Support bare-identifier field initializers in Rust

In Rust one can initialize a struct member from an identically-named
local variable by simply mentioning the member name in the
initializer, like:

    let x = 0;
    let y = Struct { x };

This initializes "Struct::x" from "x".

This patch adds this form of initializer to the Rust expression parser
and adds a test.

Tested on x86-64 Fedora 26 using rustc 1.23.

2018-03-19  Tom Tromey  <tom@tromey.com>

	* rust-exp.y (struct_expr_tail, struct_expr_list): Add plain
	"IDENT" production.

2018-03-19  Tom Tromey  <tom@tromey.com>

	* gdb.rust/simple.rs (main): Add local variables field1, field2,
	y0.
	* gdb.rust/simple.exp: Test bare identifier form of struct
	initializer.

2 files changed, 7 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.rust/simple.exp b/gdb/testsuite/gdb.rust/simple.exp
index 230e6a7..2db596b 100644
--- a/gdb/testsuite/gdb.rust/simple.exp
+++ b/gdb/testsuite/gdb.rust/simple.exp
@@ -166,6 +166,9 @@ gdb_test "print simple::HiBob + 5" \
 gdb_test "print nosuchsymbol" \
     "No symbol 'nosuchsymbol' in current context"
 
+gdb_test "print simple::HiBob{field1, field2}" \
+    " = simple::HiBob \\{field1: 77, field2: 88\\}"
+
 gdb_test "print e" " = simple::MoreComplicated::Two\\(73\\)"
 gdb_test "print e2" \
     " = simple::MoreComplicated::Four\\{this: true, is: 8, a: 109 'm', struct_: 100, variant: 10\\}"
diff --git a/gdb/testsuite/gdb.rust/simple.rs b/gdb/testsuite/gdb.rust/simple.rs
index 7e43cd8..b2b5dfe 100644
--- a/gdb/testsuite/gdb.rust/simple.rs
+++ b/gdb/testsuite/gdb.rust/simple.rs
@@ -112,6 +112,10 @@ fn main () {
     let y = HiBob {field1: 7, field2: 8};
     let z = ByeBob(7, 8);
 
+    let field1 = 77;
+    let field2 = 88;
+    let y0 = HiBob { field1, field2 };
+
     let univariant = Univariant::Foo {a : 1};
     let univariant_anon = UnivariantAnon::Foo(1);