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Don't use match-destructuring for derived ops on structs.
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All derive ops currently use match-destructuring to access fields. This
is reasonable for enums, but sub-optimal for structs. E.g.:
```
fn eq(&self, other: &Point) -> bool {
    match *other {
	Self { x: ref __self_1_0, y: ref __self_1_1 } =>
	    match *self {
		Self { x: ref __self_0_0, y: ref __self_0_1 } =>
		    (*__self_0_0) == (*__self_1_0) &&
			(*__self_0_1) == (*__self_1_1),
	    },
    }
}
```
This commit changes derive ops on structs to use field access instead, e.g.:
```
fn eq(&self, other: &Point) -> bool {
    self.x == other.x && self.y == other.y
}
```
This is faster to compile, results in smaller binaries, and is simpler to
generate. Unfortunately, we have to keep the old pattern generating code around
for `repr(packed)` structs because something like `&self.x` (which doesn't show
up in `PartialEq` ops, but does show up in `Debug` and `Hash` ops) isn't
allowed. But this commit at least changes those cases to use let-destructuring
instead of match-destructuring, e.g.:
```
fn hash<__H: ::core::hash::Hasher>(&self, state: &mut __H) -> () {
    {
	let Self(ref __self_0_0) = *self;
	{ ::core::hash::Hash::hash(&(*__self_0_0), state) }
    }
}
```
There are some unnecessary blocks remaining in the generated code, but I
will fix them in a follow-up PR.
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@nnethercote
nnethercote committed Jul 4, 2022
1 parent 528343f commit ecc6e95
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Showing 5 changed files with 314 additions and 470 deletions.
124 changes: 80 additions & 44 deletions compiler/rustc_builtin_macros/src/deriving/generic/mod.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -429,6 +429,7 @@ impl<'a> TraitDef<'a> {
generics,
from_scratch,
use_temporaries,
is_packed,
),
ast::ItemKind::Enum(ref enum_def, ref generics) => {
// We ignore `use_temporaries` here, because
Expand All @@ -448,6 +449,7 @@ impl<'a> TraitDef<'a> {
generics,
from_scratch,
use_temporaries,
is_packed,
)
} else {
cx.span_err(mitem.span, "this trait cannot be derived for unions");
Expand Down Expand Up @@ -729,6 +731,7 @@ impl<'a> TraitDef<'a> {
generics: &Generics,
from_scratch: bool,
use_temporaries: bool,
is_packed: bool,
) -> P<ast::Item> {
let field_tys: Vec<P<ast::Ty>> =
struct_def.fields().iter().map(|field| field.ty.clone()).collect();
Expand Down Expand Up @@ -757,6 +760,7 @@ impl<'a> TraitDef<'a> {
&self_args,
&nonself_args,
use_temporaries,
is_packed,
)
};

Expand Down Expand Up @@ -945,6 +949,7 @@ impl<'a> MethodDef<'a> {
})
}

/// The normal case uses field access.
/// ```
/// #[derive(PartialEq)]
/// # struct Dummy;
Expand All @@ -953,33 +958,21 @@ impl<'a> MethodDef<'a> {
/// // equivalent to:
/// impl PartialEq for A {
/// fn eq(&self, other: &A) -> bool {
/// match *self {
/// A {x: ref __self_0_0, y: ref __self_0_1} => {
/// match *other {
/// A {x: ref __self_1_0, y: ref __self_1_1} => {
/// __self_0_0.eq(__self_1_0) && __self_0_1.eq(__self_1_1)
/// }
/// }
/// }
/// }
/// self.x == other.x && self.y == other.y
/// }
/// }
/// ```
/// or if A is repr(packed) - note fields are matched by-value
/// instead of by-reference.
/// But if the struct is `repr(packed)`, we can't use something like
/// `&self.x` on a packed type (as required for e.g. `Debug` and `Hash`)
/// because that might cause an unaligned ref. So we use let-destructuring
/// instead.
/// ```
/// # struct A { x: i32, y: i32 }
/// impl PartialEq for A {
/// fn eq(&self, other: &A) -> bool {
/// match *self {
/// A {x: __self_0_0, y: __self_0_1} => {
/// match other {
/// A {x: __self_1_0, y: __self_1_1} => {
/// __self_0_0.eq(&__self_1_0) && __self_0_1.eq(&__self_1_1)
/// }
/// }
/// }
/// }
/// let Self { x: ref __self_0_0, y: ref __self_0_1 } = *self;
/// let Self { x: ref __self_1_0, y: ref __self_1_1 } = *other;
/// *__self_0_0 == *__self_1_0 && *__self_0_1 == *__self_1_1
/// }
/// }
/// ```
Expand All @@ -992,24 +985,33 @@ impl<'a> MethodDef<'a> {
self_args: &[P<Expr>],
nonself_args: &[P<Expr>],
use_temporaries: bool,
is_packed: bool,
) -> P<Expr> {
let mut raw_fields = Vec::new(); // Vec<[fields of self], [fields of next Self arg], [etc]>
let span = trait_.span;
let mut patterns = Vec::new();
for i in 0..self_args.len() {
// We could use `type_ident` instead of `Self`, but in the case of a type parameter
// shadowing the struct name, that causes a second, unnecessary E0578 error. #97343
let struct_path = cx.path(span, vec![Ident::new(kw::SelfUpper, type_ident.span)]);
let (pat, ident_expr) = trait_.create_struct_pattern(
cx,
struct_path,
struct_def,
&format!("__self_{}", i),
ast::Mutability::Not,
use_temporaries,
);
patterns.push(pat);
raw_fields.push(ident_expr);

for (i, self_arg) in self_args.iter().enumerate() {
let ident_exprs = if !is_packed {
trait_.create_struct_field_accesses(cx, self_arg, struct_def)
} else {
// Get the pattern for the let-destructuring.
//
// We could use `type_ident` instead of `Self`, but in the case of a type parameter
// shadowing the struct name, that causes a second, unnecessary E0578 error. #97343
let struct_path = cx.path(span, vec![Ident::new(kw::SelfUpper, type_ident.span)]);
let (pat, ident_exprs) = trait_.create_struct_pattern(
cx,
struct_path,
struct_def,
&format!("__self_{}", i),
ast::Mutability::Not,
use_temporaries,
);
patterns.push(pat);
ident_exprs
};
raw_fields.push(ident_exprs);
}

// transpose raw_fields
Expand All @@ -1036,7 +1038,6 @@ impl<'a> MethodDef<'a> {
cx.span_bug(span, "no `self` parameter for method in generic `derive`")
};

// body of the inner most destructuring match
let mut body = self.call_substructure_method(
cx,
trait_,
Expand All @@ -1045,14 +1046,18 @@ impl<'a> MethodDef<'a> {
&Struct(struct_def, fields),
);

// make a series of nested matches, to destructure the
// structs. This is actually right-to-left, but it shouldn't
// matter.
for (arg_expr, pat) in iter::zip(self_args, patterns) {
body = cx.expr_match(span, arg_expr.clone(), vec![cx.arm(span, pat.clone(), body)])
}
if !is_packed {
body.span = span;
body
} else {
// Do the let-destructuring.
let mut stmts: Vec<_> = iter::zip(self_args, patterns)
.map(|(arg_expr, pat)| cx.stmt_let_pat(span, pat, arg_expr.clone()))
.collect();
stmts.push(cx.stmt_expr(body));

body
cx.expr_block(cx.block(span, stmts))
}
}

fn expand_static_struct_method_body(
Expand Down Expand Up @@ -1522,8 +1527,6 @@ impl<'a> TraitDef<'a> {
paths.push(ident.with_span_pos(sp));
let val = cx.expr_path(cx.path_ident(sp, ident));
let val = if use_temporaries { val } else { cx.expr_deref(sp, val) };
let val = cx.expr(sp, ast::ExprKind::Paren(val));

ident_exprs.push((sp, struct_field.ident, val, &struct_field.attrs[..]));
}

Expand Down Expand Up @@ -1555,6 +1558,39 @@ impl<'a> TraitDef<'a> {
(pattern, ident_exprs)
}

fn create_struct_field_accesses(
&self,
cx: &mut ExtCtxt<'_>,
mut self_arg: &P<Expr>,
struct_def: &'a VariantData,
) -> Vec<(Span, Option<Ident>, P<Expr>, &'a [ast::Attribute])> {
let mut ident_exprs = Vec::new();
for (i, struct_field) in struct_def.fields().iter().enumerate() {
let sp = struct_field.span.with_ctxt(self.span.ctxt());

// We don't the need the deref, if there is one.
if let ast::ExprKind::Unary(ast::UnOp::Deref, inner) = &self_arg.kind {
self_arg = inner;
}

// Note: we must use `struct_field.span` rather than `span` in the
// `unwrap_or_else` case otherwise the hygiene is wrong and we get
// "field `0` of struct `Point` is private" errors on tuple
// structs.
let val = cx.expr(
sp,
ast::ExprKind::Field(
self_arg.clone(),
struct_field.ident.unwrap_or_else(|| {
Ident::from_str_and_span(&i.to_string(), struct_field.span)
}),
),
);
ident_exprs.push((sp, struct_field.ident, val, &struct_field.attrs[..]));
}
ident_exprs
}

fn create_enum_variant_pattern(
&self,
cx: &mut ExtCtxt<'_>,
Expand Down
13 changes: 13 additions & 0 deletions compiler/rustc_expand/src/build.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -152,6 +152,19 @@ impl<'a> ExtCtxt<'a> {
ast::Stmt { id: ast::DUMMY_NODE_ID, span: expr.span, kind: ast::StmtKind::Expr(expr) }
}

pub fn stmt_let_pat(&self, sp: Span, pat: P<ast::Pat>, ex: P<ast::Expr>) -> ast::Stmt {
let local = P(ast::Local {
pat,
ty: None,
id: ast::DUMMY_NODE_ID,
kind: LocalKind::Init(ex),
span: sp,
attrs: AttrVec::new(),
tokens: None,
});
self.stmt_local(local, sp)
}

pub fn stmt_let(&self, sp: Span, mutbl: bool, ident: Ident, ex: P<ast::Expr>) -> ast::Stmt {
self.stmt_let_ty(sp, mutbl, ident, None, ex)
}
Expand Down
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