Add missing coercion rule from array to slice

Arrays are coercible into slices, this adds the missing type-resolution
the rule which works for now. The other part of this fix is described in
so that we reuse as much code as possible and handle complex coercion
sites.

Fixes #1129

Author: philberty

gcc/rust/typecheck/rust-tyty-coercion.h

@@ -886,6 +886,22 @@ class SliceCoercionRules : public BaseCoercionRules
 			      TyVar (base_resolved->get_ref ()));
   }
 
+  void visit (ArrayType &type) override
+  {
+    // check base type
+    auto base_resolved
+      = base->get_element_type ()->unify (type.get_element_type ());
+    if (base_resolved == nullptr)
+      {
+	BaseCoercionRules::visit (type);
+	return;
+      }
+
+    resolved = new SliceType (type.get_ref (), type.get_ty_ref (),
+			      type.get_ident ().locus,
+			      TyVar (base_resolved->get_ref ()));
+  }
+
 private:
   BaseType *get_base () override { return base; }
 

gcc/testsuite/rust/compile/issue-1129-1.rs

@@ -0,0 +1,4 @@
+// { dg-additional-options "-w" }
+fn write_u8(i: u8) {
+    let x: &[u8] = &[i];
+}

gcc/testsuite/rust/compile/issue-1129-2.rs

@@ -0,0 +1,22 @@
+// { dg-additional-options "-w" }
+pub trait Hasher {
+    fn finish(&self) -> u64;
+    fn write(&mut self, bytes: &[u8]);
+    fn write_u8(&mut self, i: u8) {
+        self.write(&[i])
+    }
+}
+
+struct SipHasher;
+
+impl Hasher for SipHasher {
+    #[inline]
+    fn write(&mut self, msg: &[u8]) {
+        loop {}
+    }
+
+    #[inline]
+    fn finish(&self) -> u64 {
+        0
+    }
+}