byos: Ease `GeneralPurposeAllocator` integration

These changes enable me to use `GeneralPurposeAllocator` with my "Bring
Your Own OS" package. The previous checks for a freestanding target have
been expanded to `@hasDecl` checks.

- `root.os.heap.page_allocator` is used if it exists.
- `debug.isValidMemory` only calls `os.msync` if it's supported.

1 files changed, 15 insertions, 14 deletions

diff --git a/lib/std/debug.zig b/lib/std/debug.zig
index 4670c49dfa..5976e52684 100644
--- a/lib/std/debug.zig
+++ b/lib/std/debug.zig
@@ -667,20 +667,7 @@ pub const StackIterator = struct {
         if (aligned_address == 0) return false;
         const aligned_memory = @as([*]align(mem.page_size) u8, @ptrFromInt(aligned_address))[0..mem.page_size];
 
-        if (native_os != .windows) {
-            if (native_os != .wasi) {
-                os.msync(aligned_memory, os.MSF.ASYNC) catch |err| {
-                    switch (err) {
-                        os.MSyncError.UnmappedMemory => {
-                            return false;
-                        },
-                        else => unreachable,
-                    }
-                };
-            }
-
-            return true;
-        } else {
+        if (native_os == .windows) {
             const w = os.windows;
             var memory_info: w.MEMORY_BASIC_INFORMATION = undefined;
 
@@ -701,6 +688,20 @@ pub const StackIterator = struct {
             }
 
             return true;
+        } else if (@hasDecl(os.system, "msync") and native_os != .wasi) {
+            os.msync(aligned_memory, os.MSF.ASYNC) catch |err| {
+                switch (err) {
+                    os.MSyncError.UnmappedMemory => {
+                        return false;
+                    },
+                    else => unreachable,
+                }
+            };
+
+            return true;
+        } else {
+            // We are unable to determine validity of memory on this target.
+            return true;
         }
     }