gdb/rust: Handle printing structures containing strings

When printing a rust structure that contains a string GDB can
currently fail to read the fields that define the string. This is
because GDB mistakenly treats a value that is the parent structure as
though it is the structure that defines the string, and then fails to
find the fields needed to extract a string.

The solution is to create a new value to represent the string field of
the parent value.

gdb/ChangeLog:

	* rust-lang.c (val_print_struct): Handle printing structures
	containing strings.

gdb/testsuite/ChangeLog:

	* gdb.rust/simple.exp: Add new test case.
	* gdb.rust/simple.rs (struct StringAtOffset): New struct.
	(main): Initialise an instance of the new struct.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2019-05-02  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	* rust-lang.c (val_print_struct): Handle printing structures
+	containing strings.
+
 2019-05-02  Tom Tromey  <tromey@adacore.com>
 
 	* valarith.c (_initialize_valarith): Remove.

gdb/rust-lang.c

@@ -378,6 +378,14 @@ val_print_struct (struct type *type, int embedded_offset,
 
   if (rust_slice_type_p (type) && strcmp (TYPE_NAME (type), "&str") == 0)
     {
+      /* If what we are printing here is actually a string within a
+	 structure then VAL will be the original parent value, while TYPE
+	 will be the type of the structure representing the string we want
+	 to print.
+	 However, RUST_VAL_PRINT_STR looks up the fields of the string
+	 inside VAL, assuming that VAL is the string.
+	 So, recreate VAL as a value representing just the string.  */
+      val = value_at_lazy (type, value_address (val) + embedded_offset);
       rust_val_print_str (stream, val, options);
       return;
     }

gdb/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2019-05-02  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	* gdb.rust/simple.exp: Add new test case.
+	* gdb.rust/simple.rs (struct StringAtOffset): New struct.
+	(main): Initialise an instance of the new struct.
+
 2019-05-01  Tom Tromey  <tromey@adacore.com>
 
 	* gdb.ada/packed_array_assign/aggregates.ads (Nested_Packed): New

gdb/testsuite/gdb.rust/simple.exp

@@ -239,6 +239,9 @@ gdb_test "print custom_some" \
     " = simple::NonZeroOptimized::Value\\(\[a-z\]+::string::String .*"
 gdb_test "print custom_none" " = simple::NonZeroOptimized::Empty"
 
+gdb_test "print st" \
+    " = simple::StringAtOffset {field1: \"hello\", field2: 1, field3: \"world\"}"
+
 proc test_one_slice {svar length base range} {
     global hex
 

gdb/testsuite/gdb.rust/simple.rs

@@ -85,6 +85,12 @@ union Union {
     f2: u8,
 }
 
+struct StringAtOffset {
+    pub field1: &'static str,
+    pub field2: i32,
+    pub field3: &'static str,
+}
+
 // A simple structure whose layout won't be changed by the compiler,
 // so that ptype/o testing will work on any platform.
 struct SimpleLayout {
@@ -146,6 +152,8 @@ fn main () {
     let to1 = &w[..3];
     let to2 = &slice[..1];
 
+    let st = StringAtOffset { field1: "hello", field2: 1, field3: "world" };
+
     // tests for enum optimizations
 
     let str_some = Some("hi".to_string());