Add install scripts (#1186)

I think this will work on linux/mac, not windows obviously. But the essence is that running `bazel run -c opt //installers/local:install` will create:

Symlink: /usr/bin/carbon-explorer -> /usr/lib/carbon/carbon
BusyBox-style binary: /usr/lib/carbon/carbon
File: /usr/lib/carbon/data/prelude.carbon

And then just running `carbon-explorer foo.carbon` with no flags (in particular, not --prelude) will work.

Since this removes the need for prelude_file in most cases, I'm removing the relative path logic added in #1179 -- it would otherwise conflict with what I'm doing here. I *think* overall this will result in something even simpler and more reliable for compiler explorer to use, and hopefully with enough flexibility that it's easy for us to change how it's implemented later without breaking much.

executable_semantics/main.cpp

@@ -2,6 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
+#include "executable_semantics/main.h"
+
 #include <unistd.h>
 
 #include <cstdio>
@@ -16,21 +18,15 @@
 #include "executable_semantics/interpreter/exec_program.h"
 #include "executable_semantics/syntax/parse.h"
 #include "executable_semantics/syntax/prelude.h"
-#include "llvm/ADT/SmallString.h"
 #include "llvm/Support/CommandLine.h"
-#include "llvm/Support/ErrorOr.h"
-#include "llvm/Support/FileSystem.h"
 #include "llvm/Support/InitLLVM.h"
-#include "llvm/Support/Path.h"
-#include "llvm/Support/Program.h"
 
-// Prints an error message and returns error code value.
-auto PrintError(const Carbon::Error& error) -> int {
-  llvm::errs() << error.message() << "\n";
-  return EXIT_FAILURE;
-}
+namespace Carbon {
 
-auto main(int argc, char* argv[]) -> int {
+namespace cl = llvm::cl;
+
+static auto Main(llvm::StringRef default_prelude_file, int argc, char* argv[])
+    -> ErrorOr<Success> {
   llvm::setBugReportMsg(
       "Please report issues to "
       "https://github.com/carbon-language/carbon-lang/issues and include the "
@@ -41,48 +37,33 @@ auto main(int argc, char* argv[]) -> int {
   // is piped to stdout.
   llvm::errs().tie(&llvm::outs());
 
-  using llvm::cl::desc;
-  using llvm::cl::opt;
-  opt<bool> trace_option("trace", desc("Enable tracing"));
-  opt<std::string> input_file_name(llvm::cl::Positional, desc("<input file>"),
-                                   llvm::cl::Required);
+  cl::opt<bool> trace_option("trace", cl::desc("Enable tracing"));
+  cl::opt<std::string> input_file_name(cl::Positional, cl::desc("<input file>"),
+                                       cl::Required);
 
   // Find the path of the executable if possible and use that as a relative root
-  // for finding the prelude. FIXME: Currently, this assumes a Bazel-like
-  // runfiles tree rather than any kind of installation tree.
-  llvm::SmallString<256> exe_path(argv[0]);
-  if (!llvm::sys::fs::exists(exe_path)) {
-    // Try to lookup the program name in the `PATH`.
-    if (llvm::ErrorOr<std::string> path =
-            llvm::sys::findProgramByName(exe_path)) {
-      exe_path = *path;
-    }
-  }
-  // If we have a valid path, find the parent directory. Otherwise, just use an
-  // empty string to get the current working directory.
-  if (llvm::sys::fs::exists(exe_path)) {
-    llvm::sys::path::remove_filename(exe_path);
-  } else {
-    exe_path = "";
-  }
-  llvm::SmallString<256> prelude_path = exe_path;
-  llvm::sys::path::append(prelude_path, "data/prelude.carbon");
-  opt<std::string> prelude_file_name("prelude", desc("<prelude file>"),
-                                     llvm::cl::init(prelude_path.str().str()));
+  cl::opt<std::string> prelude_file_name("prelude", cl::desc("<prelude file>"),
+                                         cl::init(default_prelude_file.str()));
+  cl::ParseCommandLineOptions(argc, argv);
 
-  llvm::cl::ParseCommandLineOptions(argc, argv);
-
-  Carbon::Arena arena;
-  Carbon::ErrorOr<Carbon::AST> ast =
-      Carbon::Parse(&arena, input_file_name, trace_option);
-  if (!ast.ok()) {
-    return PrintError(ast.error());
-  }
-  AddPrelude(prelude_file_name, &arena, &ast->declarations);
+  Arena arena;
+  ASSIGN_OR_RETURN(AST ast, Parse(&arena, input_file_name, trace_option));
+  AddPrelude(prelude_file_name, &arena, &ast.declarations);
 
   // Typecheck and run the parsed program.
-  Carbon::ErrorOr<int> result = Carbon::ExecProgram(&arena, *ast, trace_option);
-  if (!result.ok()) {
-    return PrintError(result.error());
-  }
+  ASSIGN_OR_RETURN(int unused_return_code,
+                   ExecProgram(&arena, ast, trace_option));
+  (void)unused_return_code;
+  return Success();
 }
+
+auto ExecutableSemanticsMain(llvm::StringRef default_prelude_file, int argc,
+                             char** argv) -> int {
+  if (auto result = Main(default_prelude_file, argc, argv); !result.ok()) {
+    llvm::errs() << result.error().message() << "\n";
+    return EXIT_FAILURE;
+  }
+  return EXIT_SUCCESS;
+}
+
+}  // namespace Carbon