Backport "noexcept" function modifier to Cython 0.29.x (GH-4903)

As a no-op, but it parses fine.

Also add some basic compile tests, and some brief documentation.

Cython/Compiler/Parsing.py

@@ -2820,6 +2820,8 @@ def p_c_func_declarator(s, pos, ctx, base, cmethod_flag):
     s.expect(')')
     nogil = p_nogil(s)
     exc_val, exc_check = p_exception_value_clause(s)
+    # TODO - warning to enforce preferred exception specification order
+    nogil = nogil or p_nogil(s)
     with_gil = p_with_gil(s)
     return Nodes.CFuncDeclaratorNode(pos,
         base = base, args = args, has_varargs = ellipsis,
@@ -2938,7 +2940,11 @@ def p_with_gil(s):
 def p_exception_value_clause(s):
     exc_val = None
     exc_check = 0
-    if s.sy == 'except':
+
+    if s.sy == 'IDENT' and s.systring == 'noexcept':
+        s.next()
+        exc_check = False  # No-op in Cython 0.29.x
+    elif s.sy == 'except':
         s.next()
         if s.sy == '*':
             exc_check = 1

docs/src/userguide/language_basics.rst

@@ -414,6 +414,17 @@ use this form, since there isn't any error return value to test.
 Otherwise, an explicit error return value allows the C compiler to generate
 more efficient code and is thus generally preferable.
 
+To explicitly mark a function as not returning an exception use
+``noexcept``.
+
+    cdef int spam() noexcept:
+        ...
+
+This is worth doing because (a) "explicit is better than implicit", and
+(b) the default behaviour for ``cdef`` functions will change in Cython 3.0
+so that functions will propagate exceptions by default. Therefore, it is
+best to mark them now if you want them to swallow exceptions in the future.
+
 An external C++ function that may raise an exception can be declared with::
 
     cdef int spam() except +

tests/compile/excvaldecl.pyx

@@ -18,9 +18,17 @@ cdef int brian() except? 0:
 cdef int silly() except -1:
     pass
 
+cdef int not_so_silly() noexcept:
+    pass
+
+cdef int not_so_silly_and_gilless() noexcept nogil:
+    pass
+
 spam()
 eggs()
 grail()
 tomato()
 brian()
 silly()
+not_so_silly()
+not_so_silly_and_gilless()