diff --git a/src/arch/riscv64/CodeGen.zig b/src/arch/riscv64/CodeGen.zig
index 316b29734ac2ead682908f0e2b59a4f059ea50a5..8a40c61cdc3e1aca3ffdb79fb583e482a29d553f 100644
--- a/src/arch/riscv64/CodeGen.zig
+++ b/src/arch/riscv64/CodeGen.zig
@@ -51,7 +51,7 @@ const Instruction = encoding.Instruction;
 
 const InnerError = CodeGenError || error{OutOfRegisters};
 
-pub fn legalizeFeatures(_: *const std.Target) ?*const Air.Legalize.Features {
+pub fn legalizeFeatures(_: *const std.Target) *const Air.Legalize.Features {
     return comptime &.initMany(&.{
         .expand_intcast_safe,
         .expand_add_safe,
diff --git a/src/arch/wasm/CodeGen.zig b/src/arch/wasm/CodeGen.zig
index 0704557cbbdb0901a57889415df9a22a61281e6d..36908eb236aca131ba7bec2aec05014eb5193027 100644
--- a/src/arch/wasm/CodeGen.zig
+++ b/src/arch/wasm/CodeGen.zig
@@ -31,7 +31,7 @@ const libcFloatSuffix = target_util.libcFloatSuffix;
 const compilerRtFloatAbbrev = target_util.compilerRtFloatAbbrev;
 const compilerRtIntAbbrev = target_util.compilerRtIntAbbrev;
 
-pub fn legalizeFeatures(_: *const std.Target) ?*const Air.Legalize.Features {
+pub fn legalizeFeatures(_: *const std.Target) *const Air.Legalize.Features {
     return comptime &.initMany(&.{
         .expand_intcast_safe,
         .expand_add_safe,
diff --git a/src/codegen/c.zig b/src/codegen/c.zig
index e76c8e069d14e4fff9c591fc876797664b20c2a6..8d947ce56a6b1486977cd08c01f89965a580efda 100644
--- a/src/codegen/c.zig
+++ b/src/codegen/c.zig
@@ -4,6 +4,7 @@ const assert = std.debug.assert;
 const mem = std.mem;
 const log = std.log.scoped(.c);
 
+const dev = @import("../dev.zig");
 const link = @import("../link.zig");
 const Zcu = @import("../Zcu.zig");
 const Module = @import("../Package/Module.zig");
@@ -21,7 +22,12 @@ const BigIntLimb = std.math.big.Limb;
 const BigInt = std.math.big.int;
 
 pub fn legalizeFeatures(_: *const std.Target) ?*const Air.Legalize.Features {
-    return null;
+    return if (dev.env.supports(.legalize)) comptime &.initMany(&.{
+        .expand_intcast_safe,
+        .expand_add_safe,
+        .expand_sub_safe,
+        .expand_mul_safe,
+    }) else null; // we don't currently ask zig1 to use safe optimization modes
 }
 
 pub const CType = @import("c/Type.zig");
diff --git a/src/codegen/spirv.zig b/src/codegen/spirv.zig
index e23ffbf66aaaafb2fa017b6a4e33a09b927dbc26..1381a7907508bdcccfb3c7bc7f87827918a837d8 100644
--- a/src/codegen/spirv.zig
+++ b/src/codegen/spirv.zig
@@ -28,7 +28,7 @@ const SpvAssembler = @import("spirv/Assembler.zig");
 
 const InstMap = std.AutoHashMapUnmanaged(Air.Inst.Index, IdRef);
 
-pub fn legalizeFeatures(_: *const std.Target) ?*const Air.Legalize.Features {
+pub fn legalizeFeatures(_: *const std.Target) *const Air.Legalize.Features {
     return comptime &.initMany(&.{
         .expand_intcast_safe,
         .expand_add_safe,
diff --git a/src/dev.zig b/src/dev.zig
index 019f18daeb447003ef6deac1f8d10921b12e8987..1dc8264ebc2c74ad670a016da0087a8ccc77e2a5 100644
--- a/src/dev.zig
+++ b/src/dev.zig
@@ -1,5 +1,8 @@
 pub const Env = enum {
     /// zig1 features
+    /// - `-ofmt=c` only
+    /// - `-OReleaseFast` or `-OReleaseSmall` only
+    /// - no `@setRuntimeSafety(true)`
     bootstrap,
 
     /// zig2 features