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.

compiler/rustc_ast/src/ast.rs

@@ -981,6 +981,75 @@ impl BinOpKind {
 
 pub type BinOp = Spanned<BinOpKind>;
 
+// Sometimes `BinOpKind` and `AssignOpKind` need the same treatment. The
+// operations covered by `AssignOpKind` are a subset of those covered by
+// `BinOpKind`, so it makes sense to convert `AssignOpKind` to `BinOpKind`.
+impl From<AssignOpKind> for BinOpKind {
+    fn from(op: AssignOpKind) -> BinOpKind {
+        match op {
+            AssignOpKind::AddAssign => BinOpKind::Add,
+            AssignOpKind::SubAssign => BinOpKind::Sub,
+            AssignOpKind::MulAssign => BinOpKind::Mul,
+            AssignOpKind::DivAssign => BinOpKind::Div,
+            AssignOpKind::RemAssign => BinOpKind::Rem,
+            AssignOpKind::BitXorAssign => BinOpKind::BitXor,
+            AssignOpKind::BitAndAssign => BinOpKind::BitAnd,
+            AssignOpKind::BitOrAssign => BinOpKind::BitOr,
+            AssignOpKind::ShlAssign => BinOpKind::Shl,
+            AssignOpKind::ShrAssign => BinOpKind::Shr,
+        }
+    }
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Encodable, Decodable, HashStable_Generic)]
+pub enum AssignOpKind {
+    /// The `+=` operator (addition)
+    AddAssign,
+    /// The `-=` operator (subtraction)
+    SubAssign,
+    /// The `*=` operator (multiplication)
+    MulAssign,
+    /// The `/=` operator (division)
+    DivAssign,
+    /// The `%=` operator (modulus)
+    RemAssign,
+    /// The `^=` operator (bitwise xor)
+    BitXorAssign,
+    /// The `&=` operator (bitwise and)
+    BitAndAssign,
+    /// The `|=` operator (bitwise or)
+    BitOrAssign,
+    /// The `<<=` operator (shift left)
+    ShlAssign,
+    /// The `>>=` operator (shift right)
+    ShrAssign,
+}
+
+impl AssignOpKind {
+    pub fn as_str(&self) -> &'static str {
+        use AssignOpKind::*;
+        match self {
+            AddAssign => "+=",
+            SubAssign => "-=",
+            MulAssign => "*=",
+            DivAssign => "/=",
+            RemAssign => "%=",
+            BitXorAssign => "^=",
+            BitAndAssign => "&=",
+            BitOrAssign => "|=",
+            ShlAssign => "<<=",
+            ShrAssign => ">>=",
+        }
+    }
+
+    /// AssignOps are always by value.
+    pub fn is_by_value(self) -> bool {
+        true
+    }
+}
+
+pub type AssignOp = Spanned<AssignOpKind>;
+
 /// Unary operator.
 ///
 /// Note that `&data` is not an operator, it's an `AddrOf` expression.
@@ -1593,7 +1662,7 @@ pub enum ExprKind {
     /// An assignment with an operator.
     ///
     /// E.g., `a += 1`.
-    AssignOp(BinOp, P<Expr>, P<Expr>),
+    AssignOp(AssignOp, P<Expr>, P<Expr>),
     /// Access of a named (e.g., `obj.foo`) or unnamed (e.g., `obj.0`) struct field.
     Field(P<Expr>, Ident),
     /// An indexing operation (e.g., `foo[2]`).

compiler/rustc_ast/src/util/parser.rs

@@ -1,6 +1,6 @@
 use rustc_span::kw;
 
-use crate::ast::{self, BinOpKind, RangeLimits};
+use crate::ast::{self, AssignOpKind, BinOpKind, RangeLimits};
 use crate::token::{self, Token};
 
 /// Associative operator.
@@ -9,7 +9,7 @@ pub enum AssocOp {
     /// A binary op.
     Binary(BinOpKind),
     /// `?=` where ? is one of the assignable BinOps
-    AssignOp(BinOpKind),
+    AssignOp(AssignOpKind),
     /// `=`
     Assign,
     /// `as`
@@ -44,16 +44,16 @@ impl AssocOp {
             token::Or => Some(Binary(BinOpKind::BitOr)),
             token::Shl => Some(Binary(BinOpKind::Shl)),
             token::Shr => Some(Binary(BinOpKind::Shr)),
-            token::PlusEq => Some(AssignOp(BinOpKind::Add)),
-            token::MinusEq => Some(AssignOp(BinOpKind::Sub)),
-            token::StarEq => Some(AssignOp(BinOpKind::Mul)),
-            token::SlashEq => Some(AssignOp(BinOpKind::Div)),
-            token::PercentEq => Some(AssignOp(BinOpKind::Rem)),
-            token::CaretEq => Some(AssignOp(BinOpKind::BitXor)),
-            token::AndEq => Some(AssignOp(BinOpKind::BitAnd)),
-            token::OrEq => Some(AssignOp(BinOpKind::BitOr)),
-            token::ShlEq => Some(AssignOp(BinOpKind::Shl)),
-            token::ShrEq => Some(AssignOp(BinOpKind::Shr)),
+            token::PlusEq => Some(AssignOp(AssignOpKind::AddAssign)),
+            token::MinusEq => Some(AssignOp(AssignOpKind::SubAssign)),
+            token::StarEq => Some(AssignOp(AssignOpKind::MulAssign)),
+            token::SlashEq => Some(AssignOp(AssignOpKind::DivAssign)),
+            token::PercentEq => Some(AssignOp(AssignOpKind::RemAssign)),
+            token::CaretEq => Some(AssignOp(AssignOpKind::BitXorAssign)),
+            token::AndEq => Some(AssignOp(AssignOpKind::BitAndAssign)),
+            token::OrEq => Some(AssignOp(AssignOpKind::BitOrAssign)),
+            token::ShlEq => Some(AssignOp(AssignOpKind::ShlAssign)),
+            token::ShrEq => Some(AssignOp(AssignOpKind::ShrAssign)),
             token::Lt => Some(Binary(BinOpKind::Lt)),
             token::Le => Some(Binary(BinOpKind::Le)),
             token::Ge => Some(Binary(BinOpKind::Ge)),