allow specifying mode in --debug-rt

The motivation is that libfuzzer is slow in Debug mode and bugs usually
manifest late into fuzzing, which makes testing it in ReleaseSafe
useful.

lib/compiler/build_runner.zig

@@ -304,7 +304,11 @@ pub fn main() !void {
             } else if (mem.eql(u8, arg, "--debug-pkg-config")) {
                 builder.debug_pkg_config = true;
             } else if (mem.eql(u8, arg, "--debug-rt")) {
-                graph.debug_compiler_runtime_libs = true;
+                graph.debug_compiler_runtime_libs = .Debug;
+            } else if (mem.cutPrefix(u8, arg, "--debug-rt=")) |rest| {
+                graph.debug_compiler_runtime_libs =
+                    std.meta.stringToEnum(std.builtin.OptimizeMode, rest) orelse
+                    fatal("unrecognized optimization mode: '{s}'", .{rest});
             } else if (mem.eql(u8, arg, "--debug-compile-errors")) {
                 builder.debug_compile_errors = true;
             } else if (mem.eql(u8, arg, "--debug-incremental")) {

lib/std/Build.zig

@@ -116,7 +116,7 @@ pub const Graph = struct {
     arena: Allocator,
     system_library_options: std.StringArrayHashMapUnmanaged(SystemLibraryMode) = .empty,
     system_package_mode: bool = false,
-    debug_compiler_runtime_libs: bool = false,
+    debug_compiler_runtime_libs: ?std.builtin.OptimizeMode = null,
     cache: Cache,
     zig_exe: [:0]const u8,
     env_map: EnvMap,

lib/std/Build/Step/Compile.zig

@@ -1572,7 +1572,8 @@ fn getZigArgs(compile: *Compile, fuzz: bool) ![][]const u8 {
     try zig_args.append("--global-cache-dir");
     try zig_args.append(b.graph.global_cache_root.path orelse ".");
 
-    if (b.graph.debug_compiler_runtime_libs) try zig_args.append("--debug-rt");
+    if (b.graph.debug_compiler_runtime_libs) |mode|
+        try zig_args.append(b.fmt("--debug-rt={t}", .{mode}));
 
     try zig_args.append("--name");
     try zig_args.append(compile.name);

src/Compilation.zig

@@ -175,7 +175,7 @@ verbose_llvm_cpu_features: bool,
 verbose_link: bool,
 disable_c_depfile: bool,
 stack_report: bool,
-debug_compiler_runtime_libs: bool,
+debug_compiler_runtime_libs: ?std.builtin.OptimizeMode,
 debug_compile_errors: bool,
 /// Do not check this field directly. Instead, use the `debugIncremental` wrapper function.
 debug_incremental: bool,
@@ -1734,7 +1734,7 @@ pub const CreateOptions = struct {
     verbose_llvm_bc: ?[]const u8 = null,
     verbose_cimport: bool = false,
     verbose_llvm_cpu_features: bool = false,
-    debug_compiler_runtime_libs: bool = false,
+    debug_compiler_runtime_libs: ?std.builtin.OptimizeMode = null,
     debug_compile_errors: bool = false,
     debug_incremental: bool = false,
     /// Normally when you create a `Compilation`, Zig will automatically build
@@ -2134,7 +2134,8 @@ pub fn create(gpa: Allocator, arena: Allocator, io: Io, diag: *CreateDiagnostic,
         cache.hash.addBytes(options.root_name);
         cache.hash.add(options.config.wasi_exec_model);
         cache.hash.add(options.config.san_cov_trace_pc_guard);
-        cache.hash.add(options.debug_compiler_runtime_libs);
+        cache.hash.add(options.debug_compiler_runtime_libs != null);
+        if (options.debug_compiler_runtime_libs) |mode| cache.hash.add(mode);
         // The actual emit paths don't matter. They're only user-specified if we aren't using the
         // cache! However, it does matter whether the files are emitted at all.
         cache.hash.add(options.emit_bin != .no);
@@ -8152,8 +8153,8 @@ pub fn addLinkLib(comp: *Compilation, lib_name: []const u8) !void {
 /// This decides the optimization mode for all zig-provided libraries, including
 /// compiler-rt, libcxx, libc, libunwind, etc.
 pub fn compilerRtOptMode(comp: Compilation) std.builtin.OptimizeMode {
-    if (comp.debug_compiler_runtime_libs) {
+    if (comp.debug_compiler_runtime_libs) |mode| {
-        return .Debug;
+        return mode;
     }
     const target = &comp.root_mod.resolved_target.result;
     switch (comp.root_mod.optimize_mode) {

src/main.zig

@@ -678,7 +678,8 @@ const usage_build_generic =
     \\  --debug-log [scope]          Enable printing debug/info log messages for scope
     \\  --debug-compile-errors       Crash with helpful diagnostics at the first compile error
     \\  --debug-link-snapshot        Enable dumping of the linker's state in JSON format
-    \\  --debug-rt                   Debug compiler runtime libraries
+    \\  --debug-rt[=mode]            Build compiler runtime libraries with [mode] optimization
+    \\                               (Debug if [=mode] is omitted)
     \\  --debug-incremental          Enable incremental compilation debug features
     \\
 ;
@@ -895,7 +896,7 @@ fn buildOutputType(
     var minor_subsystem_version: ?u16 = null;
     var mingw_unicode_entry_point: bool = false;
     var enable_link_snapshots: bool = false;
-    var debug_compiler_runtime_libs = false;
+    var debug_compiler_runtime_libs: ?std.builtin.OptimizeMode = null;
     var install_name: ?[]const u8 = null;
     var hash_style: link.File.Lld.Elf.HashStyle = .both;
     var entitlements: ?[]const u8 = null;
@@ -1350,7 +1351,9 @@ fn buildOutputType(
                             enable_link_snapshots = true;
                         }
                     } else if (mem.eql(u8, arg, "--debug-rt")) {
-                        debug_compiler_runtime_libs = true;
+                        debug_compiler_runtime_libs = .Debug;
+                    } else if (mem.cutPrefix(u8, arg, "--debug-rt=")) |rest| {
+                        debug_compiler_runtime_libs = parseOptimizeMode(rest);
                     } else if (mem.eql(u8, arg, "--debug-incremental")) {
                         if (build_options.enable_debug_extensions) {
                             debug_incremental = true;