RFC: Introduce concat_bytes!() to join [u8] and byte str analogous to concat! for str

text/0000-byte-concat.md

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+- Feature Name: byte_concat
+- Start Date: 2018-07-31
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+Add a macro `concat_bytes!()` to join byte sequences onto an `u8` array,
+the same way `concat!()` currently supports for `str` literals.
+
+# Motivation
+[motivation]: #motivation
+
+`concat!()` is convenient and useful to create compile time `str` literals
+from `str`, `bool`, numeric and `char` literals in the code. This RFC adds an
+equivalent capability for `[u8]` instead of `str`.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+The `concat_bytes!()` macro concatenates literals into a byte string literal
+(an expression of the type `&[u8; N]`). The following literal types are
+supported as inputs:
+
+- byte string literals (`b"..."`)
+- byte literals (`b'b'`)
+- numeric array literals – if any literal is outside of `u8` range, it will
+  cause a compile time error:
+
+  ```
+  error: cannot concatenate a non-`u8` literal in a byte string literal
+    --> $FILE:XX:YY
+     |
+  XX |     concat_bytes!([300, 1, 2, 256], b"val");
+     |                    ^^^        ^^^ this value is larger than `255`
+     |                    |
+     |                    this value is larger than `255`
+  ```
+
+For example, `concat_bytes!(42, b"va", b'l', [1, 2])` evaluates to
+`[42, 118, 97, 108, 1, 2]`.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+None known.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+`concat!` could instead be changed to sometimes produce byte literals instead of
+string literals, like a previous revision of this RFC proposed. This would make
+it hard to ensure the right output type is produced – users would have to use
+hacks like adding a dummy `b""` argument to force a byte literal output.
+
+An earlier version of this RFC proposed to support integer literals outside of
+arrays, but that was rejected since it would make the output of
+`byte_concat!(123, b"\n")` inconsistent with the equivalent `concat!`
+invocation.
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+- Should additional literal types be supported? Byte string literals are
+  basically the same thing as byte slice references, so it might make sense to
+  support those as well (support `&[0, 1, 2]` in addition to `[0, 1, 2]`).
+- What to do with string and character literals? They could either be supported
+  with their underlying UTF-8 representation being concatenated, or rejected.