diff --git a/build.zig b/build.zig
index c57c3e713a..7208d1fbb9 100644
--- a/build.zig
+++ b/build.zig
@@ -260,7 +260,7 @@ pub fn build(b: *std.Build) !void {
         };
         const git_describe = mem.trim(u8, git_describe_untrimmed, " \n\r");
 
-        switch (mem.count(u8, git_describe, "-")) {
+        switch (mem.countScalar(u8, git_describe, '-')) {
             0 => {
                 // Tagged release version (e.g. 0.10.0).
                 if (!mem.eql(u8, git_describe, version_string)) {
diff --git a/lib/std/fs/Dir.zig b/lib/std/fs/Dir.zig
index d097573122..f5df4a969f 100644
--- a/lib/std/fs/Dir.zig
+++ b/lib/std/fs/Dir.zig
@@ -663,35 +663,17 @@ fn iterateImpl(self: Dir, first_iter_start_value: bool) Iterator {
     }
 }
 
-pub const Walker = struct {
-    stack: std.ArrayListUnmanaged(StackItem),
+pub const SelectiveWalker = struct {
+    stack: std.ArrayListUnmanaged(Walker.StackItem),
     name_buffer: std.ArrayListUnmanaged(u8),
     allocator: Allocator,
 
-    pub const Entry = struct {
-        /// The containing directory. This can be used to operate directly on `basename`
-        /// rather than `path`, avoiding `error.NameTooLong` for deeply nested paths.
-        /// The directory remains open until `next` or `deinit` is called.
-        dir: Dir,
-        basename: [:0]const u8,
-        path: [:0]const u8,
-        kind: Dir.Entry.Kind,
-    };
-
-    const StackItem = struct {
-        iter: Dir.Iterator,
-        dirname_len: usize,
-    };
-
     /// After each call to this function, and on deinit(), the memory returned
     /// from this function becomes invalid. A copy must be made in order to keep
     /// a reference to the path.
-    pub fn next(self: *Walker) !?Walker.Entry {
-        const gpa = self.allocator;
-        while (self.stack.items.len != 0) {
-            // `top` and `containing` become invalid after appending to `self.stack`
-            var top = &self.stack.items[self.stack.items.len - 1];
-            var containing = top;
+    pub fn next(self: *SelectiveWalker) !?Walker.Entry {
+        while (self.stack.items.len > 0) {
+            const top = &self.stack.items[self.stack.items.len - 1];
             var dirname_len = top.dirname_len;
             if (top.iter.next() catch |err| {
                 // If we get an error, then we want the user to be able to continue
@@ -703,36 +685,22 @@ pub const Walker = struct {
                     item.iter.dir.close();
                 }
                 return err;
-            }) |base| {
+            }) |entry| {
                 self.name_buffer.shrinkRetainingCapacity(dirname_len);
                 if (self.name_buffer.items.len != 0) {
-                    try self.name_buffer.append(gpa, fs.path.sep);
+                    try self.name_buffer.append(self.allocator, fs.path.sep);
                     dirname_len += 1;
                 }
-                try self.name_buffer.ensureUnusedCapacity(gpa, base.name.len + 1);
-                self.name_buffer.appendSliceAssumeCapacity(base.name);
+                try self.name_buffer.ensureUnusedCapacity(self.allocator, entry.name.len + 1);
+                self.name_buffer.appendSliceAssumeCapacity(entry.name);
                 self.name_buffer.appendAssumeCapacity(0);
-                if (base.kind == .directory) {
-                    var new_dir = top.iter.dir.openDir(base.name, .{ .iterate = true }) catch |err| switch (err) {
-                        error.NameTooLong => unreachable, // no path sep in base.name
-                        else => |e| return e,
-                    };
-                    {
-                        errdefer new_dir.close();
-                        try self.stack.append(gpa, .{
-                            .iter = new_dir.iterateAssumeFirstIteration(),
-                            .dirname_len = self.name_buffer.items.len - 1,
-                        });
-                        top = &self.stack.items[self.stack.items.len - 1];
-                        containing = &self.stack.items[self.stack.items.len - 2];
-                    }
-                }
-                return .{
-                    .dir = containing.iter.dir,
+                const walker_entry: Walker.Entry = .{
+                    .dir = top.iter.dir,
                     .basename = self.name_buffer.items[dirname_len .. self.name_buffer.items.len - 1 :0],
                     .path = self.name_buffer.items[0 .. self.name_buffer.items.len - 1 :0],
-                    .kind = base.kind,
+                    .kind = entry.kind,
                 };
+                return walker_entry;
             } else {
                 var item = self.stack.pop().?;
                 if (self.stack.items.len != 0) {
@@ -743,16 +711,116 @@ pub const Walker = struct {
         return null;
     }
 
-    pub fn deinit(self: *Walker) void {
-        const gpa = self.allocator;
-        // Close any remaining directories except the initial one (which is always at index 0)
-        if (self.stack.items.len > 1) {
-            for (self.stack.items[1..]) |*item| {
-                item.iter.dir.close();
-            }
+    /// Traverses into the directory, continuing walking one level down.
+    pub fn enter(self: *SelectiveWalker, entry: Walker.Entry) !void {
+        if (entry.kind != .directory) {
+            @branchHint(.cold);
+            return;
         }
-        self.stack.deinit(gpa);
-        self.name_buffer.deinit(gpa);
+
+        var new_dir = entry.dir.openDir(entry.basename, .{ .iterate = true }) catch |err| {
+            switch (err) {
+                error.NameTooLong => unreachable,
+                else => |e| return e,
+            }
+        };
+        errdefer new_dir.close();
+
+        try self.stack.append(self.allocator, .{
+            .iter = new_dir.iterateAssumeFirstIteration(),
+            .dirname_len = self.name_buffer.items.len - 1,
+        });
+    }
+
+    pub fn deinit(self: *SelectiveWalker) void {
+        self.name_buffer.deinit(self.allocator);
+        self.stack.deinit(self.allocator);
+    }
+
+    /// Leaves the current directory, continuing walking one level up.
+    /// If the current entry is a directory entry, then the "current directory"
+    /// will pertain to that entry if `enter` is called before `leave`.
+    pub fn leave(self: *SelectiveWalker) void {
+        var item = self.stack.pop().?;
+        if (self.stack.items.len != 0) {
+            @branchHint(.likely);
+            item.iter.dir.close();
+        }
+    }
+};
+
+/// Recursively iterates over a directory, but requires the user to
+/// opt-in to recursing into each directory entry.
+///
+/// `self` must have been opened with `OpenOptions{.iterate = true}`.
+///
+/// `Walker.deinit` releases allocated memory and directory handles.
+///
+/// The order of returned file system entries is undefined.
+///
+/// `self` will not be closed after walking it.
+///
+/// See also `walk`.
+pub fn walkSelectively(self: Dir, allocator: Allocator) !SelectiveWalker {
+    var stack: std.ArrayListUnmanaged(Walker.StackItem) = .empty;
+
+    try stack.append(allocator, .{
+        .iter = self.iterate(),
+        .dirname_len = 0,
+    });
+
+    return .{
+        .stack = stack,
+        .name_buffer = .{},
+        .allocator = allocator,
+    };
+}
+
+pub const Walker = struct {
+    inner: SelectiveWalker,
+
+    pub const Entry = struct {
+        /// The containing directory. This can be used to operate directly on `basename`
+        /// rather than `path`, avoiding `error.NameTooLong` for deeply nested paths.
+        /// The directory remains open until `next` or `deinit` is called.
+        dir: Dir,
+        basename: [:0]const u8,
+        path: [:0]const u8,
+        kind: Dir.Entry.Kind,
+
+        /// Returns the depth of the entry relative to the initial directory.
+        /// Returns 1 for a direct child of the initial directory, 2 for an entry
+        /// within a direct child of the initial directory, etc.
+        pub fn depth(self: Walker.Entry) usize {
+            return mem.countScalar(u8, self.path, fs.path.sep) + 1;
+        }
+    };
+
+    const StackItem = struct {
+        iter: Dir.Iterator,
+        dirname_len: usize,
+    };
+
+    /// After each call to this function, and on deinit(), the memory returned
+    /// from this function becomes invalid. A copy must be made in order to keep
+    /// a reference to the path.
+    pub fn next(self: *Walker) !?Walker.Entry {
+        const entry = try self.inner.next();
+        if (entry != null and entry.?.kind == .directory) {
+            try self.inner.enter(entry.?);
+        }
+        return entry;
+    }
+
+    pub fn deinit(self: *Walker) void {
+        self.inner.deinit();
+    }
+
+    /// Leaves the current directory, continuing walking one level up.
+    /// If the current entry is a directory entry, then the "current directory"
+    /// is the directory pertaining to the current entry.
+    pub fn leave(self: *Walker) void {
+        self.inner.leave();
     }
 };
 
@@ -765,18 +833,11 @@ pub const Walker = struct {
 /// The order of returned file system entries is undefined.
 ///
 /// `self` will not be closed after walking it.
+///
+/// See also `walkSelectively`.
 pub fn walk(self: Dir, allocator: Allocator) Allocator.Error!Walker {
-    var stack: std.ArrayListUnmanaged(Walker.StackItem) = .empty;
-
-    try stack.append(allocator, .{
-        .iter = self.iterate(),
-        .dirname_len = 0,
-    });
-
     return .{
-        .stack = stack,
-        .name_buffer = .{},
-        .allocator = allocator,
+        .inner = try walkSelectively(self, allocator),
     };
 }
 
diff --git a/lib/std/fs/test.zig b/lib/std/fs/test.zig
index 65e86e4c2e..b88456ef7c 100644
--- a/lib/std/fs/test.zig
+++ b/lib/std/fs/test.zig
@@ -1765,14 +1765,14 @@ test "walker" {
 
     // iteration order of walker is undefined, so need lookup maps to check against
 
-    const expected_paths = std.StaticStringMap(void).initComptime(.{
-        .{"dir1"},
-        .{"dir2"},
-        .{"dir3"},
-        .{"dir4"},
-        .{"dir3" ++ fs.path.sep_str ++ "sub1"},
-        .{"dir3" ++ fs.path.sep_str ++ "sub2"},
-        .{"dir3" ++ fs.path.sep_str ++ "sub2" ++ fs.path.sep_str ++ "subsub1"},
+    const expected_paths = std.StaticStringMap(usize).initComptime(.{
+        .{ "dir1", 1 },
+        .{ "dir2", 1 },
+        .{ "dir3", 1 },
+        .{ "dir4", 1 },
+        .{ "dir3" ++ fs.path.sep_str ++ "sub1", 2 },
+        .{ "dir3" ++ fs.path.sep_str ++ "sub2", 2 },
+        .{ "dir3" ++ fs.path.sep_str ++ "sub2" ++ fs.path.sep_str ++ "subsub1", 3 },
     });
 
     const expected_basenames = std.StaticStringMap(void).initComptime(.{
@@ -1802,6 +1802,76 @@ test "walker" {
             std.debug.print("found unexpected path: {f}\n", .{std.ascii.hexEscape(entry.path, .lower)});
             return err;
         };
+        testing.expectEqual(expected_paths.get(entry.path).?, entry.depth()) catch |err| {
+            std.debug.print("path reported unexpected depth: {f}\n", .{std.ascii.hexEscape(entry.path, .lower)});
+            return err;
+        };
+        // make sure that the entry.dir is the containing dir
+        var entry_dir = try entry.dir.openDir(entry.basename, .{});
+        defer entry_dir.close();
+        num_walked += 1;
+    }
+    try testing.expectEqual(expected_paths.kvs.len, num_walked);
+}
+
+test "selective walker, skip entries that start with ." {
+    var tmp = tmpDir(.{ .iterate = true });
+    defer tmp.cleanup();
+
+    const paths_to_create: []const []const u8 = &.{
+        "dir1/foo/.git/ignored",
+        ".hidden/bar",
+        "a/b/c",
+        "a/baz",
+    };
+
+    // iteration order of walker is undefined, so need lookup maps to check against
+
+    const expected_paths = std.StaticStringMap(usize).initComptime(.{
+        .{ "dir1", 1 },
+        .{ "dir1" ++ fs.path.sep_str ++ "foo", 2 },
+        .{ "a", 1 },
+        .{ "a" ++ fs.path.sep_str ++ "b", 2 },
+        .{ "a" ++ fs.path.sep_str ++ "b" ++ fs.path.sep_str ++ "c", 3 },
+        .{ "a" ++ fs.path.sep_str ++ "baz", 2 },
+    });
+
+    const expected_basenames = std.StaticStringMap(void).initComptime(.{
+        .{"dir1"},
+        .{"foo"},
+        .{"a"},
+        .{"b"},
+        .{"c"},
+        .{"baz"},
+    });
+
+    for (paths_to_create) |path| {
+        try tmp.dir.makePath(path);
+    }
+
+    var walker = try tmp.dir.walkSelectively(testing.allocator);
+    defer walker.deinit();
+
+    var num_walked: usize = 0;
+    while (try walker.next()) |entry| {
+        if (entry.basename[0] == '.') continue;
+        if (entry.kind == .directory) {
+            try walker.enter(entry);
+        }
+
+        testing.expect(expected_basenames.has(entry.basename)) catch |err| {
+            std.debug.print("found unexpected basename: {f}\n", .{std.ascii.hexEscape(entry.basename, .lower)});
+            return err;
+        };
+        testing.expect(expected_paths.has(entry.path)) catch |err| {
+            std.debug.print("found unexpected path: {f}\n", .{std.ascii.hexEscape(entry.path, .lower)});
+            return err;
+        };
+        testing.expectEqual(expected_paths.get(entry.path).?, entry.depth()) catch |err| {
+            std.debug.print("path reported unexpected depth: {f}\n", .{std.ascii.hexEscape(entry.path, .lower)});
+            return err;
+        };
+
         // make sure that the entry.dir is the containing dir
         var entry_dir = try entry.dir.openDir(entry.basename, .{});
         defer entry_dir.close();
diff --git a/lib/std/mem.zig b/lib/std/mem.zig
index 3cd0507bee..3763de5180 100644
--- a/lib/std/mem.zig
+++ b/lib/std/mem.zig
@@ -1704,6 +1704,26 @@ test count {
     try testing.expect(count(u8, "owowowu", "owowu") == 1);
 }
 
+/// Returns the number of needles inside the haystack
+pub fn countScalar(comptime T: type, haystack: []const T, needle: T) usize {
+    var i: usize = 0;
+    var found: usize = 0;
+
+    while (findScalarPos(T, haystack, i, needle)) |idx| {
+        i = idx + 1;
+        found += 1;
+    }
+
+    return found;
+}
+
+test countScalar {
+    try testing.expectEqual(0, countScalar(u8, "", 'h'));
+    try testing.expectEqual(1, countScalar(u8, "h", 'h'));
+    try testing.expectEqual(2, countScalar(u8, "hh", 'h'));
+    try testing.expectEqual(3, countScalar(u8, "   abcabc   abc", 'b'));
+}
+
 /// Returns true if the haystack contains expected_count or more needles
 /// needle.len must be > 0
 /// does not count overlapping needles
diff --git a/tools/update_mingw.zig b/tools/update_mingw.zig
index 9c67b27375..6288807cf1 100644
--- a/tools/update_mingw.zig
+++ b/tools/update_mingw.zig
@@ -109,13 +109,29 @@ pub fn main() !void {
 
     {
         // Also add all new def and def.in files.
-        var walker = try src_crt_dir.walk(arena);
+        var walker = try src_crt_dir.walkSelectively(arena);
         defer walker.deinit();
 
         var fail = false;
 
         while (try walker.next()) |entry| {
-            if (entry.kind != .file) continue;
+            switch (entry.kind) {
+                .directory => {
+                    switch (entry.depth()) {
+                        1 => if (def_dirs.has(entry.basename)) {
+                            try walker.enter(entry);
+                            continue;
+                        },
+                        else => {
+                            // The top-level directory was already validated
+                            try walker.enter(entry);
+                            continue;
+                        },
+                    }
+                },
+                .file => {},
+                else => continue,
+            }
 
             const ok_ext = for (def_exts) |ext| {
                 if (std.mem.endsWith(u8, entry.path, ext)) break true;
@@ -123,12 +139,6 @@ pub fn main() !void {
 
             if (!ok_ext) continue;
 
-            const ok_prefix = for (def_dirs) |p| {
-                if (std.mem.startsWith(u8, entry.path, p)) break true;
-            } else false;
-
-            if (!ok_prefix) continue;
-
             const blacklisted = for (blacklisted_defs) |item| {
                 if (std.mem.eql(u8, entry.basename, item)) break true;
             } else false;
@@ -162,14 +172,14 @@ const def_exts = [_][]const u8{
     ".def.in",
 };
 
-const def_dirs = [_][]const u8{
-    "lib32" ++ std.fs.path.sep_str,
-    "lib64" ++ std.fs.path.sep_str,
-    "libarm32" ++ std.fs.path.sep_str,
-    "libarm64" ++ std.fs.path.sep_str,
-    "lib-common" ++ std.fs.path.sep_str,
-    "def-include" ++ std.fs.path.sep_str,
-};
+const def_dirs = std.StaticStringMap(void).initComptime(.{
+    .{"lib32"},
+    .{"lib64"},
+    .{"libarm32"},
+    .{"libarm64"},
+    .{"lib-common"},
+    .{"def-include"},
+});
 
 const blacklisted_defs = [_][]const u8{
     "crtdll.def.in",