diff --git a/lib/std/json.zig b/lib/std/json.zig
index 1e4f19fda..74e008d76 100644
--- a/lib/std/json.zig
+++ b/lib/std/json.zig
@@ -1598,21 +1598,21 @@ test "write json then parse it" {
     testing.expect(mem.eql(u8, tree.root.Object.get("str").?.value.String, "hello"));
 }
 
-fn test_parse(memory: []u8, json_str: []const u8) !Value {
-    // buf_alloc goes out of scope, but we don't use it after parsing
-    var buf_alloc = std.heap.FixedBufferAllocator.init(memory);
-    var p = Parser.init(&buf_alloc.allocator, false);
+fn test_parse(arena_allocator: *std.mem.Allocator, json_str: []const u8) !Value {
+    var p = Parser.init(arena_allocator, false);
     return (try p.parse(json_str)).root;
 }
 
 test "parsing empty string gives appropriate error" {
-    var memory: [1024 * 4]u8 = undefined;
-    testing.expectError(error.UnexpectedEndOfJson, test_parse(&memory, ""));
+    var arena_allocator = std.heap.ArenaAllocator.init(std.testing.allocator);
+    defer arena_allocator.deinit();
+    testing.expectError(error.UnexpectedEndOfJson, test_parse(&arena_allocator.allocator, ""));
 }
 
 test "integer after float has proper type" {
-    var memory: [1024 * 8]u8 = undefined;
-    const json = try test_parse(&memory,
+    var arena_allocator = std.heap.ArenaAllocator.init(std.testing.allocator);
+    defer arena_allocator.deinit();
+    const json = try test_parse(&arena_allocator.allocator,
         \\{
         \\  "float": 3.14,
         \\  "ints": [1, 2, 3]
@@ -1622,7 +1622,8 @@ test "integer after float has proper type" {
 }
 
 test "escaped characters" {
-    var memory: [1024 * 16]u8 = undefined;
+    var arena_allocator = std.heap.ArenaAllocator.init(std.testing.allocator);
+    defer arena_allocator.deinit();
     const input =
         \\{
         \\  "backslash": "\\",
@@ -1638,7 +1639,7 @@ test "escaped characters" {
         \\}
     ;
 
-    const obj = (try test_parse(&memory, input)).Object;
+    const obj = (try test_parse(&arena_allocator.allocator, input)).Object;
 
     testing.expectEqualSlices(u8, obj.get("backslash").?.value.String, "\\");
     testing.expectEqualSlices(u8, obj.get("forwardslash").?.value.String, "/");
@@ -1662,13 +1663,13 @@ test "string copy option" {
         \\}
     ;
 
-    var mem_buffer: [1024 * 16]u8 = undefined;
-    var buf_alloc = std.heap.FixedBufferAllocator.init(&mem_buffer);
+    var arena_allocator = std.heap.ArenaAllocator.init(std.testing.allocator);
+    defer arena_allocator.deinit();
 
-    const tree_nocopy = try Parser.init(&buf_alloc.allocator, false).parse(input);
+    const tree_nocopy = try Parser.init(&arena_allocator.allocator, false).parse(input);
     const obj_nocopy = tree_nocopy.root.Object;
 
-    const tree_copy = try Parser.init(&buf_alloc.allocator, true).parse(input);
+    const tree_copy = try Parser.init(&arena_allocator.allocator, true).parse(input);
     const obj_copy = tree_copy.root.Object;
 
     for ([_][]const u8{ "noescape", "simple", "unicode", "surrogatepair" }) |field_name| {
diff --git a/lib/std/json/write_stream.zig b/lib/std/json/write_stream.zig
index 51afe7fcb..213b6768d 100644
--- a/lib/std/json/write_stream.zig
+++ b/lib/std/json/write_stream.zig
@@ -254,11 +254,11 @@ test "json write stream" {
     var slice_stream = std.io.SliceOutStream.init(&out_buf);
     const out = &slice_stream.stream;
 
-    var mem_buf: [1024 * 10]u8 = undefined;
-    const allocator = &std.heap.FixedBufferAllocator.init(&mem_buf).allocator;
+    var arena_allocator = std.heap.ArenaAllocator.init(std.testing.allocator);
+    defer arena_allocator.deinit();
 
     var w = std.json.WriteStream(@TypeOf(out).Child, 10).init(out);
-    try w.emitJson(try getJson(allocator));
+    try w.emitJson(try getJson(&arena_allocator.allocator));
 
     const result = slice_stream.getWritten();
     const expected =
diff --git a/lib/std/math/big/rational.zig b/lib/std/math/big/rational.zig
index 27a3b7300..438c6c94f 100644
--- a/lib/std/math/big/rational.zig
+++ b/lib/std/math/big/rational.zig
@@ -587,14 +587,13 @@ fn gcdLehmer(r: *Int, xa: Int, ya: Int) !void {
     r.swap(&x);
 }
 
-var buffer: [64 * 8192]u8 = undefined;
-var fixed = std.heap.FixedBufferAllocator.init(buffer[0..]);
-var al = &fixed.allocator;
-
 test "big.rational gcd non-one small" {
-    var a = try Int.initSet(al, 17);
-    var b = try Int.initSet(al, 97);
-    var r = try Int.init(al);
+    var a = try Int.initSet(testing.allocator, 17);
+    defer a.deinit();
+    var b = try Int.initSet(testing.allocator, 97);
+    defer b.deinit();
+    var r = try Int.init(testing.allocator);
+    defer r.deinit();
 
     try gcd(&r, a, b);
 
@@ -602,9 +601,12 @@ test "big.rational gcd non-one small" {
 }
 
 test "big.rational gcd non-one small" {
-    var a = try Int.initSet(al, 4864);
-    var b = try Int.initSet(al, 3458);
-    var r = try Int.init(al);
+    var a = try Int.initSet(testing.allocator, 4864);
+    defer a.deinit();
+    var b = try Int.initSet(testing.allocator, 3458);
+    defer b.deinit();
+    var r = try Int.init(testing.allocator);
+    defer r.deinit();
 
     try gcd(&r, a, b);
 
@@ -612,9 +614,12 @@ test "big.rational gcd non-one small" {
 }
 
 test "big.rational gcd non-one large" {
-    var a = try Int.initSet(al, 0xffffffffffffffff);
-    var b = try Int.initSet(al, 0xffffffffffffffff7777);
-    var r = try Int.init(al);
+    var a = try Int.initSet(testing.allocator, 0xffffffffffffffff);
+    defer a.deinit();
+    var b = try Int.initSet(testing.allocator, 0xffffffffffffffff7777);
+    defer b.deinit();
+    var r = try Int.init(testing.allocator);
+    defer r.deinit();
 
     try gcd(&r, a, b);
 
@@ -622,9 +627,12 @@ test "big.rational gcd non-one large" {
 }
 
 test "big.rational gcd large multi-limb result" {
-    var a = try Int.initSet(al, 0x12345678123456781234567812345678123456781234567812345678);
-    var b = try Int.initSet(al, 0x12345671234567123456712345671234567123456712345671234567);
-    var r = try Int.init(al);
+    var a = try Int.initSet(testing.allocator, 0x12345678123456781234567812345678123456781234567812345678);
+    defer a.deinit();
+    var b = try Int.initSet(testing.allocator, 0x12345671234567123456712345671234567123456712345671234567);
+    defer b.deinit();
+    var r = try Int.init(testing.allocator);
+    defer r.deinit();
 
     try gcd(&r, a, b);
 
@@ -632,9 +640,12 @@ test "big.rational gcd large multi-limb result" {
 }
 
 test "big.rational gcd one large" {
-    var a = try Int.initSet(al, 1897056385327307);
-    var b = try Int.initSet(al, 2251799813685248);
-    var r = try Int.init(al);
+    var a = try Int.initSet(testing.allocator, 1897056385327307);
+    defer a.deinit();
+    var b = try Int.initSet(testing.allocator, 2251799813685248);
+    defer b.deinit();
+    var r = try Int.init(testing.allocator);
+    defer r.deinit();
 
     try gcd(&r, a, b);
 
@@ -661,7 +672,8 @@ fn extractLowBits(a: Int, comptime T: type) T {
 }
 
 test "big.rational extractLowBits" {
-    var a = try Int.initSet(al, 0x11112222333344441234567887654321);
+    var a = try Int.initSet(testing.allocator, 0x11112222333344441234567887654321);
+    defer a.deinit();
 
     const a1 = extractLowBits(a, u8);
     testing.expect(a1 == 0x21);
@@ -680,7 +692,8 @@ test "big.rational extractLowBits" {
 }
 
 test "big.rational set" {
-    var a = try Rational.init(al);
+    var a = try Rational.init(testing.allocator);
+    defer a.deinit();
 
     try a.setInt(5);
     testing.expect((try a.p.to(u32)) == 5);
@@ -708,7 +721,8 @@ test "big.rational set" {
 }
 
 test "big.rational setFloat" {
-    var a = try Rational.init(al);
+    var a = try Rational.init(testing.allocator);
+    defer a.deinit();
 
     try a.setFloat(f64, 2.5);
     testing.expect((try a.p.to(i32)) == 5);
@@ -732,7 +746,8 @@ test "big.rational setFloat" {
 }
 
 test "big.rational setFloatString" {
-    var a = try Rational.init(al);
+    var a = try Rational.init(testing.allocator);
+    defer a.deinit();
 
     try a.setFloatString("72.14159312071241458852455252781510353");
 
@@ -742,7 +757,8 @@ test "big.rational setFloatString" {
 }
 
 test "big.rational toFloat" {
-    var a = try Rational.init(al);
+    var a = try Rational.init(testing.allocator);
+    defer a.deinit();
 
     // = 3.14159297943115234375
     try a.setRatio(3294199, 1048576);
@@ -754,7 +770,8 @@ test "big.rational toFloat" {
 }
 
 test "big.rational set/to Float round-trip" {
-    var a = try Rational.init(al);
+    var a = try Rational.init(testing.allocator);
+    defer a.deinit();
     var prng = std.rand.DefaultPrng.init(0x5EED);
     var i: usize = 0;
     while (i < 512) : (i += 1) {
@@ -765,23 +782,29 @@ test "big.rational set/to Float round-trip" {
 }
 
 test "big.rational copy" {
-    var a = try Rational.init(al);
+    var a = try Rational.init(testing.allocator);
+    defer a.deinit();
 
-    const b = try Int.initSet(al, 5);
+    const b = try Int.initSet(testing.allocator, 5);
+    defer b.deinit();
 
     try a.copyInt(b);
     testing.expect((try a.p.to(u32)) == 5);
     testing.expect((try a.q.to(u32)) == 1);
 
-    const c = try Int.initSet(al, 7);
-    const d = try Int.initSet(al, 3);
+    const c = try Int.initSet(testing.allocator, 7);
+    defer c.deinit();
+    const d = try Int.initSet(testing.allocator, 3);
+    defer d.deinit();
 
     try a.copyRatio(c, d);
     testing.expect((try a.p.to(u32)) == 7);
     testing.expect((try a.q.to(u32)) == 3);
 
-    const e = try Int.initSet(al, 9);
-    const f = try Int.initSet(al, 3);
+    const e = try Int.initSet(testing.allocator, 9);
+    defer e.deinit();
+    const f = try Int.initSet(testing.allocator, 3);
+    defer f.deinit();
 
     try a.copyRatio(e, f);
     testing.expect((try a.p.to(u32)) == 3);
@@ -789,7 +812,8 @@ test "big.rational copy" {
 }
 
 test "big.rational negate" {
-    var a = try Rational.init(al);
+    var a = try Rational.init(testing.allocator);
+    defer a.deinit();
 
     try a.setInt(-50);
     testing.expect((try a.p.to(i32)) == -50);
@@ -805,7 +829,8 @@ test "big.rational negate" {
 }
 
 test "big.rational abs" {
-    var a = try Rational.init(al);
+    var a = try Rational.init(testing.allocator);
+    defer a.deinit();
 
     try a.setInt(-50);
     testing.expect((try a.p.to(i32)) == -50);
@@ -821,8 +846,10 @@ test "big.rational abs" {
 }
 
 test "big.rational swap" {
-    var a = try Rational.init(al);
-    var b = try Rational.init(al);
+    var a = try Rational.init(testing.allocator);
+    defer a.deinit();
+    var b = try Rational.init(testing.allocator);
+    defer b.deinit();
 
     try a.setRatio(50, 23);
     try b.setRatio(17, 3);
@@ -843,8 +870,10 @@ test "big.rational swap" {
 }
 
 test "big.rational cmp" {
-    var a = try Rational.init(al);
-    var b = try Rational.init(al);
+    var a = try Rational.init(testing.allocator);
+    defer a.deinit();
+    var b = try Rational.init(testing.allocator);
+    defer b.deinit();
 
     try a.setRatio(500, 231);
     try b.setRatio(18903, 8584);
@@ -856,8 +885,10 @@ test "big.rational cmp" {
 }
 
 test "big.rational add single-limb" {
-    var a = try Rational.init(al);
-    var b = try Rational.init(al);
+    var a = try Rational.init(testing.allocator);
+    defer a.deinit();
+    var b = try Rational.init(testing.allocator);
+    defer b.deinit();
 
     try a.setRatio(500, 231);
     try b.setRatio(18903, 8584);
@@ -869,9 +900,12 @@ test "big.rational add single-limb" {
 }
 
 test "big.rational add" {
-    var a = try Rational.init(al);
-    var b = try Rational.init(al);
-    var r = try Rational.init(al);
+    var a = try Rational.init(testing.allocator);
+    defer a.deinit();
+    var b = try Rational.init(testing.allocator);
+    defer b.deinit();
+    var r = try Rational.init(testing.allocator);
+    defer r.deinit();
 
     try a.setRatio(78923, 23341);
     try b.setRatio(123097, 12441414);
@@ -882,9 +916,12 @@ test "big.rational add" {
 }
 
 test "big.rational sub" {
-    var a = try Rational.init(al);
-    var b = try Rational.init(al);
-    var r = try Rational.init(al);
+    var a = try Rational.init(testing.allocator);
+    defer a.deinit();
+    var b = try Rational.init(testing.allocator);
+    defer b.deinit();
+    var r = try Rational.init(testing.allocator);
+    defer r.deinit();
 
     try a.setRatio(78923, 23341);
     try b.setRatio(123097, 12441414);
@@ -895,9 +932,12 @@ test "big.rational sub" {
 }
 
 test "big.rational mul" {
-    var a = try Rational.init(al);
-    var b = try Rational.init(al);
-    var r = try Rational.init(al);
+    var a = try Rational.init(testing.allocator);
+    defer a.deinit();
+    var b = try Rational.init(testing.allocator);
+    defer b.deinit();
+    var r = try Rational.init(testing.allocator);
+    defer r.deinit();
 
     try a.setRatio(78923, 23341);
     try b.setRatio(123097, 12441414);
@@ -908,9 +948,12 @@ test "big.rational mul" {
 }
 
 test "big.rational div" {
-    var a = try Rational.init(al);
-    var b = try Rational.init(al);
-    var r = try Rational.init(al);
+    var a = try Rational.init(testing.allocator);
+    defer a.deinit();
+    var b = try Rational.init(testing.allocator);
+    defer b.deinit();
+    var r = try Rational.init(testing.allocator);
+    defer r.deinit();
 
     try a.setRatio(78923, 23341);
     try b.setRatio(123097, 12441414);
@@ -921,8 +964,10 @@ test "big.rational div" {
 }
 
 test "big.rational div" {
-    var a = try Rational.init(al);
-    var r = try Rational.init(al);
+    var a = try Rational.init(testing.allocator);
+    defer a.deinit();
+    var r = try Rational.init(testing.allocator);
+    defer r.deinit();
 
     try a.setRatio(78923, 23341);
     a.invert();
diff --git a/lib/std/mem.zig b/lib/std/mem.zig
index 8f4987e0c..a1b4a3c80 100644
--- a/lib/std/mem.zig
+++ b/lib/std/mem.zig
@@ -1011,11 +1011,21 @@ pub fn join(allocator: *Allocator, separator: []const u8, slices: []const []cons
 }
 
 test "mem.join" {
-    var buf: [1024]u8 = undefined;
-    const a = &std.heap.FixedBufferAllocator.init(&buf).allocator;
-    testing.expect(eql(u8, try join(a, ",", &[_][]const u8{ "a", "b", "c" }), "a,b,c"));
-    testing.expect(eql(u8, try join(a, ",", &[_][]const u8{"a"}), "a"));
-    testing.expect(eql(u8, try join(a, ",", &[_][]const u8{ "a", "", "b", "", "c" }), "a,,b,,c"));
+    {
+        const str = try join(testing.allocator, ",", &[_][]const u8{ "a", "b", "c" });
+        defer testing.allocator.free(str);
+        testing.expect(eql(u8, str, "a,b,c"));
+    }
+    {
+        const str = try join(testing.allocator, ",", &[_][]const u8{"a"});
+        defer testing.allocator.free(str);
+        testing.expect(eql(u8, str, "a"));
+    }
+    {
+        const str = try join(testing.allocator, ",", &[_][]const u8{ "a", "", "b", "", "c" });
+        defer testing.allocator.free(str);
+        testing.expect(eql(u8, str, "a,,b,,c"));
+    }
 }
 
 /// Copies each T from slices into a new slice that exactly holds all the elements.
@@ -1044,15 +1054,21 @@ pub fn concat(allocator: *Allocator, comptime T: type, slices: []const []const T
 }
 
 test "concat" {
-    var buf: [1024]u8 = undefined;
-    const a = &std.heap.FixedBufferAllocator.init(&buf).allocator;
-    testing.expect(eql(u8, try concat(a, u8, &[_][]const u8{ "abc", "def", "ghi" }), "abcdefghi"));
-    testing.expect(eql(u32, try concat(a, u32, &[_][]const u32{
-        &[_]u32{ 0, 1 },
-        &[_]u32{ 2, 3, 4 },
-        &[_]u32{},
-        &[_]u32{5},
-    }), &[_]u32{ 0, 1, 2, 3, 4, 5 }));
+    {
+        const str = try concat(testing.allocator, u8, &[_][]const u8{ "abc", "def", "ghi" });
+        defer testing.allocator.free(str);
+        testing.expect(eql(u8, str, "abcdefghi"));
+    }
+    {
+        const str = try concat(testing.allocator, u32, &[_][]const u32{
+            &[_]u32{ 0, 1 },
+            &[_]u32{ 2, 3, 4 },
+            &[_]u32{},
+            &[_]u32{5},
+        });
+        defer testing.allocator.free(str);
+        testing.expect(eql(u32, str, &[_]u32{ 0, 1, 2, 3, 4, 5 }));
+    }
 }
 
 test "testStringEquality" {
diff --git a/lib/std/net/test.zig b/lib/std/net/test.zig
index 703a804bf..45d8b1cff 100644
--- a/lib/std/net/test.zig
+++ b/lib/std/net/test.zig
@@ -67,10 +67,8 @@ test "resolve DNS" {
         // DNS resolution not implemented on Windows yet.
         return error.SkipZigTest;
     }
-    var buf: [1000 * 10]u8 = undefined;
-    const a = &std.heap.FixedBufferAllocator.init(&buf).allocator;
 
-    const address_list = net.getAddressList(a, "example.com", 80) catch |err| switch (err) {
+    const address_list = net.getAddressList(testing.allocator, "example.com", 80) catch |err| switch (err) {
         // The tests are required to work even when there is no Internet connection,
         // so some of these errors we must accept and skip the test.
         error.UnknownHostName => return error.SkipZigTest,