RFC: Elide array size

text/0000-elide-array-size.md

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+- Feature Name: elide_array_size
+- Start Date: 2018-09-18
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+In arrays declared in `static`s, `const`s and `let` bindings with initializer, 
+allow to elide the size in the type and put an underscore there instead if it 
+can be inferred. For example: `static BLORP_NUMBERS: [u32; _] = [0, 8, 15];`.
+
+# Motivation
+[motivation]: #motivation
+
+This will make it easier to set up static data. With the current syntax, one 
+needs to count the elements of an array manually to determine its size. Letting 
+the compiler find the size reduces the potential for error. In `let` bindings 
+it also allows us to ascribe the array component type without requiring the 
+size (which is – perhaps surprisingly – not currently allowed).
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+Requiring to write `[MyType; 42]` for a 42-element array gets tedious,
+especially since the compiler already knows the size from the initializer. To
+reduce the strain, Rust allows you to omit the `42` in those type ascriptions.
+
+For example, you can write:
+
+```rust
+const CONST_CHARS: [u8; _] = *b"This is really a byte array";
+static STATIC_MASKS: [u8; _] = [0, 1, 3, 7, 15, 31, 63, 127, 255];
+
+fn main() {
+    let local_strs: [&'static str; _] = ["Hello", "Rust"];
+    ..
+}
+```
+
+In all other positions and on `let` bindings without initializer, the exact
+number keeps being required.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+This feature is a simple extension of the parser plus AST transformation. In
+its proposed form, there are no corner cases, because the only thing it enables
+that is currently disallowed is to put a `_` wildcard in place of the number of
+elements in array types in `let`s, `const`s and `static`s.
+
+In the parser, at the array length position, we allow underscores. The AST
+may need to be extended to allow underscore; In principle `AnonConst` can
+contain any expression, so `_` could be represented by a `Path`, or the whole
+`AnonConst` could be made `Option`al.
+
+To ensure locality of reasoning, this RFC proposes a lint for `const` or
+`static` items with initializers other than `ExprKind::Repeat` or
+`ExprKind::Array`.
+
+The length can simply be inferred from the initializer. Care should be taken to 
+keep the error messages useful.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+There is a modicum of complexity to extend the parser and AST which needs to be
+done for every Rust parser in existence. However, the feature is minor, so the
+cost should be acceptable.
+
+Also for longer array declarations the actual size may no longer be obvious
+from the code. However, putting any probable or improbable length in and
+observing the compiler error (if any) is enough to find out; also the author
+hopes that the programmers will put in the lengths if they are essential.
+
+Finally, it's possible to add a clippy lint that suggests replacing the 
+underscore with the actual length.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+This feature is very useful and has been requested numerous times. A draft of
+this RFC actually proposed a very resticted type of inference, which would
+however clash with RFC [#2000](https://github.com/rust-lang/rfcs/pull/2000).
+So the solution is to add the restriction as a lint (either in rustc or
+clippy).
+
+We could leave out the wildcard entirely, but `[u32; ]` looks strange and could
+possibly indicate an error, so it's better to use the more balanced `[u32; _]`.
+
+We could do nothing, and waste Rustacean's time by counting or parsing compiler
+errors (unless their accuracy of estimating array length is 100%. I'm sure mine
+isn't) or use a macro (see below).
+
+# Prior art
+[prior-art]: #prior-art
+
+We already allow wildcard lifetimes, which have been beneficial. I'll defer to
+[RFC 2115](https://rust-lang.github.io/rfcs/2115-argument-lifetimes.html) for
+more information.
+
+Following the tradition of using macros to prototype language features, Alex
+Durka has built the [counted-array](https://crates.io/crates/counted-array)
+crate which contains a macro to enable the proposed syntax.
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+Bikeshedding the lint name