♻️ reorganize PR

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json/format_as.md

@@ -49,21 +49,24 @@ std::string format_as(const BasicJsonType& j)
     `fmt` only picks up a `format_as` overload that returns a `std::string` in fmt **10.0.0 through
     11.0.2**. Starting with fmt **11.1.0**, `fmt` restricts automatic `format_as` pickup to overloads that
     return an arithmetic type, so this function has no effect there (it is simply unused, not a compile
-    error). If you use fmt \>= 11.1.0 (or want the same `#!cpp "{:#}"` pretty-print support that
-    [`std::formatter<basic_json>`](std_formatter.md) has), define your own `fmt::formatter` specialization,
-    for example:
+    error).
+
+    If you use fmt \>= 11.1.0, or want the same pretty-print spec support that
+    [`std::formatter<basic_json>`](std_formatter.md) has (`#!cpp "{:#}"`, a width to set the indent such
+    as `#!cpp "{:2}"`/`#!cpp "{:#2}"`, and fill-and-align to pick the indent character such as
+    `#!cpp "{:.>#}"`), define your own `fmt::formatter` specialization mirroring the same logic:
 
     ```cpp
-    template <>
-    struct fmt::formatter<nlohmann::json> : fmt::formatter<std::string>
-    {
-        auto format(const nlohmann::json& j, format_context& ctx) const
-        {
-            return fmt::formatter<std::string>::format(j.dump(), ctx);
-        }
-    };
+    --8<-- "../../../tests/fmt_formatter/project/main.cpp:formatter_recipe"
     ```
 
+    This recipe isn't shipped by the library itself, since doing so would make `fmt` a build dependency
+    (see the FAQ entry on
+    [using JSON values with `std::format` or `fmt`](../../home/faq.md#using-json-values-with-stdformat-or-fmt)
+    for more background) — but it *is* compiled and exercised against a real, current `fmt` release as
+    part of the library's own test suite (`tests/fmt_formatter`, via CMake `FetchContent`), so it's kept in
+    sync with `std::formatter<basic_json>` and verified to actually work, not just illustrative.
+
 ## Examples
 
 ??? example

docs/mkdocs/docs/api/basic_json/std_formatter.md

@@ -10,11 +10,21 @@ namespace std {
 Specialization to make JSON values formattable with [`std::format`](https://en.cppreference.com/w/cpp/utility/format/format)
 (and the other members of C++20's `<format>` header, such as `std::format_to`).
 
-Only an empty format spec (`#!cpp "{}"`) or the single flag `#!cpp "{:#}"` are accepted; any other spec
-throws [`std::format_error`](https://en.cppreference.com/w/cpp/utility/format/format_error).
+A subset of the [standard format spec grammar](https://en.cppreference.com/w/cpp/utility/format/spec) is
+supported, repurposed for JSON pretty-printing; any other spec component (sign, the `0` flag, precision,
+`L`, a dynamic width such as `#!cpp "{:{}}"`, or a trailing type character) throws
+[`std::format_error`](https://en.cppreference.com/w/cpp/utility/format/format_error):
 
 - `#!cpp "{}"` serializes the value the same way as [`dump()`](dump.md) (compact, no whitespace).
-- `#!cpp "{:#}"` serializes the value the same way as `#!cpp dump(4)` (pretty-printed with an indent of 4).
+- `#!cpp "{:#}"` ("alternate form") serializes the value the same way as `#!cpp dump(4)` (pretty-printed
+  with an indent of 4).
+- A width, with or without `#!cpp "#"` (e.g. `#!cpp "{:2}"` or `#!cpp "{:#2}"`), serializes the value the
+  same way as `#!cpp dump(width)` — a width on its own implies pretty-printing, since an indent size has
+  no meaning for compact output.
+- `fill-and-align` (e.g. `#!cpp "{:.>#}"` or `#!cpp "{:.>3}"`) picks a custom indent character, the same
+  way as `#!cpp dump(indent, indent_char)`. The alignment direction itself (`#!cpp '<'`, `#!cpp '>'`,
+  `#!cpp '^'`) has no separate meaning for JSON values — only the fill character before it is used, and
+  any of the three directions is accepted.
 
 This specialization is only available for `#!cpp char`-based JSON values and only if the standard library
 provides `<format>`, controlled by the [`JSON_HAS_STD_FORMAT`](../macros/json_has_std_format.md) macro.

docs/mkdocs/docs/examples/std_formatter.c++20.cpp

@@ -12,5 +12,11 @@ int main()
     std::cout << std::format("{}", j) << "\n\n";
 
     // pretty-printed formatting, like dump(4)
-    std::cout << std::format("{:#}", j) << std::endl;
+    std::cout << std::format("{:#}", j) << "\n\n";
+
+    // a width sets the indent, like dump(2)
+    std::cout << std::format("{:2}", j) << "\n\n";
+
+    // fill-and-align sets the indent character, like dump(4, '.')
+    std::cout << std::format("{:.>#}", j) << std::endl;
 }

docs/mkdocs/docs/examples/std_formatter.c++20.output

@@ -4,3 +4,13 @@
     "one": 1,
     "two": 2
 }
+
+{
+  "one": 1,
+  "two": 2
+}
+
+{
+...."one": 1,
+...."two": 2
+}

docs/mkdocs/docs/home/faq.md

@@ -181,13 +181,14 @@ See [this section](../features/types/number_handling.md#number-serialization) on
 `std::format` works out of the box since version 3.12.x, as long as the standard library provides
 `<format>` (see [`JSON_HAS_STD_FORMAT`](../api/macros/json_has_std_format.md)); see
 [`std::formatter<basic_json>`](../api/basic_json/std_formatter.md) for details, including the `#!cpp "{:#}"`
-pretty-print spec.
+pretty-print spec, indent widths (`#!cpp "{:2}"`), and custom indent characters (`#!cpp "{:.>#}"`).
 
 For `fmt`, the library ships [`format_as`](../api/basic_json/format_as.md), a small customization point
 `fmt` looks for via argument-dependent lookup. It only has an effect on fmt 10.0.0 through 11.0.2 — from
 fmt 11.1.0 onwards, `fmt` no longer picks up a `format_as` overload that returns a `std::string`. On such
-versions (or any version, if you also want `#!cpp "{:#}"` pretty-print support), define your own
-`fmt::formatter` specialization; see [`format_as`](../api/basic_json/format_as.md) for a two-line recipe.
+versions (or any version, if you also want the same `#!cpp "{:#}"`/width/fill-and-align spec support that
+`std::formatter<basic_json>` has), define your own `fmt::formatter` specialization; see
+[`format_as`](../api/basic_json/format_as.md) for a recipe that mirrors it.
 
 If you get ambiguous-overload errors when passing a JSON value to `fmt::format`/`fmt::print` without any
 `fmt::formatter<json>` specialization in scope, that's `fmt` picking up `basic_json`'s implicit