Add more correct grammar for cfg_select!

Macros can be renamed when imported and can delimit their bodies with
different tokens, so we don't want to document the outer part of the
call; we just want to document the input to the macro.

Doing this for `cfg_select!` is a bit tricky since an expression with
a block doesn't need to be followed by a comma while an expression
without a block does unless it's the last one.  We don't want to
handle this the way that the `match` grammar does, currently, since we
want to move toward finite lookahead.  So, instead, we handle this
with right recursion in `CfgSelectArms`.

Unlike `match`, outer attributes aren't allowed on the RHS
expressions.  To handle this, we need to refactor
`ExpressionWithoutBlock` and `ExpressionWithBlock`.

We also document that the outer braces are removed during expansion
when the payload is a block expression.

(We called it the arm "payload" after running out of other options.)

Author: traviscross

src/conditional-compilation.md

@@ -471,19 +471,22 @@ r[cfg.cfg_select]
 
 r[cfg.cfg_select.syntax]
 ```grammar,configuration
-CfgSelect ->
-    cfg_select! `{` CfgSelectBranch* `}`
+@root CfgSelect -> CfgSelectArms?
+
+CfgSelectArms ->
+    CfgSelectConfigurationPredicate `=>`
+    (
+        `{` ^ TokenTree `}` CfgSelectArms?
+      | ExpressionWithBlockNoAttrs `,`? CfgSelectArms?
+      | ExpressionWithoutBlockNoAttrs ( `,` CfgSelectArms? )?
+    )
 
 CfgSelectConfigurationPredicate ->
     ConfigurationPredicate | `_`
-
-CfgSelectBranch ->
-    CfgSelectConfigurationPredicate `=>` `{` TokenTree `}`
-  | CfgSelectConfigurationPredicate `=>` TokenTree `,`
 ```
 
 r[cfg.cfg_select.general]
-The built-in `cfg_select` macro expands to the `TokenTree` on the right-hand side of the first configuration predicate that evaluates to `true`.
+The built-in `cfg_select` macro expands to the arm payload of the first configuration predicate that evaluates to `true`. If the payload is wrapped in curly braces, those are removed during expansion.
 
 For example:
 

src/expressions.md

@@ -8,41 +8,41 @@ Expression ->
     | ExpressionWithBlock
 
 ExpressionWithoutBlock ->
-    OuterAttribute*
-    (
-        LiteralExpression
-      | PathExpression
-      | OperatorExpression
-      | GroupedExpression
-      | ArrayExpression
-      | AwaitExpression
-      | IndexExpression
-      | TupleExpression
-      | TupleIndexingExpression
-      | StructExpression
-      | CallExpression
-      | MethodCallExpression
-      | FieldExpression
-      | ClosureExpression
-      | AsyncBlockExpression
-      | ContinueExpression
-      | BreakExpression
-      | RangeExpression
-      | ReturnExpression
-      | UnderscoreExpression
-      | MacroInvocation
-    )
+    OuterAttribute* ExpressionWithoutBlockNoAttrs
+
+ExpressionWithoutBlockNoAttrs ->
+      LiteralExpression
+    | PathExpression
+    | OperatorExpression
+    | GroupedExpression
+    | ArrayExpression
+    | AwaitExpression
+    | IndexExpression
+    | TupleExpression
+    | TupleIndexingExpression
+    | StructExpression
+    | CallExpression
+    | MethodCallExpression
+    | FieldExpression
+    | ClosureExpression
+    | AsyncBlockExpression
+    | ContinueExpression
+    | BreakExpression
+    | RangeExpression
+    | ReturnExpression
+    | UnderscoreExpression
+    | MacroInvocation
 
 ExpressionWithBlock ->
-    OuterAttribute*
-    (
-        BlockExpression
-      | ConstBlockExpression
-      | UnsafeBlockExpression
-      | LoopExpression
-      | IfExpression
-      | MatchExpression
-    )
+    OuterAttribute* ExpressionWithBlockNoAttrs
+
+ExpressionWithBlockNoAttrs ->
+      BlockExpression
+    | ConstBlockExpression
+    | UnsafeBlockExpression
+    | LoopExpression
+    | IfExpression
+    | MatchExpression
 ```
 
 r[expr.intro]