Merge pull request #3 from lcnr/generic-const-param-types

generic-const-param-types design doc

Author: lcnr

design-docs/generic-const-param-types.md

@@ -0,0 +1,36 @@
+# Generic const param types
+
+We want to support the types of const parameters
+to depend on other generic parameters.
+```rust
+fn foo<const LEN: usize, const ARR: [u8; LEN]>() -> [u8; LEN] {
+    ARR
+}
+```
+
+This is currently forbidden during name resolution.
+
+Probably the biggest blocker is type-checking const arguments
+for generic parameters. This currently uses [a hack][WithOptConstParam]
+to supply the `DefId` of the corresponding const parameter.
+
+Now, let's look at the following:
+```rust
+fn foo<const N: usize, const M: [u8; N]>() {}
+
+fn bar() {
+    foo::<3, { [0; 3] }>();
+}
+```
+Here the expected type of `{ [0; 3] }` should be `[u8; 3]`. With the
+current approach it is `[u8; N]` instead. To fix this we would have to
+supply the affected queries the expected type of the const argument itself
+or use a `(DefId, SubstsRef<'tcx>)` pair instead.
+
+Doing so isn't trivial because we have to worry about accidentially
+ending up with different types for the same const argument which would
+break stuff. 
+
+Potential dangers include partially resolved types and projections.
+
+[WithOptConstParam]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.WithOptConstParam.html