Add useful comments on ExprKind::If variants.

Things that aren't obvious and took me a while to work out.

compiler/rustc_ast/src/ast.rs

@@ -1633,6 +1633,9 @@ pub enum ExprKind {
     /// An `if` block, with an optional `else` block.
     ///
     /// `if expr { block } else { expr }`
+    ///
+    /// If present, the "else" expr is always `ExprKind::Block` (for `else`) or
+    /// `ExprKind::If` (for `else if`).
     If(P<Expr>, P<Block>, Option<P<Expr>>),
     /// A while loop, with an optional label.
     ///

compiler/rustc_hir/src/hir.rs

@@ -2730,6 +2730,9 @@ pub enum ExprKind<'hir> {
     /// An `if` block, with an optional else block.
     ///
     /// I.e., `if <expr> { <expr> } else { <expr> }`.
+    ///
+    /// The "then" expr is always `ExprKind::Block`. If present, the "else" expr is always
+    /// `ExprKind::Block` (for `else`) or `ExprKind::If` (for `else if`).
     If(&'hir Expr<'hir>, &'hir Expr<'hir>, Option<&'hir Expr<'hir>>),
     /// A conditionless loop (can be exited with `break`, `continue`, or `return`).
     ///

compiler/rustc_middle/src/thir.rs

@@ -292,7 +292,10 @@ pub enum ExprKind<'tcx> {
     If {
         if_then_scope: region::Scope,
         cond: ExprId,
+        /// `then` is always `ExprKind::Block`.
         then: ExprId,
+        /// If present, the `else_opt` expr is always `ExprKind::Block` (for
+        /// `else`) or `ExprKind::If` (for `else if`).
         else_opt: Option<ExprId>,
     },
     /// A function call. Method calls and overloaded operators are converted to plain function calls.