Hexadecimal integers with fmt::Debug, including within larger types

text/0000-fmt-debug-hex.md

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+- Feature Name: fmt-debug-hex
+- Start Date: 2017-11-24
+- RFC PR:
+- Rust Issue:
+
+# Summary
+[summary]: #summary
+
+Add support for formatting integers as hexadecimal with the `fmt::Debug` trait,
+including when the occur within larger types.
+
+```rust
+println!("{:02X?}", b"AZaz\0")
+```
+```
+[41, 5A, 61, 7A, 00]
+```
+
+# Motivation
+[motivation]: #motivation
+
+Sometimes the bits that make up an integer are more meaningful than its purely numerical value.
+For example, an RGBA color encoded in `u32` with 8 bits per channel is easier to understand
+when shown as `00CC44FF` than `13387007`.
+
+The `std::fmt::UpperHex` and `std::fmt::LowerHex` provide hexadecimal formatting
+through `{:X}` and `{:x}` in formatting strings,
+but they’re only implemented for plain integer types
+and not other types like slices that might contain integers.
+
+The `std::fmt::Debug` trait (used with `{:?}`) however is intended for
+formatting “in a programmer-facing, debugging context”.
+It can be derived, and doing so is recommended for most types.
+
+This RFC proposes adding the missing combination of:
+
+* Output intended primarily for end-users (`Display`) v.s. for programmers (`Debug`)
+* Numbers shown in decimal v.s. hexadecimal
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+In formatting strings like in the `format!` and `println!` macros,
+the formatting parameters `x` or `X` − to select lower-case or upper-case hexadecimal −
+can now be combined with `?` which select the `Debug` trait.
+
+For example, `format!("{:X?}", [65280].first())` returns `Some(FF00)`.
+
+This can also be combined with other formatting parameters.
+For example, `format!("{:02X?}", b"AZaz\0")` zero-pads each byte to two hexadecimal digits
+and return `[41, 5A, 61, 7A, 00]`.
+
+An API returning `Vec<u32>` might be tested like this:
+
+```rust
+let return_value = foo(bar);
+let expected = &[ /* ... */ ][..];
+assert!(return_value == expected, "{:08X?} != {:08X?}", return_value, expected);
+```
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+## Formatting strings
+
+The syntax of formatting strings
+is [specified with a grammar](https://doc.rust-lang.org/std/fmt/#syntax)
+which at the moment is as follows:
+
+```
+format_string := <text> [ maybe-format <text> ] *
+maybe-format := '{' '{' | '}' '}' | <format>
+format := '{' [ argument ] [ ':' format_spec ] '}'
+argument := integer | identifier
+
+format_spec := [[fill]align][sign]['#']['0'][width]['.' precision][type]
+fill := character
+align := '<' | '^' | '>'
+sign := '+' | '-'
+width := count
+precision := count | '*'
+type := identifier | ''
+count := parameter | integer
+parameter := argument '$'
+```
+
+This RFC adds an optional *radix* immediately before *type*:
+
+```
+format_spec := [[fill]align][sign]['#']['0'][width]['.' precision][radix][type]
+radix: 'x' | 'X'
+```
+
+## `Formatter` API
+
+Note that `x` and `X` are already valid *types*.
+They are only interpreted as a radix when the type is `?`,
+since combining them with other types doesn’t make sense.
+
+This radix is exposed indirectly in two additional methods of `std::fmt::Formatter`:
+
+```rust
+impl<'a> Formatter<'a> {
+    // ...
+
+    /// Based on the radix and type: 16, 10, 8, or 2.
+    ///
+    /// This is mostly useful in `Debug` impls,
+    /// where the trait itself doesn’t imply a radix.
+    fn number_radix(&self) -> u32
+
+    /// true for `X` or `E`
+    ///
+    /// This is mostly useful in `Debug` impls,
+    /// where the trait itself doesn’t imply a case.
+    fn number_uppercase(&self) -> bool
+}
+```
+
+Although the radix and type are separate in the formatting string grammar,
+they are intentionally conflated in this new API.
+
+## `Debug` impls
+
+The `Debug` implementation for primitive integer types `{u,i}{8,16,32,64,128,size}`
+is modified to defer to `LowerHex` or `UpperHex` instead of `Display`,
+based on `formatter.number_radix()` and `formatter.number_uppercase()`.
+The *alternate* `#` flag is ignored, since it already has a separate meaning for `Debug`:
+the `0x` prefix is *not* included.
+
+As of Rust 1.22, impls using the `Formatter::debug_*` methods do not forward
+formatting parameters such as *width* when formatting keys/values/items.
+Doing so is important for this RFC to be useful.
+This is fixed by [PR #46233](https://github.com/rust-lang/rust/pull/46233).
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+The hexadecimal flag in the the `Debug` trait is superficially redundant
+with the `LowerHex` and `UpperHex` traits.
+If these traits were not stable yet, we could have considered a more unified design.
+
+# Rationale and alternatives
+[alternatives]: #alternatives
+
+Implementing `LowerHex` and `UpperHex` was proposed and rejected
+in [PR #44751](https://github.com/rust-lang/rust/pull/44751).
+
+The status quo is that debugging or testing code that could be a one-liner
+requires manual `Debug` impls and/or concatenating the results of separate
+string formatting operations.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+* Should this be extended to octal and binary (as `{:o?}` and `{:b?}`)?
+  Other formatting types/traits too?
+* Details of the new `Formatter` API