Mark move constructors as "noexcept"

I recently learned that move constructors generally should be marked
"noexcept".  This ensures that standard containers will move objects
when possible, rather than copy them.

This patch fixes the cases I could find.  Note that implicitly-defined
or defaulted move constructors will automatically do what you'd
expect; that is, they are noexcept if all the members have noexcept
move constructors.

While doing this, I noticed a couple of odd cases where the move
constructor seemed to assume that the object being constructed could
have state requiring destruction.  I've fixed these as well.  See
completion_result and scoped_mmap.

gdb/ChangeLog
2020-04-20  Tom Tromey  <tromey@adacore.com>

	* python/python.c (struct gdbpy_event): Mark move constructor as
	noexcept.
	* python/py-tui.c (class gdbpy_tui_window_maker): Mark move
	constructor as noexcept.
	* completer.h (struct completion_result): Mark move constructor as
	noexcept.
	* completer.c (completion_result::completion_result): Use
	initialization style.  Don't call reset_match_list.

gdbsupport/ChangeLog
2020-04-20  Tom Tromey  <tromey@adacore.com>

	* scoped_mmap.h (scoped_mmap): Mark move constructor as noexcept.
	Use initialization style.  Don't call destroy.
	* scoped_fd.h (class scoped_fd): Mark move constructor as
	noexcept.
	* gdb_ref_ptr.h (class ref_ptr): Mark move constructor as
	noexcept.

4 files changed, 14 insertions, 8 deletions

diff --git a/gdbsupport/ChangeLog b/gdbsupport/ChangeLog
index cd7033d..78fbbe6 100644
--- a/gdbsupport/ChangeLog
+++ b/gdbsupport/ChangeLog
@@ -1,3 +1,12 @@
+2020-04-20  Tom Tromey  <tromey@adacore.com>
+
+	* scoped_mmap.h (scoped_mmap): Mark move constructor as noexcept.
+	Use initialization style.  Don't call destroy.
+	* scoped_fd.h (class scoped_fd): Mark move constructor as
+	noexcept.
+	* gdb_ref_ptr.h (class ref_ptr): Mark move constructor as
+	noexcept.
+
 2020-04-13  Tom Tromey  <tom@tromey.com>
 
 	* event-loop.c: Move comment.  Remove obsolete 	comment.
diff --git a/gdbsupport/gdb_ref_ptr.h b/gdbsupport/gdb_ref_ptr.h
index c5ef13f..de387f5 100644
--- a/gdbsupport/gdb_ref_ptr.h
+++ b/gdbsupport/gdb_ref_ptr.h
@@ -78,7 +78,7 @@ class ref_ptr
   }
 
   /* Transfer ownership from OTHER.  */
-  ref_ptr (ref_ptr &&other)
+  ref_ptr (ref_ptr &&other) noexcept
     : m_obj (other.m_obj)
   {
     other.m_obj = NULL;
diff --git a/gdbsupport/scoped_fd.h b/gdbsupport/scoped_fd.h
index f40ce8b..ec654df 100644
--- a/gdbsupport/scoped_fd.h
+++ b/gdbsupport/scoped_fd.h
@@ -30,7 +30,7 @@ class scoped_fd
 public:
   explicit scoped_fd (int fd = -1) noexcept : m_fd (fd) {}
 
-  scoped_fd (scoped_fd &&other)
+  scoped_fd (scoped_fd &&other) noexcept
     : m_fd (other.m_fd)
   {
     other.m_fd = -1;
diff --git a/gdbsupport/scoped_mmap.h b/gdbsupport/scoped_mmap.h
index bab988f..9b74383 100644
--- a/gdbsupport/scoped_mmap.h
+++ b/gdbsupport/scoped_mmap.h
@@ -42,13 +42,10 @@ public:
     destroy ();
   }
 
-  scoped_mmap (scoped_mmap &&rhs)
+  scoped_mmap (scoped_mmap &&rhs) noexcept
+    : m_mem (rhs.m_mem),
+      m_length (rhs.m_length)
   {
-    destroy ();
-
-    m_mem = rhs.m_mem;
-    m_length = rhs.m_length;
-
     rhs.m_mem = MAP_FAILED;
     rhs.m_length = 0;
   }