diff --git a/lib/std/SemanticVersion.zig b/lib/std/SemanticVersion.zig
index cd0d48fba7..99072eeea5 100644
--- a/lib/std/SemanticVersion.zig
+++ b/lib/std/SemanticVersion.zig
@@ -157,7 +157,7 @@ pub fn format(
     out_stream: anytype,
 ) !void {
     _ = options;
-    if (fmt.len != 0) @compileError("Unknown format string: '" ++ fmt ++ "'");
+    if (fmt.len != 0) std.fmt.invalidFmtError(fmt, self);
     try std.fmt.format(out_stream, "{d}.{d}.{d}", .{ self.major, self.minor, self.patch });
     if (self.pre) |pre| try std.fmt.format(out_stream, "-{s}", .{pre});
     if (self.build) |build| try std.fmt.format(out_stream, "+{s}", .{build});
diff --git a/lib/std/build/CheckObjectStep.zig b/lib/std/build/CheckObjectStep.zig
index 63b361473b..7bebea54a0 100644
--- a/lib/std/build/CheckObjectStep.zig
+++ b/lib/std/build/CheckObjectStep.zig
@@ -187,7 +187,7 @@ const ComputeCompareExpected = struct {
         options: std.fmt.FormatOptions,
         writer: anytype,
     ) !void {
-        _ = fmt;
+        if (fmt.len != 0) std.fmt.invalidFmtError(fmt, value);
         _ = options;
         try writer.print("{s} ", .{@tagName(value.op)});
         switch (value.value) {
@@ -360,7 +360,7 @@ fn make(step: *Step) !void {
                         std.debug.print(
                             \\
                             \\========= Comparison failed for action: ===========
-                            \\{s} {s}
+                            \\{s} {}
                             \\========= From parsed file: =======================
                             \\{s}
                             \\
diff --git a/lib/std/builtin.zig b/lib/std/builtin.zig
index 430a29c9d7..33604dfee7 100644
--- a/lib/std/builtin.zig
+++ b/lib/std/builtin.zig
@@ -38,12 +38,13 @@ pub const StackTrace = struct {
         options: std.fmt.FormatOptions,
         writer: anytype,
     ) !void {
+        if (fmt.len != 0) std.fmt.invalidFmtError(fmt, self);
+
         // TODO: re-evaluate whether to use format() methods at all.
         // Until then, avoid an error when using GeneralPurposeAllocator with WebAssembly
         // where it tries to call detectTTYConfig here.
         if (builtin.os.tag == .freestanding) return;
 
-        _ = fmt;
         _ = options;
         var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
         defer arena.deinit();
@@ -534,7 +535,7 @@ pub const Version = struct {
                 return std.fmt.format(out_stream, "{d}.{d}.{d}", .{ self.major, self.minor, self.patch });
             }
         } else {
-            @compileError("Unknown format string: '" ++ fmt ++ "'");
+            std.fmt.invalidFmtError(fmt, self);
         }
     }
 };
diff --git a/lib/std/debug.zig b/lib/std/debug.zig
index 205be60b11..75e8426e2c 100644
--- a/lib/std/debug.zig
+++ b/lib/std/debug.zig
@@ -2123,7 +2123,7 @@ pub fn ConfigurableTrace(comptime size: usize, comptime stack_frame_count: usize
             options: std.fmt.FormatOptions,
             writer: anytype,
         ) !void {
-            _ = fmt;
+            if (fmt.len != 0) std.fmt.invalidFmtError(fmt, t);
             _ = options;
             if (enabled) {
                 try writer.writeAll("\n");
diff --git a/lib/std/fmt.zig b/lib/std/fmt.zig
index 0fcb2ae5b1..2af293194a 100644
--- a/lib/std/fmt.zig
+++ b/lib/std/fmt.zig
@@ -454,7 +454,7 @@ fn stripOptionalOrErrorUnionSpec(comptime fmt: []const u8) []const u8 {
         fmt[1..];
 }
 
-fn invalidFmtErr(comptime fmt: []const u8, value: anytype) void {
+pub fn invalidFmtError(comptime fmt: []const u8, value: anytype) void {
     @compileError("invalid format string '" ++ fmt ++ "' for type '" ++ @typeName(@TypeOf(value)) ++ "'");
 }
 
@@ -486,11 +486,11 @@ pub fn formatType(
             return formatValue(value, actual_fmt, options, writer);
         },
         .Void => {
-            if (actual_fmt.len != 0) invalidFmtErr(fmt, value);
+            if (actual_fmt.len != 0) invalidFmtError(fmt, value);
             return formatBuf("void", options, writer);
         },
         .Bool => {
-            if (actual_fmt.len != 0) invalidFmtErr(fmt, value);
+            if (actual_fmt.len != 0) invalidFmtError(fmt, value);
             return formatBuf(if (value) "true" else "false", options, writer);
         },
         .Optional => {
@@ -514,14 +514,14 @@ pub fn formatType(
             }
         },
         .ErrorSet => {
-            if (actual_fmt.len != 0) invalidFmtErr(fmt, value);
+            if (actual_fmt.len != 0) invalidFmtError(fmt, value);
             try writer.writeAll("error.");
             return writer.writeAll(@errorName(value));
         },
         .Enum => |enumInfo| {
             try writer.writeAll(@typeName(T));
             if (enumInfo.is_exhaustive) {
-                if (actual_fmt.len != 0) invalidFmtErr(fmt, value);
+                if (actual_fmt.len != 0) invalidFmtError(fmt, value);
                 try writer.writeAll(".");
                 try writer.writeAll(@tagName(value));
                 return;
@@ -542,7 +542,7 @@ pub fn formatType(
             try writer.writeAll(")");
         },
         .Union => |info| {
-            if (actual_fmt.len != 0) invalidFmtErr(fmt, value);
+            if (actual_fmt.len != 0) invalidFmtError(fmt, value);
             try writer.writeAll(@typeName(T));
             if (max_depth == 0) {
                 return writer.writeAll("{ ... }");
@@ -562,7 +562,7 @@ pub fn formatType(
             }
         },
         .Struct => |info| {
-            if (actual_fmt.len != 0) invalidFmtErr(fmt, value);
+            if (actual_fmt.len != 0) invalidFmtError(fmt, value);
             if (info.is_tuple) {
                 // Skip the type and field names when formatting tuples.
                 if (max_depth == 0) {
@@ -618,7 +618,7 @@ pub fn formatType(
                         }
                         return;
                     }
-                    invalidFmtErr(fmt, value);
+                    invalidFmtError(fmt, value);
                 },
                 .Enum, .Union, .Struct => {
                     return formatType(value.*, actual_fmt, options, writer, max_depth);
@@ -640,7 +640,7 @@ pub fn formatType(
                         else => {},
                     }
                 }
-                invalidFmtErr(fmt, value);
+                invalidFmtError(fmt, value);
             },
             .Slice => {
                 if (actual_fmt.len == 0)
@@ -703,20 +703,20 @@ pub fn formatType(
             try writer.writeAll(" }");
         },
         .Fn => {
-            if (actual_fmt.len != 0) invalidFmtErr(fmt, value);
+            if (actual_fmt.len != 0) invalidFmtError(fmt, value);
             return format(writer, "{s}@{x}", .{ @typeName(T), @ptrToInt(value) });
         },
         .Type => {
-            if (actual_fmt.len != 0) invalidFmtErr(fmt, value);
+            if (actual_fmt.len != 0) invalidFmtError(fmt, value);
             return formatBuf(@typeName(value), options, writer);
         },
         .EnumLiteral => {
-            if (actual_fmt.len != 0) invalidFmtErr(fmt, value);
+            if (actual_fmt.len != 0) invalidFmtError(fmt, value);
             const buffer = [_]u8{'.'} ++ @tagName(value);
             return formatBuf(buffer, options, writer);
         },
         .Null => {
-            if (actual_fmt.len != 0) invalidFmtErr(fmt, value);
+            if (actual_fmt.len != 0) invalidFmtError(fmt, value);
             return formatBuf("null", options, writer);
         },
         else => @compileError("unable to format type '" ++ @typeName(T) ++ "'"),
@@ -786,7 +786,7 @@ pub fn formatIntValue(
         radix = 8;
         case = .lower;
     } else {
-        invalidFmtErr(fmt, value);
+        invalidFmtError(fmt, value);
     }
 
     return formatInt(int_value, radix, case, options, writer);
@@ -815,7 +815,7 @@ fn formatFloatValue(
             error.NoSpaceLeft => unreachable,
         };
     } else {
-        invalidFmtErr(fmt, value);
+        invalidFmtError(fmt, value);
     }
 
     return formatBuf(buf_stream.getWritten(), options, writer);
diff --git a/lib/std/fs/wasi.zig b/lib/std/fs/wasi.zig
index 522731ef02..ca766bbd29 100644
--- a/lib/std/fs/wasi.zig
+++ b/lib/std/fs/wasi.zig
@@ -62,7 +62,7 @@ pub const PreopenType = union(PreopenTypeTag) {
     }
 
     pub fn format(self: Self, comptime fmt: []const u8, options: std.fmt.FormatOptions, out_stream: anytype) !void {
-        _ = fmt;
+        if (fmt.len != 0) std.fmt.invalidFmtError(fmt, self);
         _ = options;
         try out_stream.print("PreopenType{{ ", .{});
         switch (self) {
diff --git a/lib/std/heap/general_purpose_allocator.zig b/lib/std/heap/general_purpose_allocator.zig
index 2180b87942..11bc9d0d11 100644
--- a/lib/std/heap/general_purpose_allocator.zig
+++ b/lib/std/heap/general_purpose_allocator.zig
@@ -326,7 +326,7 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
                             const slot_index = @intCast(SlotIndex, used_bits_byte * 8 + bit_index);
                             const stack_trace = bucketStackTrace(bucket, size_class, slot_index, .alloc);
                             const addr = bucket.page + slot_index * size_class;
-                            log.err("memory address 0x{x} leaked: {s}", .{
+                            log.err("memory address 0x{x} leaked: {}", .{
                                 @ptrToInt(addr), stack_trace,
                             });
                             leaks = true;
@@ -358,7 +358,7 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
             while (it.next()) |large_alloc| {
                 if (config.retain_metadata and large_alloc.freed) continue;
                 const stack_trace = large_alloc.getStackTrace(.alloc);
-                log.err("memory address 0x{x} leaked: {s}", .{
+                log.err("memory address 0x{x} leaked: {}", .{
                     @ptrToInt(large_alloc.bytes.ptr), stack_trace,
                 });
                 leaks = true;
@@ -443,7 +443,7 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
                 .index = 0,
             };
             std.debug.captureStackTrace(ret_addr, &second_free_stack_trace);
-            log.err("Double free detected. Allocation: {s} First free: {s} Second free: {s}", .{
+            log.err("Double free detected. Allocation: {} First free: {} Second free: {}", .{
                 alloc_stack_trace, free_stack_trace, second_free_stack_trace,
             });
         }
@@ -533,7 +533,7 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
                     .index = 0,
                 };
                 std.debug.captureStackTrace(ret_addr, &free_stack_trace);
-                log.err("Allocation size {d} bytes does not match free size {d}. Allocation: {s} Free: {s}", .{
+                log.err("Allocation size {d} bytes does not match free size {d}. Allocation: {} Free: {}", .{
                     entry.value_ptr.bytes.len,
                     old_mem.len,
                     entry.value_ptr.getStackTrace(.alloc),
@@ -606,7 +606,7 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
                     .index = 0,
                 };
                 std.debug.captureStackTrace(ret_addr, &free_stack_trace);
-                log.err("Allocation size {d} bytes does not match free size {d}. Allocation: {s} Free: {s}", .{
+                log.err("Allocation size {d} bytes does not match free size {d}. Allocation: {} Free: {}", .{
                     entry.value_ptr.bytes.len,
                     old_mem.len,
                     entry.value_ptr.getStackTrace(.alloc),
diff --git a/lib/std/math/big/int.zig b/lib/std/math/big/int.zig
index ac2f089ea1..189093aa84 100644
--- a/lib/std/math/big/int.zig
+++ b/lib/std/math/big/int.zig
@@ -2061,7 +2061,7 @@ pub const Const = struct {
             radix = 16;
             case = .upper;
         } else {
-            @compileError("Unknown format string: '" ++ fmt ++ "'");
+            std.fmt.invalidFmtError(fmt, self);
         }
 
         var limbs: [128]Limb = undefined;
diff --git a/lib/std/net.zig b/lib/std/net.zig
index 21fa36c4eb..def6e0e46f 100644
--- a/lib/std/net.zig
+++ b/lib/std/net.zig
@@ -149,6 +149,7 @@ pub const Address = extern union {
         options: std.fmt.FormatOptions,
         out_stream: anytype,
     ) !void {
+        if (fmt.len != 0) std.fmt.invalidFmtError(fmt, self);
         switch (self.any.family) {
             os.AF.INET => try self.in.format(fmt, options, out_stream),
             os.AF.INET6 => try self.in6.format(fmt, options, out_stream),
@@ -274,7 +275,7 @@ pub const Ip4Address = extern struct {
         options: std.fmt.FormatOptions,
         out_stream: anytype,
     ) !void {
-        _ = fmt;
+        if (fmt.len != 0) std.fmt.invalidFmtError(fmt, self);
         _ = options;
         const bytes = @ptrCast(*const [4]u8, &self.sa.addr);
         try std.fmt.format(out_stream, "{}.{}.{}.{}:{}", .{
@@ -563,7 +564,7 @@ pub const Ip6Address = extern struct {
         options: std.fmt.FormatOptions,
         out_stream: anytype,
     ) !void {
-        _ = fmt;
+        if (fmt.len != 0) std.fmt.invalidFmtError(fmt, self);
         _ = options;
         const port = mem.bigToNative(u16, self.sa.port);
         if (mem.eql(u8, self.sa.addr[0..12], &[_]u8{ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xff, 0xff })) {
diff --git a/lib/std/os/uefi.zig b/lib/std/os/uefi.zig
index 25964e09fa..f51caaa86f 100644
--- a/lib/std/os/uefi.zig
+++ b/lib/std/os/uefi.zig
@@ -68,7 +68,7 @@ pub const Guid = extern struct {
                 fmt(std.mem.asBytes(&self.node)),
             });
         } else {
-            @compileError("Unknown format character: '" ++ f ++ "'");
+            std.fmt.invalidFmtError(f, self);
         }
     }
 
diff --git a/lib/std/target.zig b/lib/std/target.zig
index cbab23644b..745be7dbff 100644
--- a/lib/std/target.zig
+++ b/lib/std/target.zig
@@ -167,19 +167,21 @@ pub const Target = struct {
                 _: std.fmt.FormatOptions,
                 out_stream: anytype,
             ) !void {
-                if (fmt.len > 0 and fmt[0] == 's') {
+                if (comptime std.mem.eql(u8, fmt, "s")) {
                     if (@enumToInt(self) >= @enumToInt(WindowsVersion.nt4) and @enumToInt(self) <= @enumToInt(WindowsVersion.latest)) {
                         try std.fmt.format(out_stream, ".{s}", .{@tagName(self)});
                     } else {
                         // TODO this code path breaks zig triples, but it is used in `builtin`
                         try std.fmt.format(out_stream, "@intToEnum(Target.Os.WindowsVersion, 0x{X:0>8})", .{@enumToInt(self)});
                     }
-                } else {
+                } else if (fmt.len == 0) {
                     if (@enumToInt(self) >= @enumToInt(WindowsVersion.nt4) and @enumToInt(self) <= @enumToInt(WindowsVersion.latest)) {
                         try std.fmt.format(out_stream, "WindowsVersion.{s}", .{@tagName(self)});
                     } else {
                         try std.fmt.format(out_stream, "WindowsVersion(0x{X:0>8})", .{@enumToInt(self)});
                     }
+                } else {
+                    std.fmt.invalidFmtError(fmt, self);
                 }
             }
         };
diff --git a/lib/std/testing.zig b/lib/std/testing.zig
index 60a778ad7a..f1b01b0bd0 100644
--- a/lib/std/testing.zig
+++ b/lib/std/testing.zig
@@ -700,7 +700,7 @@ pub fn checkAllAllocationFailures(backing_allocator: std.mem.Allocator, comptime
             error.OutOfMemory => {
                 if (failing_allocator_inst.allocated_bytes != failing_allocator_inst.freed_bytes) {
                     print(
-                        "\nfail_index: {d}/{d}\nallocated bytes: {d}\nfreed bytes: {d}\nallocations: {d}\ndeallocations: {d}\nallocation that was made to fail: {s}",
+                        "\nfail_index: {d}/{d}\nallocated bytes: {d}\nfreed bytes: {d}\nallocations: {d}\ndeallocations: {d}\nallocation that was made to fail: {}",
                         .{
                             fail_index,
                             needed_alloc_count,
diff --git a/lib/std/wasm.zig b/lib/std/wasm.zig
index 9af0156239..3d47d43487 100644
--- a/lib/std/wasm.zig
+++ b/lib/std/wasm.zig
@@ -356,7 +356,7 @@ pub const Type = struct {
     returns: []const Valtype,
 
     pub fn format(self: Type, comptime fmt: []const u8, opt: std.fmt.FormatOptions, writer: anytype) !void {
-        _ = fmt;
+        if (fmt.len != 0) std.fmt.invalidFmtError(fmt, self);
         _ = opt;
         try writer.writeByte('(');
         for (self.params) |param, i| {
diff --git a/lib/std/x/net/bpf.zig b/lib/std/x/net/bpf.zig
index 8fd318b03b..bee930c332 100644
--- a/lib/std/x/net/bpf.zig
+++ b/lib/std/x/net/bpf.zig
@@ -282,8 +282,7 @@ pub const Insn = extern struct {
         writer: anytype,
     ) !void {
         _ = opts;
-        if (comptime layout.len != 0 and layout[0] != 's')
-            @compileError("Unsupported format specifier for BPF Insn type '" ++ layout ++ "'.");
+        if (layout.len != 0) std.fmt.invalidFmtError(layout, self);
 
         try std.fmt.format(
             writer,
diff --git a/lib/std/x/net/ip.zig b/lib/std/x/net/ip.zig
index a3f5dfecf6..b3da9725d8 100644
--- a/lib/std/x/net/ip.zig
+++ b/lib/std/x/net/ip.zig
@@ -47,8 +47,8 @@ pub const Address = union(enum) {
         opts: fmt.FormatOptions,
         writer: anytype,
     ) !void {
+        if (layout.len != 0) std.fmt.invalidFmtError(layout, self);
         _ = opts;
-        _ = layout;
         switch (self) {
             .ipv4 => |address| try fmt.format(writer, "{}:{}", .{ address.host, address.port }),
             .ipv6 => |address| try fmt.format(writer, "{}:{}", .{ address.host, address.port }),
diff --git a/lib/std/x/os/net.zig b/lib/std/x/os/net.zig
index d8af2b3577..e00299e243 100644
--- a/lib/std/x/os/net.zig
+++ b/lib/std/x/os/net.zig
@@ -168,9 +168,7 @@ pub const IPv4 = extern struct {
         writer: anytype,
     ) !void {
         _ = opts;
-        if (comptime layout.len != 0 and layout[0] != 's') {
-            @compileError("Unsupported format specifier for IPv4 type '" ++ layout ++ "'.");
-        }
+        if (layout.len != 0) std.fmt.invalidFmtError(layout, self);
 
         try fmt.format(writer, "{}.{}.{}.{}", .{
             self.octets[0],
@@ -382,7 +380,7 @@ pub const IPv6 = extern struct {
             'x', 'X' => |specifier| specifier,
             's' => 'x',
             'S' => 'X',
-            else => @compileError("Unsupported format specifier for IPv6 type '" ++ layout ++ "'."),
+            else => std.fmt.invalidFmtError(layout, self),
         }};
 
         if (mem.startsWith(u8, &self.octets, &v4_mapped_prefix)) {
diff --git a/lib/std/x/os/socket.zig b/lib/std/x/os/socket.zig
index ed354681ed..3de9359cc5 100644
--- a/lib/std/x/os/socket.zig
+++ b/lib/std/x/os/socket.zig
@@ -127,8 +127,8 @@ pub const Socket = struct {
             opts: fmt.FormatOptions,
             writer: anytype,
         ) !void {
+            if (layout.len != 0) std.fmt.invalidFmtError(layout, self);
             _ = opts;
-            _ = layout;
             switch (self) {
                 .ipv4 => |address| try fmt.format(writer, "{}:{}", .{ address.host, address.port }),
                 .ipv6 => |address| try fmt.format(writer, "{}:{}", .{ address.host, address.port }),
diff --git a/src/main.zig b/src/main.zig
index d0edbbed6d..71325b18d9 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -3744,6 +3744,7 @@ pub fn cmdBuild(gpa: Allocator, arena: Allocator, args: []const []const u8) !voi
         var override_global_cache_dir: ?[]const u8 = try optionalStringEnvVar(arena, "ZIG_GLOBAL_CACHE_DIR");
         var override_local_cache_dir: ?[]const u8 = try optionalStringEnvVar(arena, "ZIG_LOCAL_CACHE_DIR");
         var child_argv = std.ArrayList([]const u8).init(arena);
+        var reference_trace: ?u32 = null;
 
         const argv_index_exe = child_argv.items.len;
         _ = try child_argv.addOne();
@@ -3795,10 +3796,16 @@ pub fn cmdBuild(gpa: Allocator, arena: Allocator, args: []const []const u8) !voi
                         try child_argv.append(arg);
                     } else if (mem.eql(u8, arg, "-freference-trace")) {
                         try child_argv.append(arg);
+                        reference_trace = 256;
                     } else if (mem.startsWith(u8, arg, "-freference-trace=")) {
                         try child_argv.append(arg);
+                        const num = arg["-freference-trace=".len..];
+                        reference_trace = std.fmt.parseUnsigned(u32, num, 10) catch |err| {
+                            fatal("unable to parse reference_trace count '{s}': {s}", .{ num, @errorName(err) });
+                        };
                     } else if (mem.eql(u8, arg, "-fno-reference-trace")) {
                         try child_argv.append(arg);
+                        reference_trace = null;
                     }
                 }
                 try child_argv.append(arg);
@@ -3932,6 +3939,7 @@ pub fn cmdBuild(gpa: Allocator, arena: Allocator, args: []const []const u8) !voi
             .thread_pool = &thread_pool,
             .use_stage1 = use_stage1,
             .cache_mode = .whole,
+            .reference_trace = reference_trace,
         }) catch |err| {
             fatal("unable to create compilation: {s}", .{@errorName(err)});
         };