miscompilation regarding pointer to optional slice

[andrewrk]

Zig Version

0.11.0-dev.2148+b3af5d076

Steps to Reproduce and Observed Behavior

Extracted from #14647.

Apply this diff:

diff --git a/lib/std/Build/RunStep.zig b/lib/std/Build/RunStep.zig
index d671ff7ae..30f5ed968 100644
--- a/lib/std/Build/RunStep.zig
+++ b/lib/std/Build/RunStep.zig
@@ -587,8 +587,7 @@ fn runCommand(self: *RunStep, argv: []const []const u8, has_side_effects: bool)
     switch (self.stdio) {
         .check => |checks| for (checks.items) |check| switch (check) {
             .expect_stderr_exact => |expected_bytes| {
-                assert(!result.stderr_null);
-                if (!mem.eql(u8, expected_bytes, result.stderr)) {
+                if (!mem.eql(u8, expected_bytes, result.stderr.?)) {
                     return step.fail(
                         \\
                         \\========= expected this stderr: =========
@@ -599,14 +598,13 @@ fn runCommand(self: *RunStep, argv: []const []const u8, has_side_effects: bool)
                         \\{s}
                     , .{
                         expected_bytes,
-                        result.stderr,
+                        result.stderr.?,
                         try Step.allocPrintCmd(arena, self.cwd, argv),
                     });
                 }
             },
             .expect_stderr_match => |match| {
-                assert(!result.stderr_null);
-                if (mem.indexOf(u8, result.stderr, match) == null) {
+                if (mem.indexOf(u8, result.stderr.?, match) == null) {
                     return step.fail(
                         \\
                         \\========= expected to find in stderr: =========
@@ -617,14 +615,13 @@ fn runCommand(self: *RunStep, argv: []const []const u8, has_side_effects: bool)
                         \\{s}
                     , .{
                         match,
-                        result.stderr,
+                        result.stderr.?,
                         try Step.allocPrintCmd(arena, self.cwd, argv),
                     });
                 }
             },
             .expect_stdout_exact => |expected_bytes| {
-                assert(!result.stdout_null);
-                if (!mem.eql(u8, expected_bytes, result.stdout)) {
+                if (!mem.eql(u8, expected_bytes, result.stdout.?)) {
                     return step.fail(
                         \\
                         \\========= expected this stdout: =========
@@ -635,14 +632,13 @@ fn runCommand(self: *RunStep, argv: []const []const u8, has_side_effects: bool)
                         \\{s}
                     , .{
                         expected_bytes,
-                        result.stdout,
+                        result.stdout.?,
                         try Step.allocPrintCmd(arena, self.cwd, argv),
                     });
                 }
             },
             .expect_stdout_match => |match| {
-                assert(!result.stdout_null);
-                if (mem.indexOf(u8, result.stdout, match) == null) {
+                if (mem.indexOf(u8, result.stdout.?, match) == null) {
                     return step.fail(
                         \\
                         \\========= expected to find in stdout: =========
@@ -653,7 +649,7 @@ fn runCommand(self: *RunStep, argv: []const []const u8, has_side_effects: bool)
                         \\{s}
                     , .{
                         match,
-                        result.stdout,
+                        result.stdout.?,
                         try Step.allocPrintCmd(arena, self.cwd, argv),
                     });
                 }
@@ -675,12 +671,8 @@ fn runCommand(self: *RunStep, argv: []const []const u8, has_side_effects: bool)
 }
 
 const ChildProcResult = struct {
-    // These use boolean flags instead of optionals as a workaround for
-    // https://github.com/ziglang/zig/issues/14783
-    stdout: []const u8,
-    stderr: []const u8,
-    stdout_null: bool,
-    stderr_null: bool,
+    stdout: ?[]const u8,
+    stderr: ?[]const u8,
     term: std.process.Child.Term,
     elapsed_ns: u64,
     peak_rss: usize,
@@ -725,12 +717,8 @@ fn spawnChildAndCollect(
     });
     var timer = try std.time.Timer.start();
 
-    // These are not optionals, as a workaround for
-    // https://github.com/ziglang/zig/issues/14783
-    var stdout_bytes: []const u8 = undefined;
-    var stderr_bytes: []const u8 = undefined;
-    var stdout_null = true;
-    var stderr_null = true;
+    var stdout_bytes: ?[]const u8 = null;
+    var stderr_bytes: ?[]const u8 = null;
 
     if (child.stdout) |stdout| {
         if (child.stderr) |stderr| {
@@ -749,24 +737,22 @@ fn spawnChildAndCollect(
 
             stdout_bytes = try poller.fifo(.stdout).toOwnedSlice();
             stderr_bytes = try poller.fifo(.stderr).toOwnedSlice();
-            stdout_null = false;
-            stderr_null = false;
         } else {
             stdout_bytes = try stdout.reader().readAllAlloc(arena, self.max_stdio_size);
-            stdout_null = false;
         }
     } else if (child.stderr) |stderr| {
         stderr_bytes = try stderr.reader().readAllAlloc(arena, self.max_stdio_size);
-        stderr_null = false;
     }
 
-    if (!stderr_null and stderr_bytes.len > 0) {
-        const stderr_is_diagnostic = switch (self.stdio) {
-            .check => |checks| !checksContainStderr(checks.items),
-            else => true,
-        };
-        if (stderr_is_diagnostic) {
-            try self.step.result_error_msgs.append(arena, stderr_bytes);
+    if (stderr_bytes) |stderr| {
+        if (stderr.len > 0) {
+            const stderr_is_diagnostic = switch (self.stdio) {
+                .check => |checks| !checksContainStderr(checks.items),
+                else => true,
+            };
+            if (stderr_is_diagnostic) {
+                try self.step.result_error_msgs.append(arena, stderr);
+            }
         }
     }
 
@@ -776,8 +762,6 @@ fn spawnChildAndCollect(
     return .{
         .stdout = stdout_bytes,
         .stderr = stderr_bytes,
-        .stdout_null = stdout_null,
-        .stderr_null = stderr_null,
         .term = term,
         .elapsed_ns = elapsed_ns,
         .peak_rss = child.resource_usage_statistics.getMaxRss() orelse 0,

If the diff no longer applies, look for this code:

zig/lib/std/Build/RunStep.zig

Lines 912 to 917 in b4d58e9

	// These use boolean flags instead of optionals as a workaround for
	// https://github.com/ziglang/zig/issues/14783
	stdout: []const u8,
	stderr: []const u8,
	stdout_null: bool,
	stderr_null: bool,

zig/lib/std/Build/RunStep.zig

Lines 1117 to 1122 in b4d58e9

	// These are not optionals, as a workaround for
	// https://github.com/ziglang/zig/issues/14783
	var stdout_bytes: []const u8 = undefined;
	var stderr_bytes: []const u8 = undefined;
	var stdout_null = true;
	var stderr_null = true;

Next, run zig build test-behavior and observe the stderr.? safety check trip, even though non-null was assigned to that variable.

I originally reported this as #14783, which has been fixed. But it seems there is another miscompilation.

Expected Behavior

With the diff applied, zig build test should have all passing tests.