriving_partial_eq(
     push: &mut dyn FnMut(Annotatable),
     is_const: bool,
 ) {
-    fn cs_eq(cx: &ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> BlockOrExpr {
-        let base = true;
-        let expr = cs_fold(
-            true, // use foldl
-            cx,
-            span,
-            substr,
-            |cx, fold| match fold {
-                CsFold::Single(field) => {
-                    let [other_expr] = &field.other_selflike_exprs[..] else {
-                        cx.dcx()
-                            .span_bug(field.span, "not exactly 2 arguments in `derive(PartialEq)`");
-                    };
-
-                    // We received arguments of type `&T`. Convert them to type `T` by stripping
-                    // any leading `&`. This isn't necessary for type checking, but
-                    // it results in better error messages if something goes wrong.
-                    //
-                    // Note: for arguments that look like `&{ x }`, which occur with packed
-                    // structs, this would cause expressions like `{ self.x } == { other.x }`,
-                    // which isn't valid Rust syntax. This wouldn't break compilation because these
-                    // AST nodes are constructed within the compiler. But it would mean that code
-                    // printed by `-Zunpretty=expanded` (or `cargo expand`) would have invalid
-                    // syntax, which would be suboptimal. So we wrap these in parens, giving
-                    // `({ self.x }) == ({ other.x })`, which is valid syntax.
-                    let convert = |expr: &P<Expr>| {
-                        if let ExprKind::AddrOf(BorrowKind::Ref, Mutability::Not, inner) =
-                            &expr.kind
-                        {
-                            if let ExprKind::Block(..) = &inner.kind {
-                                // `&{ x }` form: remove the `&`, add parens.
-                                cx.expr_paren(field.span, inner.clone())
-                            } else {
-                                // `&x` form: remove the `&`.
-                                inner.clone()
-                            }
-                        } else {
-                            expr.clone()
-                        }
-                    };
-                    cx.expr_binary(
-                        field.span,
-                        BinOpKind::Eq,
-                        convert(&field.self_expr),
-                        convert(other_expr),
-                    )
-                }
-                CsFold::Combine(span, expr1, expr2) => {
-                    cx.expr_binary(span, BinOpKind::And, expr1, expr2)
-                }
-                CsFold::Fieldless => cx.expr_bool(span, base),
-            },
-        );
-        BlockOrExpr::new_expr(expr)
-    }
-
     let structural_trait_def = TraitDef {
         span,
         path: path_std!(marker::StructuralPartialEq),
@@ -97,7 +42,9 @@ pub(crate) fn expand_deriving_partial_eq(
         ret_ty: Path(path_local!(bool)),
         attributes: thin_vec![cx.attr_word(sym::inline, span)],
         fieldless_variants_strategy: FieldlessVariantsStrategy::Unify,
-        combine_substructure: combine_substructure(Box::new(|a, b, c| cs_eq(a, b, c))),
+        combine_substructure: combine_substructure(Box::new(|a, b, c| {
+            BlockOrExpr::new_expr(get_substructure_equality_expr(a, b, c))
+        })),
     }];
 
     let trait_def = TraitDef {
@@ -113,3 +60,138 @@ pub(crate) fn expand_deriving_partial_eq(
     };
     trait_def.expand(cx, mitem, item, push)
 }
+
+/// Generates the equality expression for a struct or enum variant when deriving `PartialEq`.
+///
+/// This function constructs an expression that compares all fields of a struct or enum variant for equality.
+/// It groups scalar and compound types separately, combining their comparisons efficiently:
+/// - If there are no fields, returns `true` (fieldless types are always equal to themselves).
+/// - Scalar fields are compared first for efficiency, then compound fields.
+/// - If only one group is non-empty, returns its comparison directly.
+/// - Otherwise, returns a conjunction (logical AND) of both groups' comparisons.
+///
+/// For enums with discriminants, compares the discriminant first, then the rest of the fields.
+///
+/// # Panics
+///
+/// If called on static or all-fieldless enums/structs, which should not occur during derive expansion.
+fn get_substructure_equality_expr(
+    cx: &ExtCtxt<'_>,
+    span: Span,
+    substructure: &Substructure<'_>,
+) -> P<Expr> {
+    /// Combines the accumulated comparison expression with the next field's comparison using logical AND.
+    ///
+    /// If this is the first field, initializes the accumulator. Otherwise, chains with logical AND.
+    fn combine(cx: &ExtCtxt<'_>, span: Span, acc: &mut Option<P<Expr>>, elem: P<Expr>) {
+        let Some(lhs) = acc.take() else {
+            *acc = Some(elem);
+            return;
+        };
+        *acc = Some(cx.expr_binary(span, BinOpKind::And, lhs, elem));
+    }
+
+    use SubstructureFields::*;
+    match substructure.fields {
+        EnumMatching(.., fields) | Struct(.., fields) => {
+            if fields.is_empty() {
+                // Fieldless structs or enum variants are always equal to themselves.
+                return cx.expr_bool(span, true);
+            }
+
+            let mut scalar_ty_cmp = None;
+            let mut compound_ty_cmp = None;
+            // Compare scalar and compound types separately for efficiency.
+            for field in fields {
+                let is_scalar = field.is_scalar;
+                let field_span = field.span;
+                let rhs = get_field_equality_expr(cx, field);
+
+                if is_scalar {
+                    // Combine scalar field comparisons first (cheaper to evaluate).
+                    combine(cx, field_span, &mut scalar_ty_cmp, rhs);
+                    continue;
+                }
+                // Combine compound (non-scalar) field comparisons.
+                combine(cx, field_span, &mut compound_ty_cmp, rhs);
+            }
+
+            // If only one group (scalar or compound) has fields, return its comparison directly.
+            if scalar_ty_cmp.is_some() ^ compound_ty_cmp.is_some() {
+                return scalar_ty_cmp.or(compound_ty_cmp).unwrap();
+            }
+
+            // If both groups are non-empty, require all fields to be equal.
+            // Scalar fields are compared first for performance.
+            return cx.expr_binary(
+                span,
+                BinOpKind::And,
+                scalar_ty_cmp.unwrap(),
+                compound_ty_cmp.unwrap(),
+            );
+        }
+        EnumDiscr(disc, match_expr) => {
+            let lhs = get_field_equality_expr(cx, disc);
+            let Some(match_expr) = match_expr else {
+                return lhs;
+            };
+            // Compare the discriminant first (cheaper), then the rest of the fields.
+            return cx.expr_binary(disc.span, BinOpKind::And, lhs, match_expr.clone());
+        }
+        StaticEnum(..) => cx.dcx().span_bug(
+            span,
+            "unexpected static enum encountered during `derive(PartialEq)` expansion",
+        ),
+        StaticStruct(..) => cx.dcx().span_bug(
+            span,
+            "unexpected static struct encountered during `derive(PartialEq)` expansion",
+        ),
+        AllFieldlessEnum(..) => cx.dcx().span_bug(
+            span,
+            "unexpected all-fieldless enum encountered during `derive(PartialEq)` expansion",
+        ),
+    }
+}
+
+/// Generates an equality comparison expression for a single struct or enum field.
+///
+/// This function produces an AST expression that compares the `self` and `other` values for a field using `==`.
+/// It removes any leading references from both sides for readability.
+/// If the field is a block expression, it is wrapped in parentheses to ensure valid syntax.
+///
+/// # Panics
+///
+/// Panics if there are not exactly two arguments to compare (should be `self` and `other`).
+fn get_field_equality_expr(cx: &ExtCtxt<'_>, field: &FieldInfo) -> P<Expr> {
+    let [rhs] = &field.other_selflike_exprs[..] else {
+        cx.dcx().span_bug(field.span, "not exactly 2 arguments in `derive(PartialEq)`");
+    };
+
+    cx.expr_binary(
+        field.span,
+        BinOpKind::Eq,
+        wrap_block_expr(cx, peel_refs(&field.self_expr)),
+        wrap_block_expr(cx, peel_refs(rhs)),
+    )
+}
+
+/// Removes all leading immutable references from an expression.
+///
+/// This is used to strip away any number of leading `&` from an expression (e.g., `&&&T` becomes `T`).
+/// Only removes immutable references; mutable references are preserved.
+fn peel_refs(mut expr: &P<Expr>) -> P<Expr> {
+    while let ExprKind::AddrOf(BorrowKind::Ref, Mutability::Not, inner) = &expr.kind {
+        expr = &inner;
+    }
+    expr.clone()
+}
+
+/// Wraps a block expression in parentheses to ensure valid AST in macro expansion output.
+///
+/// If the given expression is a block, it is wrapped in parentheses; otherwise, it is returned unchanged.
+fn wrap_block_expr(cx: &ExtCtxt<'_>, expr: P<Expr>) -> P<Expr> {
+    if matches!(&expr.kind, ExprKind::Block(..)) {
+        return cx.expr_paren(expr.span, expr);
+    }
+    expr
+}

compiler/rustc_builtin_macros/src/deriving/generic/mod.rs

@@ -284,6 +284,7 @@ pub(crate) struct FieldInfo {
     /// The expressions corresponding to references to this field in
     /// the other selflike arguments.
     pub other_selflike_exprs: Vec<P<Expr>>,
+    pub is_scalar: bool,
 }
 
 #[derive(Copy, Clone)]
@@ -1220,7 +1221,8 @@ impl<'a> MethodDef<'a> {
 
             let self_expr = discr_exprs.remove(0);
             let other_selflike_exprs = discr_exprs;
-            let discr_field = FieldInfo { span, name: None, self_expr, other_selflike_exprs };
+            let discr_field =
+                FieldInfo { span, name: None, self_expr, other_selflike_exprs, is_scalar: true };
 
             let discr_let_stmts: ThinVec<_> = iter::zip(&discr_idents, &selflike_args)
                 .map(|(&ident, selflike_arg)| {
@@ -1533,6 +1535,7 @@ impl<'a> TraitDef<'a> {
                     name: struct_field.ident,
                     self_expr,
                     other_selflike_exprs,
+                    is_scalar: struct_field.ty.peel_refs().kind.is_scalar(),
                 }
             })
             .collect()