Rename executable_semantics to explorer (#1188)

Change generated with:

```
#!/usr/bin/bash -eux

# Helper script for renaming pending work.
# Run from the repo root.

# Rename executable_semantics in code.
sed -i 's/executable_semantics/explorer/g' \
  $(git grep -l 'executable_semantics' . | grep -v proposals)
sed -i 's/executable semantics/explorer/g' \
  $(git grep -l 'executable semantics' . | grep -v proposals)
sed -i 's/Executable semantics/Explorer/g' \
  $(git grep -l 'Executable semantics' . |  grep -v proposals)
sed -i 's/Executable Semantics/Explorer/g' \
  $(git grep -l 'Executable Semantics' . |  grep -v proposals)
sed -i 's/EXECUTABLE_SEMANTICS/EXPLORER/g' \
  $(git grep -l 'EXECUTABLE_SEMANTICS' . | grep -v proposals)
sed -i 's/ExecutableSemantics/Explorer/g' \
  $(git grep -l 'ExecutableSemantics' . | grep -v proposals)
sed -i 's/executable-semantics/explorer/g' \
  $(git grep -l 'executable-semantics' . | grep -v proposals)

# This is only needed for the initial move.
mv executable_semantics explorer
mv explorer/fuzzing/executable_semantics_fuzzer.cpp explorer/fuzzing/explorer_fuzzer.cpp
```

Verified with `bazel test ...`

explorer/testdata/basic_syntax/choice.carbon

@@ -0,0 +1,48 @@
+// 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: %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: result: 0
+
+package ExplorerTest api;
+
+choice Ints {
+  None,
+  One(i32),
+  Two(i32, i32)
+}
+
+fn Main() -> i32 {
+  var x: auto = Ints.None();
+  var y: auto = Ints.One(42);
+  var n: auto = 0;
+  match (y) {
+    case Ints.None =>
+      n = n + 2;
+    case Ints.One(x: auto) =>
+      n = x + 1 - 42;
+    case Ints.Two(a: auto, b: auto) =>
+      n = 2;
+  }
+  match (x) {
+    case Ints.One(x: auto) =>
+      n = x + 2;
+    case Ints.None() =>
+      n = n - 1;
+    case Ints.Two(x: auto, y: auto) =>
+      n = 5;
+  }
+  return n;
+}
+
+// Test some alternate syntaxes
+choice MoreInts {
+  None(),
+  One(i32),
+  Two(i32, i32),
+}

explorer/testdata/basic_syntax/fail_assign_to_function.carbon

@@ -0,0 +1,20 @@
+// 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} %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_assign_to_function.carbon:18: Cannot assign to rvalue 'F'
+
+package ExplorerTest api;
+
+fn F() {}
+fn G() {}
+
+fn Main() -> i32 {
+  F = G;
+  return 0;
+}

explorer/testdata/basic_syntax/fail_assign_to_rval.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} %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_assign_to_rval.carbon:15: Cannot assign to rvalue '1'
+
+package ExplorerTest api;
+
+fn Main() -> i32 {
+  1 = 0;
+  return 0;
+}

explorer/testdata/basic_syntax/fail_invalid_char.carbon

@@ -0,0 +1,12 @@
+// 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} %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_invalid_char.carbon:12: invalid character '\xEF' in source file.
+
+�

explorer/testdata/basic_syntax/fail_invalid_integer.carbon

@@ -0,0 +1,16 @@
+// 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} %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_invalid_integer.carbon:15: Invalid integer literal: 11111111111111111111111111
+
+package ExplorerTest api;
+
+fn Main() -> i32 {
+  return 11111111111111111111111111;
+}

explorer/testdata/basic_syntax/fail_invalid_var_expression.carbon

@@ -0,0 +1,16 @@
+// 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} %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_invalid_var_expression.carbon:14: Expected expression for variable type
+
+package ExplorerTest api;
+
+var a: auto;
+
+fn Main() -> i32 { return 0; }

explorer/testdata/basic_syntax/fail_missing_var.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} %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_missing_var.carbon:16: syntax error, unexpected COLON, expecting EQUAL or SEMICOLON
+
+package ExplorerTest api;
+
+fn Main() -> i32 {
+  // error
+  x : i32;
+  return 1;
+}

explorer/testdata/basic_syntax/fail_nested_binding.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} %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_nested_binding.carbon:15: The type of a binding pattern cannot contain bindings.
+
+package ExplorerTest api;
+
+fn Main() -> i32 {
+  var x: (T: Type) = 1;
+  return 1;
+}

explorer/testdata/basic_syntax/fail_unimplemented_example.carbon

@@ -0,0 +1,16 @@
+// 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} %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_unimplemented_example.carbon:15: Unimplemented
+
+package ExplorerTest api;
+
+fn Main() -> i32 {
+  return 1 __unimplemented_example_infix 2;
+}

explorer/testdata/basic_syntax/fail_var_type.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} %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_var_type.carbon:17: Expected a type, but got 42
+
+package ExplorerTest api;
+
+fn Main () -> i32
+{
+  // 42 cannot be used as the type of a variable.
+  var x: 42 = 0;
+  return x;
+}

explorer/testdata/basic_syntax/next.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: %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: result: 0
+
+package ExplorerTest api;
+
+fn Main () -> i32
+{
+  var x: i32 = 0;
+  return x;
+}

explorer/testdata/basic_syntax/not_compare_precedence.carbon

@@ -0,0 +1,16 @@
+// 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} %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/not_compare_precedence.carbon:15: syntax error, unexpected EQUAL_EQUAL, expecting SEMICOLON
+
+package ExplorerTest api;
+
+fn CompareBools(a: Bool, b: Bool) -> Bool {
+  return not a == b;
+}

explorer/testdata/basic_syntax/placeholder_variable.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: %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: result: 0
+
+package ExplorerTest api;
+
+fn Main() -> i32 {
+  var _: auto = 1;
+  return 0;
+}

explorer/testdata/basic_syntax/print.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: %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: Hello world!
+// CHECK: result: 0
+
+package ExplorerTest api;
+
+fn Main() -> i32 {
+  var s: auto = "Hello world!\n";
+  Print(s);
+  return 0;
+}

explorer/testdata/basic_syntax/record.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: %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: result: 0
+
+package ExplorerTest api;
+
+fn Main() -> i32 {
+  var t2: {.x: i32, .y: i32} = {.x = 2, .y = 5};
+  t2.y = 3;
+  // 3 - 2 - 1
+  return t2.y - t2.x - 1;
+}

explorer/testdata/basic_syntax/star.carbon

@@ -0,0 +1,25 @@
+// 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: %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: result: 0
+
+package ExplorerTest api;
+
+// Never actually called, so this only tests typechecking and semantic analysis.
+fn F(n: i32, p: i32*, q: i32***) -> i32* {
+  var a: i32 = n * *p;
+  var b: i32 = a*n;
+  *p = b*(*p);
+  **q = p;
+  return **q;
+}
+
+fn Main() -> i32 {
+  return 0;
+}

explorer/testdata/basic_syntax/trace.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: %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+//
+// A lot of output is elided: this is only checking for a few things for simple
+// sanity checking on --parser_debug --trace_file=- output.
+//
+// NOAUTOUPDATE
+// CHECK: ********** source program **********
+// CHECK: fn Print (format_str: String) {
+// CHECK: ********** type checking **********
+// CHECK: checking pattern (format_str: String)
+// CHECK: ********** type checking complete **********
+// CHECK: fn Print (format_str: String) {
+// CHECK: ********** starting execution **********
+// CHECK: ********** initializing globals **********
+// CHECK: ********** calling main function **********
+// CHECK: {
+// CHECK: stack:
+// CHECK: heap:
+// CHECK: }
+// CHECK: --- step exp Main() (<Main()>:0) --->
+// CHECK: result: 0
+
+package ExplorerTest api;
+
+fn Main() -> i32 {
+  return 0;
+}

explorer/testdata/basic_syntax/var_tuple.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: %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: result: 0
+
+package ExplorerTest api;
+
+fn Main() -> i32 {
+  var (x: auto, y: auto) = (2, 3);
+  return y - x - 1;
+}

explorer/testdata/basic_syntax/zero.carbon

@@ -0,0 +1,16 @@
+// 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: %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: result: 0
+
+package ExplorerTest api;
+
+fn Main() -> i32 {
+  return 0;
+}