Tighten up assignment operator representations.

In the AST, currently we use `BinOpKind` within `ExprKind::AssignOp` and
`AssocOp::AssignOp`, even though this allows some nonsensical
combinations. E.g. there is no `&&=` operator. Likewise for HIR and
THIR.

This commit introduces `AssignOpKind` which only includes the ten
assignable operators, and uses it in `ExprKind::AssignOp` and
`AssocOp::AssignOp`. (And does similar things for `hir::ExprKind` and
`thir::ExprKind`.) This avoids the possibility of nonsensical
combinations, as seen by the removal of the `bug!` case in
`lang_item_for_binop`.

The commit is mostly plumbing, including:
- Adds an `impl From<AssignOpKind> for BinOpKind` (AST) and `impl
  From<AssignOp> for BinOp` (MIR/THIR).
- `BinOpCategory` can now be created from both `BinOpKind` and
  `AssignOpKind`.
- Replaces the `IsAssign` type with `Op`, which has more information and
  a few methods.
- `suggest_swapping_lhs_and_rhs`: moves the condition to the call site,
  it's easier that way.
- `check_expr_inner`: had to factor out some code into a separate
  method.

I'm on the fence about whether avoiding the nonsensical combinations is
worth the extra code.

1 files changed, 5 insertions, 1 deletions

diff --git a/compiler/rustc_ast_lowering/src/expr.rs b/compiler/rustc_ast_lowering/src/expr.rs
index 52291fdfb30..80bb1e8fc41 100644
--- a/compiler/rustc_ast_lowering/src/expr.rs
+++ b/compiler/rustc_ast_lowering/src/expr.rs
@@ -274,7 +274,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 }
                 ExprKind::Assign(el, er, span) => self.lower_expr_assign(el, er, *span, e.span),
                 ExprKind::AssignOp(op, el, er) => hir::ExprKind::AssignOp(
-                    self.lower_binop(*op),
+                    self.lower_assign_op(*op),
                     self.lower_expr(el),
                     self.lower_expr(er),
                 ),
@@ -443,6 +443,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
         Spanned { node: b.node, span: self.lower_span(b.span) }
     }
 
+    fn lower_assign_op(&mut self, a: AssignOp) -> AssignOp {
+        Spanned { node: a.node, span: self.lower_span(a.span) }
+    }
+
     fn lower_legacy_const_generics(
         &mut self,
         mut f: Expr,