Auto merge of #25570 - oli-obk:const_indexing, r=nikomatsakis

This PR allows the constant evaluation of index operations on constant arrays and repeat expressions. This allows index expressions to appear in the expression path of the length expression of a repeat expression or an array type.

An example is

```rust
const ARR: [usize; 5] = [1, 2, 3, 4, 5];
const ARR2: [usize; ARR[1]] = [42, 99];
```

In most other locations llvm's const evaluator figures it out already. This is not specific to index expressions and could be remedied in the future.

src/libsyntax/feature_gate.rs

@@ -179,6 +179,9 @@ const KNOWN_FEATURES: &'static [(&'static str, &'static str, Option<u32>, Status
     // Allows the definition of `const fn` functions.
     ("const_fn", "1.2.0", Some(24111), Active),
 
+    // Allows indexing into constant arrays.
+    ("const_indexing", "1.4.0", Some(29947), Active),
+
     // Allows using #[prelude_import] on glob `use` items.
     //
     // rustc internal
@@ -492,6 +495,7 @@ pub struct Features {
     /// #![feature] attrs for non-language (library) features
     pub declared_lib_features: Vec<(InternedString, Span)>,
     pub const_fn: bool,
+    pub const_indexing: bool,
     pub static_recursion: bool,
     pub default_type_parameter_fallback: bool,
     pub type_macros: bool,
@@ -524,6 +528,7 @@ impl Features {
             declared_stable_lang_features: Vec::new(),
             declared_lib_features: Vec::new(),
             const_fn: false,
+            const_indexing: false,
             static_recursion: false,
             default_type_parameter_fallback: false,
             type_macros: false,
@@ -1097,6 +1102,7 @@ fn check_crate_inner<F>(cm: &CodeMap, span_handler: &SpanHandler,
         declared_stable_lang_features: accepted_features,
         declared_lib_features: unknown_features,
         const_fn: cx.has_feature("const_fn"),
+        const_indexing: cx.has_feature("const_indexing"),
         static_recursion: cx.has_feature("static_recursion"),
         default_type_parameter_fallback: cx.has_feature("default_type_parameter_fallback"),
         type_macros: cx.has_feature("type_macros"),