diff --git a/lib/ubsan_rt.zig b/lib/ubsan_rt.zig
index a2e0a6c1aa..56cc591898 100644
--- a/lib/ubsan_rt.zig
+++ b/lib/ubsan_rt.zig
@@ -121,28 +121,26 @@ const Value = extern struct {
 
     pub fn format(
         value: Value,
+        bw: *std.io.BufferedWriter,
         comptime fmt: []const u8,
-        _: std.fmt.FormatOptions,
-        writer: anytype,
-    ) !void {
+    ) anyerror!usize {
         comptime assert(fmt.len == 0);
 
         // Work around x86_64 backend limitation.
         if (builtin.zig_backend == .stage2_x86_64 and builtin.os.tag == .windows) {
-            try writer.writeAll("(unknown)");
-            return;
+            return bw.writeAllCount("(unknown)");
         }
 
         switch (value.td.kind) {
             .integer => {
                 if (value.td.isSigned()) {
-                    try writer.print("{}", .{value.getSignedInteger()});
+                    return bw.printCount("{d}", .{value.getSignedInteger()});
                 } else {
-                    try writer.print("{}", .{value.getUnsignedInteger()});
+                    return bw.printCount("{d}", .{value.getUnsignedInteger()});
                 }
             },
-            .float => try writer.print("{}", .{value.getFloat()}),
-            .unknown => try writer.writeAll("(unknown)"),
+            .float => return bw.printCount("{d}", .{value.getFloat()}),
+            .unknown => return bw.writeAllCount("(unknown)"),
         }
     }
 };
@@ -174,8 +172,8 @@ fn overflowHandler(
             const rhs: Value = .{ .handle = rhs_handle, .td = data.td };
 
             const is_signed = data.td.isSigned();
-            const fmt = "{s} integer overflow: " ++ "{} " ++
-                operator ++ " {} cannot be represented in type {s}";
+            const fmt = "{s} integer overflow: " ++ "{f} " ++
+                operator ++ " {f} cannot be represented in type {s}";
 
             panic(@returnAddress(), fmt, .{
                 if (is_signed) "signed" else "unsigned",
@@ -203,7 +201,7 @@ fn negationHandler(
     const value: Value = .{ .handle = value_handle, .td = data.td };
     panic(
         @returnAddress(),
-        "negation of {} cannot be represented in type {s}",
+        "negation of {f} cannot be represented in type {s}",
         .{ value, data.td.getName() },
     );
 }
@@ -227,7 +225,7 @@ fn divRemHandler(
     if (rhs.isMinusOne()) {
         panic(
             @returnAddress(),
-            "division of {} by -1 cannot be represented in type {s}",
+            "division of {f} by -1 cannot be represented in type {s}",
             .{ lhs, data.td.getName() },
         );
     } else panic(@returnAddress(), "division by zero", .{});
@@ -269,8 +267,8 @@ fn alignmentAssumptionHandler(
     if (maybe_offset) |offset| {
         panic(
             @returnAddress(),
-            "assumption of {} byte alignment (with offset of {} byte) for pointer of type {s} failed\n" ++
-                "offset address is {} aligned, misalignment offset is {} bytes",
+            "assumption of {f} byte alignment (with offset of {d} byte) for pointer of type {s} failed\n" ++
+                "offset address is {d} aligned, misalignment offset is {d} bytes",
             .{
                 alignment,
                 @intFromPtr(offset),
@@ -282,8 +280,8 @@ fn alignmentAssumptionHandler(
     } else {
         panic(
             @returnAddress(),
-            "assumption of {} byte alignment for pointer of type {s} failed\n" ++
-                "address is {} aligned, misalignment offset is {} bytes",
+            "assumption of {f} byte alignment for pointer of type {s} failed\n" ++
+                "address is {d} aligned, misalignment offset is {d} bytes",
             .{
                 alignment,
                 data.td.getName(),
@@ -320,21 +318,21 @@ fn shiftOob(
         rhs.getPositiveInteger() >= data.lhs_type.getIntegerSize())
     {
         if (rhs.isNegative()) {
-            panic(@returnAddress(), "shift exponent {} is negative", .{rhs});
+            panic(@returnAddress(), "shift exponent {f} is negative", .{rhs});
         } else {
             panic(
                 @returnAddress(),
-                "shift exponent {} is too large for {}-bit type {s}",
+                "shift exponent {f} is too large for {d}-bit type {s}",
                 .{ rhs, data.lhs_type.getIntegerSize(), data.lhs_type.getName() },
             );
         }
     } else {
         if (lhs.isNegative()) {
-            panic(@returnAddress(), "left shift of negative value {}", .{lhs});
+            panic(@returnAddress(), "left shift of negative value {f}", .{lhs});
         } else {
             panic(
                 @returnAddress(),
-                "left shift of {} by {} places cannot be represented in type {s}",
+                "left shift of {f} by {f} places cannot be represented in type {s}",
                 .{ lhs, rhs, data.lhs_type.getName() },
             );
         }
@@ -361,7 +359,7 @@ fn outOfBounds(
     const index: Value = .{ .handle = index_handle, .td = data.index_type };
     panic(
         @returnAddress(),
-        "index {} out of bounds for type {s}",
+        "index {f} out of bounds for type {s}",
         .{ index, data.array_type.getName() },
     );
 }
@@ -387,7 +385,7 @@ fn pointerOverflow(
         if (result == 0) {
             panic(@returnAddress(), "applying zero offset to null pointer", .{});
         } else {
-            panic(@returnAddress(), "applying non-zero offset {} to null pointer", .{result});
+            panic(@returnAddress(), "applying non-zero offset {d} to null pointer", .{result});
         }
     } else {
         if (result == 0) {
@@ -483,7 +481,7 @@ fn typeMismatch(
     } else if (!std.mem.isAligned(handle, alignment)) {
         panic(
             @returnAddress(),
-            "{s} misaligned address 0x{x} for type {s}, which requires {} byte alignment",
+            "{s} misaligned address 0x{x} for type {s}, which requires {d} byte alignment",
             .{ data.kind.getName(), handle, data.td.getName(), alignment },
         );
     } else {
@@ -531,7 +529,7 @@ fn nonNullArgAbort(data: *const NonNullArgData) callconv(.c) noreturn {
 fn nonNullArg(data: *const NonNullArgData) callconv(.c) noreturn {
     panic(
         @returnAddress(),
-        "null pointer passed as argument {}, which is declared to never be null",
+        "null pointer passed as argument {d}, which is declared to never be null",
         .{data.arg_index},
     );
 }
@@ -555,7 +553,7 @@ fn loadInvalidValue(
     const value: Value = .{ .handle = value_handle, .td = data.td };
     panic(
         @returnAddress(),
-        "load of value {}, which is not valid for type {s}",
+        "load of value {f}, which is not valid for type {s}",
         .{ value, data.td.getName() },
     );
 }
@@ -598,7 +596,7 @@ fn vlaBoundNotPositive(
     const bound: Value = .{ .handle = bound_handle, .td = data.td };
     panic(
         @returnAddress(),
-        "variable length array bound evaluates to non-positive value {}",
+        "variable length array bound evaluates to non-positive value {f}",
         .{bound},
     );
 }
@@ -631,13 +629,13 @@ fn floatCastOverflow(
     if (@as(u16, ptr[0]) + @as(u16, ptr[1]) < 2 or ptr[0] == 0xFF or ptr[1] == 0xFF) {
         const data: *const FloatCastOverflowData = @ptrCast(data_handle);
         const from_value: Value = .{ .handle = from_handle, .td = data.from };
-        panic(@returnAddress(), "{} is outside the range of representable values of type {s}", .{
+        panic(@returnAddress(), "{f} is outside the range of representable values of type {s}", .{
             from_value, data.to.getName(),
         });
     } else {
         const data: *const FloatCastOverflowDataV2 = @ptrCast(data_handle);
         const from_value: Value = .{ .handle = from_handle, .td = data.from };
-        panic(@returnAddress(), "{} is outside the range of representable values of type {s}", .{
+        panic(@returnAddress(), "{f} is outside the range of representable values of type {s}", .{
             from_value, data.to.getName(),
         });
     }