Add support for if/switch expressions

As pointed out in #351.

[SE-0380](https://github.com/apple/swift-evolution/blob/main/proposals/0380-if-switch-expressions.md)
made it possible to do this, but support was lacking in this repository.
The fix turned out to be easier than anticipated, simply requiring the
statement to be added in some new places. Theoretically, it should also
be renamed, but that would make this a breaking change.

The proposal does not make clear whether labels are permitted on
expression-position `if` and `switch` statements. This assumes they are
not, for simplicity.

grammar.js

@@ -176,6 +176,10 @@ module.exports = grammar({
 
     // `lazy` is also allowed as an identifier...
     [$.property_behavior_modifier, $.simple_identifier],
+
+    // SE-0380: if/switch expressions
+    [$._expression, $.if_statement],
+    [$._expression, $.switch_statement],
   ],
   extras: ($) => [
     $.comment,
@@ -474,6 +478,8 @@ module.exports = grammar({
           $._binary_expression,
           $.ternary_expression,
           $._primary_expression,
+          $.if_statement,
+          $.switch_statement,
           $.assignment,
           seq($._expression, alias($._immediate_quest, "?")),
           alias("async", $.simple_identifier)
@@ -541,7 +547,13 @@ module.exports = grammar({
         PRECS.prefix_operations,
         seq(
           field("operation", $._prefix_unary_operator),
-          field("target", $._expression)
+          field(
+            "target",
+            choice(
+              $._expression,
+              alias(choice("async", "if", "switch"), $._expression)
+            )
+          )
         )
       ),
     as_expression: ($) =>
@@ -711,7 +723,12 @@ module.exports = grammar({
       ),
     value_argument_label: ($) =>
       prec.left(
-        choice($.simple_identifier, alias("async", $.simple_identifier))
+        choice(
+          $.simple_identifier,
+          alias("async", $.simple_identifier),
+          alias("if", $.simple_identifier),
+          alias("switch", $.simple_identifier)
+        )
       ),
     value_argument: ($) =>
       prec.left(

test/corpus/expressions.txt

@@ -1272,3 +1272,226 @@ foo()
     (call_suffix
       (value_arguments)))
   (fully_open_range))
+
+================================================================================
+If/else expression
+================================================================================
+
+let number = if Bool.random() {
+    1.0
+} else {
+    0.0
+}
+
+--------------------------------------------------------------------------------
+
+(source_file
+  (property_declaration
+    (value_binding_pattern)
+    (pattern
+      (simple_identifier))
+    (if_statement
+      (call_expression
+        (navigation_expression
+          (simple_identifier)
+          (navigation_suffix
+            (simple_identifier)))
+        (call_suffix
+          (value_arguments)))
+      (statements
+        (real_literal))
+      (else)
+      (statements
+        (real_literal)))))
+
+================================================================================
+Complex if/else expression
+================================================================================
+
+let bullet =
+    if isRoot && (count == 0 || !willExpand) { "" }
+    else if count == 0 { "- " }
+    else if maxDepth <= 0 { "▹ " }
+    else { "▿ " }
+
+--------------------------------------------------------------------------------
+
+(source_file
+  (property_declaration
+    (value_binding_pattern)
+    (pattern
+      (simple_identifier))
+    (if_statement
+      (conjunction_expression
+        (simple_identifier)
+        (tuple_expression
+          (equality_expression
+            (simple_identifier)
+            (disjunction_expression
+              (integer_literal)
+              (prefix_expression
+                (bang)
+                (simple_identifier))))))
+      (statements
+        (line_string_literal))
+      (else)
+      (if_statement
+        (equality_expression
+          (simple_identifier)
+          (integer_literal))
+        (statements
+          (line_string_literal
+            (line_str_text)))
+        (else)
+        (if_statement
+          (infix_expression
+            (simple_identifier)
+            (custom_operator)
+            (integer_literal))
+          (statements
+            (line_string_literal
+              (line_str_text)))
+          (else)
+          (statements
+            (line_string_literal
+              (line_str_text))))))))
+
+================================================================================
+Switch expression
+================================================================================
+
+let y: Float = switch x.value {
+    case 0..<0x80: 1
+    case 0x80..<0x0800: 2.0
+    case 0x0800..<0x1_0000: 3.0
+    default: 4.5
+}
+
+--------------------------------------------------------------------------------
+
+(source_file
+  (property_declaration
+    (value_binding_pattern)
+    (pattern
+      (simple_identifier))
+    (type_annotation
+      (user_type
+        (type_identifier)))
+    (switch_statement
+      (navigation_expression
+        (simple_identifier)
+        (navigation_suffix
+          (simple_identifier)))
+      (switch_entry
+        (switch_pattern
+          (pattern
+            (range_expression
+              (integer_literal)
+              (hex_literal))))
+        (statements
+          (integer_literal)))
+      (switch_entry
+        (switch_pattern
+          (pattern
+            (range_expression
+              (hex_literal)
+              (hex_literal))))
+        (statements
+          (real_literal)))
+      (switch_entry
+        (switch_pattern
+          (pattern
+            (range_expression
+              (hex_literal)
+              (hex_literal))))
+        (statements
+          (real_literal)))
+      (switch_entry
+        (default_keyword)
+        (statements
+          (real_literal))))))
+
+================================================================================
+If and switch can still be used as a value argument labels
+================================================================================
+
+atomicValue.set(to: "value + 1", if: "value <= 100")
+atomicValue.replace(from: otherValue, switch: true)
+
+--------------------------------------------------------------------------------
+
+(source_file
+  (call_expression
+    (navigation_expression
+      (simple_identifier)
+      (navigation_suffix
+        (simple_identifier)))
+    (call_suffix
+      (value_arguments
+        (value_argument
+          (value_argument_label
+            (simple_identifier))
+          (line_string_literal
+            (line_str_text)))
+        (value_argument
+          (value_argument_label
+            (simple_identifier))
+          (line_string_literal
+            (line_str_text))))))
+  (call_expression
+    (navigation_expression
+      (simple_identifier)
+      (navigation_suffix
+        (simple_identifier)))
+    (call_suffix
+      (value_arguments
+        (value_argument
+          (value_argument_label
+            (simple_identifier))
+          (simple_identifier))
+        (value_argument
+          (value_argument_label
+            (simple_identifier))
+          (boolean_literal))))))
+
+================================================================================
+If and switch can still be used as enum identifiers
+================================================================================
+
+static let statements: Set<StatementKind> = [
+    .if,
+    .switch,
+    .while
+]
+
+parseStatements(.switch)
+
+--------------------------------------------------------------------------------
+
+(source_file
+  (property_declaration
+    (modifiers
+      (property_modifier))
+    (value_binding_pattern)
+    (pattern
+      (simple_identifier))
+    (type_annotation
+      (user_type
+        (type_identifier)
+        (type_arguments
+          (user_type
+            (type_identifier)))))
+    (array_literal
+      (prefix_expression
+        (_expression))
+      (prefix_expression
+        (_expression))
+      (prefix_expression
+        (simple_identifier))))
+  (call_expression
+    (simple_identifier)
+    (call_suffix
+      (value_arguments
+        (value_argument
+          (prefix_expression
+            (_expression)))))))

test/corpus/statements.txt

@@ -564,6 +564,47 @@ someLabel: if a.isEmpty, let b = getB() {
       (call_suffix
         (value_arguments)))))
 
+================================================================================
+If let on multiple lines
+================================================================================
+
+if
+    let foo = getFoo(),
+    let bar = getBar()
+{
+    doSomethingWith(foo: foo, bar: bar)
+}
+
+--------------------------------------------------------------------------------
+
+(source_file
+  (if_statement
+    (value_binding_pattern)
+    (simple_identifier)
+    (call_expression
+      (simple_identifier)
+      (call_suffix
+        (value_arguments)))
+    (value_binding_pattern)
+    (simple_identifier)
+    (call_expression
+      (simple_identifier)
+      (call_suffix
+        (value_arguments)))
+    (statements
+      (call_expression
+        (simple_identifier)
+        (call_suffix
+          (value_arguments
+            (value_argument
+              (value_argument_label
+                (simple_identifier))
+              (simple_identifier))
+            (value_argument
+              (value_argument_label
+                (simple_identifier))
+              (simple_identifier))))))))
+
 ================================================================================
 If let with function call
 ================================================================================