Create 0000-unreachable-patterns-error.md

text/0000-unreachable-patterns-error.md

@@ -0,0 +1,62 @@
+- Feature Name: Make unreachable patterns error, not warning
+- Start Date: 2017-07-06
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+At now compiler accept such nonsense:
+
+```rust
+let x = 1;
+match x {
+  y => {}
+  z => {}
+  _ => {}
+}
+```
+
+`rustc` generate warnings, not errors about the same patterns,
+but this is 100% error of developer, so let's show this as error.
+
+# Motivation
+[motivation]: #motivation
+
+Novice in `rustc` can try to use `match` with variables,
+not literals, like this:
+
+```rust
+let x = calculate();
+let y = 2;
+let z = 3;
+match x {
+  y => {}
+  z => {}
+  _ => {}
+}
+```
+
+this code compiles and run and sometimes warning about unused
+variables (`y`, `z`) comes first before warning about unreachable pattern,
+what makes unclear what is going on.
+
+So to simplify for novice `match` usage, let's make unreachable pattern is error,
+not warning.
+
+Also not novice may made typo, for example miss `z if x == z` in some of "match arms",
+so error clear show typo.
+
+
+# How We Teach This
+[how-we-teach-this]: #how-we-teach-this
+
+I suppose that this behaviour expected from `match`.
+Compiler/language is strict enough to not allow miss some branch,
+so obviously expected that it not allow duplicate branches.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Some buggy code will be not accepted by compiler.
+