Improve error diagnostic for exception-spec mismatch (#5547)

It seems like the most common Cython 3 snag is the change in
function exception behaviour. This PR adds some extra diagnostics
to help people navigate this change, and a general mechanism
for adding this kind of diagnostics.

It needs some tests, but I wasn't to see what tests fail
to tell me where to put them.

Cython/Compiler/ExprNodes.py

@@ -1077,7 +1077,11 @@ def coerce_to(self, dst_type, env):
         return src
 
     def fail_assignment(self, dst_type):
-        error(self.pos, "Cannot assign type '%s' to '%s'" % (self.type, dst_type))
+        extra_diagnostics = dst_type.assignment_failure_extra_info(self.type)
+        if extra_diagnostics:
+            extra_diagnostics = ". " + extra_diagnostics
+        error(self.pos, "Cannot assign type '%s' to '%s'%s" % (
+            self.type, dst_type, extra_diagnostics))
 
     def check_for_coercion_error(self, dst_type, env, fail=False, default=None):
         if fail and not default:

Cython/Compiler/PyrexTypes.py

@@ -318,6 +318,11 @@ def assignable_from(self, src_type):
     def assignable_from_resolved_type(self, src_type):
         return self.same_as(src_type)
 
+    def assignment_failure_extra_info(self, src_type):
+        """Override if you can useful provide extra
+        information about why an assignment didn't work."""
+        return ""
+
     def as_argument_type(self):
         return self
 
@@ -2860,6 +2865,22 @@ def assignable_from_resolved_type(self, other_type):
             return self.base_type.is_void or self.base_type.same_as(other_type.base_type)
         return 0
 
+    def assignment_failure_extra_info(self, src_type):
+        if self.base_type.is_cfunction and src_type.is_ptr:
+            src_type = src_type.base_type.resolve()
+        if self.base_type.is_cfunction and src_type.is_cfunction:
+            copied_src_type = copy.copy(src_type)
+            # make the exception values the same as us
+            copied_src_type.exception_check = self.base_type.exception_check
+            copied_src_type.exception_value = self.base_type.exception_value
+            if self.base_type.pointer_assignable_from_resolved_type(copied_src_type):
+                # the only reason we can't assign is because of exception incompatibility
+                msg = "Exception values are incompatible."
+                if not self.base_type.exception_check and not self.base_type.exception_value:
+                    msg += " Suggest adding 'noexcept' to type '{}'.".format(src_type)
+                return msg
+        return super(CPtrType, self).assignment_failure_extra_info(src_type)
+
     def specialize(self, values):
         base_type = self.base_type.specialize(values)
         if base_type == self.base_type:

tests/errors/cfuncptr.pyx

@@ -41,14 +41,14 @@ def fail_struct_pointer():
 
 
 _ERRORS = """
-13:13: Cannot assign type 'int (int) except? -2' to 'int (*)(int) except -2'
-14:13: Cannot assign type 'int (int) except? -2' to 'int (*)(int) except -1'
-15:13: Cannot assign type 'int (int) except? -2' to 'int (*)(int) except? -1'
-29:13: Cannot assign type 'int (int) except *' to 'int (*)(int) noexcept'
-30:13: Cannot assign type 'int (int) except *' to 'int (*)(int) except -1'
-31:13: Cannot assign type 'int (int) except *' to 'int (*)(int) except? -1'
-40:32: Cannot assign type 'int (*)(int) except? -1 nogil' to 'int (*)(int) noexcept nogil'
-40:32: Cannot assign type 'int (*)(int) except? -1 nogil' to 'int (*)(int) noexcept nogil'
-40:37: Cannot assign type 'void (*)(int) except * nogil' to 'void (*)(int) noexcept nogil'
-40:37: Cannot assign type 'void (*)(int) except * nogil' to 'void (*)(int) noexcept nogil'
+13:13: Cannot assign type 'int (int) except? -2' to 'int (*)(int) except -2'. Exception values are incompatible.
+14:13: Cannot assign type 'int (int) except? -2' to 'int (*)(int) except -1'. Exception values are incompatible.
+15:13: Cannot assign type 'int (int) except? -2' to 'int (*)(int) except? -1'. Exception values are incompatible.
+29:13: Cannot assign type 'int (int) except *' to 'int (*)(int) noexcept'. Exception values are incompatible. Suggest adding 'noexcept' to type 'int (int) except *'.
+30:13: Cannot assign type 'int (int) except *' to 'int (*)(int) except -1'. Exception values are incompatible.
+31:13: Cannot assign type 'int (int) except *' to 'int (*)(int) except? -1'. Exception values are incompatible.
+40:32: Cannot assign type 'int (*)(int) except? -1 nogil' to 'int (*)(int) noexcept nogil'. Exception values are incompatible. Suggest adding 'noexcept' to type 'int (int) except? -1 nogil'.
+40:32: Cannot assign type 'int (*)(int) except? -1 nogil' to 'int (*)(int) noexcept nogil'. Exception values are incompatible. Suggest adding 'noexcept' to type 'int (int) except? -1 nogil'.
+40:37: Cannot assign type 'void (*)(int) except * nogil' to 'void (*)(int) noexcept nogil'. Exception values are incompatible. Suggest adding 'noexcept' to type 'void (int) except * nogil'.
+40:37: Cannot assign type 'void (*)(int) except * nogil' to 'void (*)(int) noexcept nogil'. Exception values are incompatible. Suggest adding 'noexcept' to type 'void (int) except * nogil'.
 """

tests/errors/e_excvalfunctype.pyx

@@ -11,6 +11,6 @@ spam = grail # type mismatch
 
 
 _ERRORS = u"""
-9:8: Cannot assign type 'spamfunc' to 'grailfunc'
-10:7: Cannot assign type 'grailfunc' to 'spamfunc'
+9:8: Cannot assign type 'spamfunc' to 'grailfunc'. Exception values are incompatible. Suggest adding 'noexcept' to type 'int (int, char *) except 42'.
+10:7: Cannot assign type 'grailfunc' to 'spamfunc'. Exception values are incompatible.
 """