Explain how patterns are compiled

Co-authored-by: Oliver Scherer <[email protected]>

Author: RalfJung

patterns.md

@@ -13,8 +13,7 @@ fn is_foo(x: i32) -> bool {
 ```
 However, that check has some loopholes, so e.g. `&T` can be used in a pattern no matter the `T`.
 
-A const-pattern is compiled by [calling `PartialEq::eq`][compile-partial-eq] to compare the subject of the `match` with the constant,
-except for TODO where the constant is treated as if it was inlined as a pattern (and the usual `match` tree is constructed).
+Any reference type const-pattern is compiled by [calling `PartialEq::eq`][compile-partial-eq] to compare the subject of the `match` with the constant. Const-patterns with other types (enum, struct, tuple, array) are treated as if the constant was inlined as a pattern (and the usual `match` tree is constructed).
 
 [struct-eq]: https://github.com/rust-lang/rust/blob/2c28244cf0fc9868f55070e55b8f332d196eaf3f/src/librustc_mir_build/hair/pattern/const_to_pat.rs#L121
 [compile-partial-eq]: https://github.com/rust-lang/rust/blob/2c28244cf0fc9868f55070e55b8f332d196eaf3f/src/librustc_mir_build/build/matches/test.rs#L355