Add missing Format.pp_print_bytes function.

This is implemented through a copy using `Bytes.to_string`. While this
seems sub-optimal, avoiding a copy (either through
`Bytes.unsafe_to_string` or by adding a new formatting action) would
"capture" the bytes but would not immediately print it (formatting
might wait before making indentation decision). During this period the
bytes might be mutated again leading to a confusing behavior.

We avoid this problem by making a copy. Users who require zero-copy
output can reimplement it easily (with `Bytes.unsafe_to_string`) and
ensure proper owernship manually.

Changes

@@ -195,6 +195,9 @@ Working version
 - #10389, #10391, #10392: Add {Int,Int32,Int64,Nativeint}.{min,max}.
   (Nicolás Ojeda Bär and Alain Frisch, review by Xavier Leroy)
 
+- #10430: Add Format.print_bytes and Format.pp_print_bytes.
+  (Gabriel Radanne, review by Gabriel Scherer and David Allsopp)
+
 ### Other libraries:
 
 - #10047: Add `Unix.realpath`

stdlib/.depend

@@ -278,6 +278,7 @@ stdlib__Format.cmo : format.ml \
     stdlib__Either.cmi \
     camlinternalFormatBasics.cmi \
     camlinternalFormat.cmi \
+    stdlib__Bytes.cmi \
     stdlib__Buffer.cmi \
     stdlib__Format.cmi
 stdlib__Format.cmx : format.ml \
@@ -291,6 +292,7 @@ stdlib__Format.cmx : format.ml \
     stdlib__Either.cmx \
     camlinternalFormatBasics.cmx \
     camlinternalFormat.cmx \
+    stdlib__Bytes.cmx \
     stdlib__Buffer.cmx \
     stdlib__Format.cmi
 stdlib__Format.cmi : format.mli \

stdlib/format.ml

@@ -636,6 +636,8 @@ let pp_print_as state isize s =
 let pp_print_string state s =
   pp_print_as state (String.length s) s
 
+let pp_print_bytes state s =
+  pp_print_as state (Bytes.length s) (Bytes.to_string s)
 
 (* To format an integer. *)
 let pp_print_int state i = pp_print_string state (Int.to_string i)
@@ -1114,6 +1116,7 @@ and open_stag = pp_open_stag std_formatter
 and close_stag = pp_close_stag std_formatter
 and print_as = pp_print_as std_formatter
 and print_string = pp_print_string std_formatter
+and print_bytes = pp_print_bytes std_formatter
 and print_int = pp_print_int std_formatter
 and print_float = pp_print_float std_formatter
 and print_char = pp_print_char std_formatter

stdlib/format.mli

@@ -221,6 +221,12 @@ val pp_print_string : formatter -> string -> unit
 val print_string : string -> unit
 (** [pp_print_string ppf s] prints [s] in the current pretty-printing box. *)
 
+val pp_print_bytes : formatter -> bytes -> unit
+val print_bytes : bytes -> unit
+(** [pp_print_bytes ppf b] prints [b] in the current pretty-printing box.
+    @since 4.13.0
+*)
+
 val pp_print_as : formatter -> int -> string -> unit
 val print_as : int -> string -> unit
 (** [pp_print_as ppf len s] prints [s] in the current pretty-printing box.