Let variables (#1095)

* Modify parser and AST nodes to include let statement

* Implement let variables

* Remove redundant code in fail_match_choice test

* Add comment for has_value_category()

* Apply suggestions from code review

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

* clang-format changes

* Update comments for new BindingPattern methods

* Implement nested vars in patterns

* Apply suggestions from @geoffromer's code review

Co-authored-by: Geoff Romer <gromer@google.com>

* Implement changes from code review.

* Remove maybe_var_pattern grammar rule

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>
Co-authored-by: Geoff Romer <gromer@google.com>

executable_semantics/testdata/let/fail_function_args.carbon

@@ -0,0 +1,23 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// RUN: %{not} %{executable_semantics} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{executable_semantics} --trace %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{executable_semantics} %s
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/let/fail_function_args.carbon:16: Cannot assign to rvalue 'y'
+
+package ExecutableSemanticsTest api;
+
+fn f((var x: i32, y: i32)) -> i32 {
+  x = 0;
+  y = 0;
+  return x - 1;
+}
+
+fn Main() -> i32 {
+  var t: auto = (1, 2);
+  return f(t);
+}

executable_semantics/testdata/let/fail_global_assign.carbon

@@ -0,0 +1,19 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// RUN: %{not} %{executable_semantics} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{executable_semantics} --trace %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{executable_semantics} %s
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/let/fail_global_assign.carbon:17: Cannot assign to rvalue 'x'
+
+package ExecutableSemanticsTest api;
+
+let x: i32 = 10;
+
+fn Main() -> i32 {
+  x = 0;
+  return 0;
+}

executable_semantics/testdata/let/fail_local_assign.carbon

@@ -0,0 +1,18 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// RUN: %{not} %{executable_semantics} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{executable_semantics} --trace %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{executable_semantics} %s
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/let/fail_local_assign.carbon:16: Cannot assign to rvalue 'x'
+
+package ExecutableSemanticsTest api;
+
+fn Main() -> i32 {
+  let x: auto = 10;
+  x = 0;
+  return 0;
+}

executable_semantics/testdata/let/fail_match_choice.carbon

@@ -0,0 +1,33 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// RUN: %{not} %{executable_semantics} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{executable_semantics} --trace %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{executable_semantics} %s
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/let/fail_match_choice.carbon:29: Cannot assign to rvalue 'x'
+
+package ExecutableSemanticsTest api;
+
+choice Ints {
+  None,
+  One(i32),
+  Two(i32,i32)
+}
+
+fn Main() -> i32 {
+  var x: auto = Ints.None();
+  var n: auto = 0;
+  match (x) {
+    case Ints.One(var x: auto) =>
+      x = 2;
+    case Ints.None() =>
+      n = n - 1;
+    case Ints.Two(x: auto, y: auto) => {
+      x = 0;
+    }
+  }
+  return 0;
+}

executable_semantics/testdata/let/fail_method_args.carbon

@@ -0,0 +1,32 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// RUN: %{not} %{executable_semantics} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{executable_semantics} --trace %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{executable_semantics} %s
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/let/fail_method_args.carbon:21: Cannot assign to rvalue 'x'
+
+package ExecutableSemanticsTest api;
+
+class Point {
+
+  fn Origin() -> Point {
+    return {.x = 0, .y = 0};
+  }
+
+  fn SetX[me: Point](x :i32) {
+    x = 10;
+  }
+
+  var x: i32;
+  var y: i32;
+}
+
+fn Main() -> i32 {
+  var p: Point = Point.Origin();
+  p.SetX(42);
+  return 0;
+}

executable_semantics/testdata/let/fail_tuple_pattern_let_context.carbon

@@ -0,0 +1,19 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// RUN: %{not} %{executable_semantics} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{executable_semantics} --trace %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{executable_semantics} %s
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/let/fail_tuple_pattern_let_context.carbon:17: Cannot assign to rvalue 'c'
+
+package ExecutableSemanticsTest api;
+
+fn Main() -> i32 {
+  let (var a: auto, b: auto, c:auto, d: auto) = (1, 2, 3, 4);
+  a = 0;
+  c = 0; // should fail
+  return 0;
+}

executable_semantics/testdata/let/fail_tuple_pattern_let_context_nested.carbon

@@ -0,0 +1,17 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// RUN: %{not} %{executable_semantics} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{executable_semantics} --trace %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{executable_semantics} %s
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/let/fail_tuple_pattern_let_context_nested.carbon:15: syntax error, unexpected LET
+
+package ExecutableSemanticsTest api;
+
+fn Main() -> i32 {
+  let (var a: auto, b: auto, c:auto, var (d: auto, let e:auto)) = (1, 2, 3, (4, 5));
+  return 0;
+}

executable_semantics/testdata/let/fail_tuple_pattern_let_in_var.carbon

@@ -0,0 +1,17 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// RUN: %{not} %{executable_semantics} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{executable_semantics} --trace %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{executable_semantics} %s
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/let/fail_tuple_pattern_let_in_var.carbon:15: syntax error, unexpected LET
+
+package ExecutableSemanticsTest api;
+
+fn Main() -> i32 {
+  var (var a: auto, b: auto, let c:auto, d: auto) = (1, 2, 3, 4);
+  return 0;
+}

executable_semantics/testdata/let/nested_tuple_pattern.carbon

@@ -0,0 +1,28 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// RUN: %{executable_semantics} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{executable_semantics} --trace %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{executable_semantics} %s
+// CHECK: result: 0
+
+package ExecutableSemanticsTest api;
+
+choice Ints {
+  None,
+  One(i32),
+  Two(i32,i32)
+}
+
+fn Main() -> i32 {
+  let (var Ints.Two(a1: auto, var a2: auto), ((b: auto, var c:auto), var (d: auto, e:auto))) = (Ints.Two(1, 10), ((2, 3), (4, 5)));
+  a1 = 0;
+  a2 = 0;
+  c = 0;
+  d = 0;
+  e = 0;
+  return 0;
+}