Merge #387

387: Don't remove unreachable statements at HIR lowering r=philberty a=lrh2000

This is a legal code segment:
```rust
fn foo() -> i32 {
    return 1;
    let a = 1;
    a
}
```
And this is illegal:
```rust
fn foo() -> i32 {
    return 1;
    let a = 1.5;
    a
}
```
So we shouldn't remove unreachable statements at HIR lowering. We can issue warnings for them, but they need to be kept so that their types can be checked.

*(Original in #364

Co-authored-by: lrh2000 <[email protected]>

Author: bors[bot]

gcc/rust/ast/rust-stmt.h

@@ -38,6 +38,8 @@ class EmptyStmt : public Stmt
 
   EmptyStmt (Location locus) : locus (locus) {}
 
+  Location get_locus_slow () const final override { return get_locus (); }
+
   Location get_locus () const { return locus; }
 
   void accept_vis (ASTVisitor &vis) override;
@@ -135,6 +137,8 @@ class LetStmt : public Stmt
   LetStmt (LetStmt &&other) = default;
   LetStmt &operator= (LetStmt &&other) = default;
 
+  Location get_locus_slow () const final override { return get_locus (); }
+
   Location get_locus () const { return locus; }
 
   void accept_vis (ASTVisitor &vis) override;

gcc/rust/hir/rust-ast-lower.cc

@@ -71,26 +71,16 @@ ASTLoweringBlock::visit (AST::BlockExpr &expr)
   std::vector<std::unique_ptr<HIR::Stmt> > block_stmts;
   bool block_did_terminate = false;
   expr.iterate_stmts ([&] (AST::Stmt *s) mutable -> bool {
+    if (block_did_terminate)
+      rust_warning_at (s->get_locus_slow (), 0, "unreachable statement");
+
     bool terminated = false;
     auto translated_stmt = ASTLoweringStmt::translate (s, &terminated);
     block_stmts.push_back (std::unique_ptr<HIR::Stmt> (translated_stmt));
-    block_did_terminate = terminated;
-    return !block_did_terminate;
-  });
-
-  // if there was a return expression everything after that becomes
-  // unreachable code. This can be detected for any AST NodeIDs that have no
-  // associated HIR Mappings
-  expr.iterate_stmts ([&] (AST::Stmt *s) -> bool {
-    HirId ref;
-    if (!mappings->lookup_node_to_hir (mappings->get_current_crate (),
-				       s->get_node_id (), &ref))
-      rust_warning_at (s->get_locus_slow (), 0, "unreachable statement");
-
+    block_did_terminate |= terminated;
     return true;
   });
 
-  bool tail_reachable = !block_did_terminate;
   if (expr.has_tail_expr () && block_did_terminate)
     {
       // warning unreachable tail expressions
@@ -101,10 +91,13 @@ ASTLoweringBlock::visit (AST::BlockExpr &expr)
   HIR::ExprWithoutBlock *tail_expr = nullptr;
   if (expr.has_tail_expr ())
     {
-      tail_expr = (HIR::ExprWithoutBlock *) ASTLoweringExpr::translate (
-	expr.get_tail_expr ().get ());
+      bool terminated = false;
+      tail_expr = (HIR::ExprWithoutBlock *)
+	ASTLoweringExpr::translate (expr.get_tail_expr ().get (), &terminated);
+      block_did_terminate |= terminated;
     }
 
+  bool tail_reachable = !block_did_terminate;
   auto crate_num = mappings->get_current_crate ();
   Analysis::NodeMapping mapping (crate_num, expr.get_node_id (),
 				 mappings->get_next_hir_id (crate_num),

gcc/testsuite/rust.test/compile/deadcode2.rs

@@ -0,0 +1,10 @@
+fn foo() -> i32 {
+    return 1;
+
+    let a = -1; // { dg-warning "unreachable statement" }
+    a // { dg-warning "unreachable expression" }
+}
+
+fn main() {
+    foo();
+}

gcc/testsuite/rust.test/xfail_compile/deadcode_err1.rs

@@ -0,0 +1,11 @@
+fn foo() -> i32 {
+    return 1;
+
+    let mut a = 1; // { dg-warning "unreachable statement" }
+    a = 1.1; // { dg-warning "unreachable statement" }
+    // { dg-error "expected .<integer>. got .<float>." "" { target { *-*-* } } .-1 }
+}
+
+fn main() {
+    foo();
+}

gcc/testsuite/rust.test/xfail_compile/deadcode_err2.rs

@@ -0,0 +1,16 @@
+fn foo() -> i32 {
+    return 1;
+    return 1.5; // { dg-error "expected .i32. got .<float>." }
+    // { dg-warning "unreachable statement" "" { target *-*-* } .-1 } 
+}
+
+fn bar() -> i32 {
+    return 1.5; // { dg-error "expected .i32. got .<float>." }
+    return 1;
+    // { dg-warning "unreachable statement" "" { target *-*-* } .-1 } 
+}
+
+fn main() {
+    foo();
+    bar();
+}