first draft, tests OK

src/expr.rs

@@ -1102,8 +1102,14 @@ impl<'a> Rewrite for ControlFlow<'a> {
         };
         let block_str = {
             let old_val = context.is_if_else_block.replace(self.else_block.is_some());
-            let result =
-                rewrite_block_with_visitor(context, "", self.block, None, None, block_shape, true);
+            let allow_single_line =
+                allow_single_line_if(&cond_str, self.block) && self.keyword == "if";
+
+            let result = if allow_single_line {
+                rewrite_block_inner(self.block, None, None, true, context, block_shape)
+            } else {
+                rewrite_block_with_visitor(context, "", self.block, None, None, block_shape, true)
+            };
             context.is_if_else_block.replace(old_val);
             result?
         };
@@ -1158,6 +1164,30 @@ impl<'a> Rewrite for ControlFlow<'a> {
     }
 }
 
+fn allow_single_line_if(result: &str, block: &ast::Block) -> bool {
+    if result.contains('\n') {
+        return false;
+    }
+
+    if block.stmts.len() == 0 {
+        return true;
+    }
+    if block.stmts.len() == 1 {
+        return is_simple_stmt(&block.stmts[0]);
+    }
+    false
+}
+
+fn is_simple_stmt(stmt: &ast::Stmt) -> bool {
+    match stmt.kind {
+        ast::StmtKind::Expr(ref expr) => match expr.kind {
+            ast::ExprKind::Ret(..) | ast::ExprKind::Continue(..) | ast::ExprKind::Break(..) => true,
+            _ => false,
+        },
+        _ => false,
+    }
+}
+
 fn rewrite_label(opt_label: Option<ast::Label>) -> Cow<'static, str> {
     match opt_label {
         Some(label) => Cow::from(format!("{}: ", label.ident)),

tests/source/expr.rs

@@ -79,7 +79,7 @@ fn bar() {
     let bar = 5 ;
     let nonsense = (10 .. 0)..(0..10);
 
-    loop{if true {break}}
+    loop{if true {break;}}
 
     let x = (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa &&
              aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa,

tests/source/single-line-if-else.rs

@@ -35,6 +35,22 @@ fn main() {
         do_something()
     }
 
+    let a = if x { 1 } else { 3 };
+
+    // if may be formatted on a single line if it is "short"
+    // and only contain a single expression
+    if true { return }
+
+    if true {
+        return
+    }
+
+    if true { return; }
+
+    if a { let y = 1; return y }
+
+    for i in 0..2 { if g == true { continue } }
+
     let x = if veeeeeeeeery_loooooong_condition() { aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa } else { bbbbbbbbbb };
 
     let x = if veeeeeeeeery_loooooong_condition()     {    aaaaaaaaaaaaaaaaaaaaaaaaa }   else  {

tests/target/single-line-if-else.rs

@@ -42,6 +42,27 @@ fn main() {
         do_something()
     }
 
+    let a = if x { 1 } else { 3 };
+
+    // if may be formatted on a single line if it is "short"
+    // and only contain a single expression
+    if true { return }
+
+    if true { return }
+
+    if true {
+        return;
+    }
+
+    if a {
+        let y = 1;
+        return y;
+    }
+
+    for i in 0..2 {
+        if g == true { continue }
+    }
+
     let x = if veeeeeeeeery_loooooong_condition() {
         aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
     } else {