Auto merge of #43854 - estebank:missing-cond, r=nikomatsakis

Point out missing if conditional

On a case where an else conditional is missing, point this out
instead of the token immediately after the (incorrect) else block:

```
error: missing condition for `if` statemementt push fork -f

  --> $DIR/issue-13483.rs:16:5
   |
13 |    } else if {
   |             ^ expected if condition here
```

instead of

```
error: expected `{`, found `else`
  --> ../../src/test/ui/issue-13483.rs:14:7
   |
14 |     } else {
   |       ^^^^
```

Fix #13483.

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

@@ -2965,6 +2965,18 @@ impl<'a> Parser<'a> {
         }
         let lo = self.prev_span;
         let cond = self.parse_expr_res(RESTRICTION_NO_STRUCT_LITERAL, None)?;
+
+        // 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");
+            err.span_label(sp, "expected if condition here");
+            return Err(err)
+        }
         let thn = self.parse_block()?;
         let mut els: Option<P<Expr>> = None;
         let mut hi = thn.span;

src/test/ui/issue-13483.rs

@@ -0,0 +1,25 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+    if true {
+    } else if {
+    } else {
+    }
+}
+
+fn foo() {
+    if true {
+    } else if {
+    }
+    bar();
+}
+
+fn bar() {}

src/test/ui/issue-13483.stderr

@@ -0,0 +1,14 @@
+error: missing condition for `if` statemement
+  --> $DIR/issue-13483.rs:13:14
+   |
+13 |     } else if {
+   |              ^ expected if condition here
+
+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
+