Fix syntax in -Zunpretty-expanded output for derived PartialEq.

If you do `derive(PartialEq)` on a packed struct, the output shown by
`-Zunpretty=expanded` includes expressions like this:
```
{ self.x } == { other.x }
```
This is invalid syntax. This doesn't break compilation, because the AST
nodes are constructed within the compiler. But it does mean anyone using
`-Zunpretty=expanded` output as a guide for hand-written impls could get
a nasty surprise.

This commit fixes things by instead using this form:
```
({ self.x }) == ({ other.x })
```

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

@@ -29,16 +29,30 @@ pub fn expand_deriving_partial_eq(
  cx.span_bug(field.span, "not exactly 2 arguments in `derive(PartialEq)`");
  };
 
- // We received `&T` arguments. Convert them to `T` by
- // stripping `&` or adding `*`. This isn't necessary for
- // type checking, but it results in much better error
- // messages if something goes wrong.
+ // We received arguments of type `&T`. Convert them to type `T` by stripping
+ // any leading `&` or adding `*`. 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
  {
- inner.clone()
+ 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 {
+ // No leading `&`: add a leading `*`.
  cx.expr_deref(field.span, expr.clone())
  }
  };

compiler/rustc_expand/src/build.rs

@@ -272,6 +272,10 @@ impl<'a> ExtCtxt<'a> {
  self.expr(sp, ast::ExprKind::AddrOf(ast::BorrowKind::Ref, ast::Mutability::Not, e))
  }
 
+ pub fn expr_paren(&self, sp: Span, e: P<ast::Expr>) -> P<ast::Expr> {
+ self.expr(sp, ast::ExprKind::Paren(e))
+ }
+
  pub fn expr_call(
  &self,
  span: Span,

tests/ui/deriving/deriving-all-codegen.stdout

@@ -209,7 +209,7 @@ impl ::core::marker::StructuralPartialEq for PackedPoint { }
 impl ::core::cmp::PartialEq for PackedPoint {
  #[inline]
  fn eq(&self, other: &PackedPoint) -> bool {
- { self.x } == { other.x } && { self.y } == { other.y }
+ ({ self.x }) == ({ other.x }) && ({ self.y }) == ({ other.y })
  }
 }
 #[automatically_derived]
@@ -718,8 +718,8 @@ impl<T: ::core::cmp::PartialEq + ::core::marker::Copy + Trait,
  ::core::marker::Copy {
  #[inline]
  fn eq(&self, other: &PackedGeneric<T, U>) -> bool {
- { self.0 } == { other.0 } && { self.1 } == { other.1 } &&
- { self.2 } == { other.2 }
+ ({ self.0 }) == ({ other.0 }) && ({ self.1 }) == ({ other.1 }) &&
+ ({ self.2 }) == ({ other.2 })
  }
 }
 #[automatically_derived]