Verify that an if condition block returns a value

src/libsyntax/ast.rs

@@ -844,6 +844,32 @@ pub struct Expr {
     pub attrs: ThinVec<Attribute>
 }
 
+impl Expr {
+    /// Wether this expression would be valid somewhere that expects a value, for example, an `if`
+    /// condition.
+    pub fn returns(&self) -> bool {
+        if let ExprKind::Block(ref block) = self.node {
+            match block.stmts.last().map(|last_stmt| &last_stmt.node) {
+                // implicit return
+                Some(&StmtKind::Expr(_)) => true,
+                Some(&StmtKind::Semi(ref expr)) => {
+                    if let ExprKind::Ret(_) = expr.node {
+                        // last statement is explicit return
+                        true
+                    } else {
+                        false
+                    }
+                }
+                // This is a block that doesn't end in either an implicit or explicit return
+                _ => false,
+            }
+        } else {
+            // This is not a block, it is a value
+            true
+        }
+    }
+}
+
 impl fmt::Debug for Expr {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         write!(f, "expr({}: {})", self.id, pprust::expr_to_string(self))

src/libsyntax/parse/parser.rs

@@ -2968,7 +2968,12 @@ impl<'a> Parser<'a> {
         }
         let lo = self.prev_span;
         let cond = self.parse_expr_res(RESTRICTION_NO_STRUCT_LITERAL, None)?;
-        if self.eat_keyword(keywords::Else) {
+
+        // Verify that the parsed `if` condition makes sense as a condition. If it is a block, then
+        // verify that the last statement is either an implicit return (no `;`) or an explicit
+        // return. This won't catch blocks with an explicit `return`, but that would be caught by
+        // the dead code lint.
+        if self.eat_keyword(keywords::Else) || !cond.returns() {
             let sp = lo.next_point();
             let mut err = self.diagnostic()
                 .struct_span_err(sp, "missing condition for `if` statemement");

src/test/ui/issue-13483.rs

@@ -10,7 +10,16 @@
 
 fn main() {
     if true {
-    } else if { //ERROR: MISSING CONDITIONAL
+    } else if {
     } else {
-    };
+    }
 }
+
+fn foo() {
+    if true {
+    } else if {
+    }
+    bar();
+}
+
+fn bar() {}

src/test/ui/issue-13483.stderr

@@ -1,8 +1,14 @@
 error: missing condition for `if` statemement
   --> $DIR/issue-13483.rs:13:14
    |
-13 |     } else if { //ERROR: MISSING CONDITIONAL
+13 |     } else if {
    |              ^ expected if condition here
 
-error: aborting due to previous error
+error: missing condition for `if` statemement
+  --> $DIR/issue-13483.rs:20:14
+   |
+20 |     } else if {
+   |              ^ expected if condition here
+
+error: aborting due to 2 previous errors