hint optional format statement for optional values

[cameronmore]

Zig Version

0.14.1

Steps to Reproduce and Observed Output

(I apologize in advance if this issue is not well written or is not substantive)

I’ve got a struct type with the following field:

programName: ?[]const u8 = null

And I could not figure out why my other fields like

otherField: []const u8, 

were printing when I formatted them as {s} but programName could only be formatted as {any}, and even then it only printed out the u8 value and not the string. I googled to find that the answer was to format optional values as {?s}. Maybe it says something in the documentation, but a compiler hint would have been helpful.

Expected Output

when detecting that a user is trying to format an optional field, it should hint to format as {?s} rather than the current message as {any} or {!}

[Vexu]

Splitting the errors would probably help:

diff --git a/lib/std/fmt.zig b/lib/std/fmt.zig
index efd5ffd3a2..5fcd6da523 100644
--- a/lib/std/fmt.zig
+++ b/lib/std/fmt.zig
@@ -528,8 +528,10 @@ pub fn formatType(
             return formatBuf(if (value) "true" else "false", options, writer);
         },
         .optional => {
-            if (actual_fmt.len == 0 or actual_fmt[0] != '?')
+            if (actual_fmt.len == 0)
                 @compileError("cannot format optional without a specifier (i.e. {?} or {any})");
+            if (actual_fmt[0] != '?')
+                @compileError("cannot format optional without optional prefix (i.e. {?" ++ actual_fmt ++ "})");
             const remaining_fmt = comptime stripOptionalOrErrorUnionSpec(actual_fmt);
             if (value) |payload| {
                 return formatType(payload, remaining_fmt, options, writer, max_depth);
@@ -538,8 +540,10 @@ pub fn formatType(
             }
         },
         .error_union => {
-            if (actual_fmt.len == 0 or actual_fmt[0] != '!')
+            if (actual_fmt.len == 0)
                 @compileError("cannot format error union without a specifier (i.e. {!} or {any})");
+            if (actual_fmt[0] != '!')
+                @compileError("cannot format error union without error union prefix (i.e. {!" ++ actual_fmt ++ "})");
             const remaining_fmt = comptime stripOptionalOrErrorUnionSpec(actual_fmt);
             if (value) |payload| {
                 return formatType(payload, remaining_fmt, options, writer, max_depth);