Fix rust-lang#5488 - prevent shorthand init for tuple struct

Closes rust-lang#5488

Fix for rust-lang#5488. Before applying shorthand initialization for structs,
check if identifier is a literal (e.g. tuple struct). If yes, then do
not apply short hand initialization.

Added test case to validate the changes for the fix.

src/expr.rs

@@ -1716,12 +1716,19 @@ pub(crate) fn rewrite_field(
         let overhead = name.len() + separator.len();
         let expr_shape = shape.offset_left(overhead)?;
         let expr = field.expr.rewrite(context, expr_shape);
-
         match expr {
-            Some(ref e) if e.as_str() == name && context.config.use_field_init_shorthand() => {
-                Some(attrs_str + name)
+            Some(ref e) => {
+                let is_lit = if let ast::ExprKind::Lit(_) = (*field.expr).kind {
+                    true
+                } else {
+                    false
+                };
+                if !is_lit && e.as_str() == name && context.config.use_field_init_shorthand() {
+                    Some(attrs_str + name)
+                } else {
+                    Some(format!("{}{}{}{}", attrs_str, name, separator, e))
+                }
             }
-            Some(e) => Some(format!("{}{}{}{}", attrs_str, name, separator, e)),
             None => {
                 let expr_offset = shape.indent.block_indent(context.config);
                 let expr = field

tests/source/issue-5488.rs

@@ -0,0 +1,11 @@
+// rustfmt-use_field_init_shorthand: true
+fn main() {
+    // Since MyStruct is a tuple struct, it should not be shorthanded to
+    // MyStruct { 0 } even if use_field_init_shorthand is enabled.
+    let instance = MyStruct { 0: 0 };
+
+    // Since AnotherStruct is not a tuple struct, the shorthand should
+    // apply.
+    let a = 10;
+    let instance = AnotherStruct { a: a };
+}

tests/target/issue-5488.rs

@@ -0,0 +1,11 @@
+// rustfmt-use_field_init_shorthand: true
+fn main() {
+    // Since MyStruct is a tuple struct, it should not be shorthanded to
+    // MyStruct { 0 } even if use_field_init_shorthand is enabled.
+    let instance = MyStruct { 0: 0 };
+
+    // Since AnotherStruct is not a tuple struct, the shorthand should
+    // apply.
+    let a = 10;
+    let instance = AnotherStruct { a };
+}