gccrs: empty match expressions should resolve to !

This is a special case in Rust and the ! type can unify with pretty much
anything its almost a inference variable and a unit-type for special cases.

Fixes #3231
Fixes #2567

gcc/rust/ChangeLog:

	* backend/rust-compile-expr.cc (check_match_scrutinee): check for empty match
	(CompileExpr::visit): fix assertion
	* checks/errors/rust-hir-pattern-analysis.cc (check_match_usefulness): check for empty
	* typecheck/rust-hir-type-check-expr.cc (TypeCheckExpr::visit): resolve to !

gcc/testsuite/ChangeLog:

	* rust/compile/exhaustiveness1.rs: remove bad check
	* rust/compile/issue-2567-1.rs: New test.
	* rust/compile/issue-2567-2.rs: New test.
	* rust/compile/issue-2567-3.rs: New test.
	* rust/compile/issue-3231.rs: New test.

Signed-off-by: Philip Herron <herron.philip@googlemail.com>

Author: philberty

gcc/rust/backend/rust-compile-expr.cc

@@ -1014,17 +1014,7 @@ check_match_scrutinee (HIR::MatchExpr &expr, Context *ctx)
 	       || scrutinee_kind == TyTy::TypeKind::TUPLE
 	       || scrutinee_kind == TyTy::TypeKind::REF);
 
-  if (scrutinee_kind == TyTy::TypeKind::ADT)
-    {
-      // this will need to change but for now the first pass implementation,
-      // lets assert this is the case
-      TyTy::ADTType *adt = static_cast<TyTy::ADTType *> (scrutinee_expr_tyty);
-      if (adt->is_enum ())
-	rust_assert (adt->number_of_variants () > 0);
-      else
-	rust_assert (adt->number_of_variants () == 1);
-    }
-  else if (scrutinee_kind == TyTy::TypeKind::FLOAT)
+  if (scrutinee_kind == TyTy::TypeKind::FLOAT)
     {
       // FIXME: CASE_LABEL_EXPR does not support floating point types.
       // Find another way to compile these.
@@ -1065,6 +1055,15 @@ CompileExpr::visit (HIR::MatchExpr &expr)
       return;
     }
 
+  // if the result of this expression is meant to be never type then we can
+  // optimise this away but there is the case where match arms resolve to !
+  // because of return statements we need to special case this
+  if (!expr.has_match_arms () && expr_tyty->is<TyTy::NeverType> ())
+    {
+      translated = unit_expression (expr.get_locus ());
+      return;
+    }
+
   fncontext fnctx = ctx->peek_fn ();
   Bvariable *tmp = NULL;
   tree enclosing_scope = ctx->peek_enclosing_scope ();

gcc/rust/checks/errors/rust-hir-pattern-analysis.cc

@@ -1530,6 +1530,9 @@ void
 check_match_usefulness (Resolver::TypeCheckContext *ctx,
 			TyTy::BaseType *scrutinee_ty, HIR::MatchExpr &expr)
 {
+  if (!expr.has_match_arms ())
+    return;
+
   // Lower the arms to a more convenient representation.
   std::vector<MatrixRow> rows;
   for (auto &arm : expr.get_match_cases ())

gcc/rust/typecheck/rust-hir-type-check-expr.cc

@@ -1456,6 +1456,18 @@ TypeCheckExpr::visit (HIR::MatchExpr &expr)
   TyTy::BaseType *scrutinee_tyty
     = TypeCheckExpr::Resolve (expr.get_scrutinee_expr ());
 
+  // https://github.com/Rust-GCC/gccrs/issues/3231#issuecomment-2462660048
+  // https://github.com/rust-lang/rust/blob/3d1dba830a564d1118361345d7ada47a05241f45/compiler/rustc_hir_typeck/src/_match.rs#L32-L36
+  if (!expr.has_match_arms ())
+    {
+      // this is a special case where rustc returns !
+      TyTy::BaseType *lookup = nullptr;
+      bool ok = context->lookup_builtin ("!", &lookup);
+      rust_assert (ok);
+      infered = lookup->clone ();
+      return;
+    }
+
   bool saw_error = false;
   std::vector<TyTy::BaseType *> kase_block_tys;
   for (auto &kase : expr.get_match_cases ())

gcc/testsuite/rust/compile/exhaustiveness1.rs

@@ -15,9 +15,7 @@ fn s2(s: S) {
 }
 
 fn s3(s: S) {
-    match s {
-        // { dg-error "non-exhaustive patterns: '_' not covered" "" { target *-*-* } .-1 }
-    }
+    match s {}
 }
 
 enum E {

gcc/testsuite/rust/compile/issue-2567-1.rs

@@ -0,0 +1,8 @@
+// { dg-options "-w" }
+enum Empty {}
+
+fn foo(x: Empty) {
+    let x: Empty = match x {
+        // empty
+    };
+}

gcc/testsuite/rust/compile/issue-2567-2.rs

@@ -0,0 +1,8 @@
+// { dg-options "-w" }
+enum Empty {}
+
+fn foo(x: Empty) {
+    let x: i32 = match x {
+        // empty
+    };
+}

gcc/testsuite/rust/compile/issue-2567-3.rs

@@ -0,0 +1,8 @@
+// { dg-options "-w" }
+enum Empty {}
+
+fn foo(x: Empty) {
+    match x {
+        // empty
+    }
+}

gcc/testsuite/rust/compile/issue-3231.rs

@@ -0,0 +1,8 @@
+// { dg-options "-w" }
+pub enum X {}
+
+pub fn foo(x: X) {
+    let _a: i32 = match x {};
+}
+
+pub fn main() {}