Make let _ = e; have the same semantics as e;

The first case was getting treated like a variable binding, meaning that
if e had a destructor, it wouldn't run until the end of the enclosing scope.
To me it seems less confusing for let _ = e; and e; to work exactly the same
way, so now, the destructor for e runs immediately in both cases.

Author: catamorphism

src/rustc/middle/trans/base.rs

@@ -4206,9 +4206,26 @@ fn build_return(bcx: block) {
     Br(bcx, bcx.fcx.llreturn);
 }
 
+fn ignore_lhs(_bcx: block, local: @ast::local) -> bool {
+    match local.node.pat.node {
+        ast::pat_wild => true, _ => false
+    }
+}
+
 fn init_local(bcx: block, local: @ast::local) -> block {
     let _icx = bcx.insn_ctxt(~"init_local");
     let ty = node_id_type(bcx, local.node.id);
+
+    if ignore_lhs(bcx, local) {
+        // Handle let _ = e; just like e;
+        match local.node.init {
+            some(init) => {
+              return trans_expr(bcx, init.expr, ignore);
+            }
+            none => { return bcx; }
+        }
+    }
+
     let llptr = match bcx.fcx.lllocals.find(local.node.id) {
       some(local_mem(v)) => v,
       _ => { bcx.tcx().sess.span_bug(local.span,

src/test/run-pass/issue-2735-2.rs

@@ -0,0 +1,14 @@
+// This test should behave exactly like issue-2735-3
+class defer {
+    let b: &mut bool;
+    new(b: &mut bool) {
+        self.b = b;
+    }   
+    drop { *(self.b) = true; }
+}
+
+fn main() {
+    let mut dtor_ran = false;
+    let _  = defer(&mut dtor_ran);
+    assert(dtor_ran);
+}

src/test/run-pass/issue-2735-3.rs

@@ -0,0 +1,14 @@
+// This test should behave exactly like issue-2735-2
+class defer {
+    let b: &mut bool;
+    new(b: &mut bool) {
+        self.b = b;
+    }   
+    drop { *(self.b) = true; }
+}
+
+fn main() {
+    let mut dtor_ran = false;
+    defer(&mut dtor_ran);
+    assert(dtor_ran);
+}