Warn on misplaced requirement function calls

conans/client/conanfile/configure.py

@@ -7,6 +7,8 @@
 def run_configure_method(conanfile, down_options, profile_options, ref):
     """ Run all the config-related functions for the given conanfile object """
 
+    initial_requires_count = len(conanfile.requires)
+
     if hasattr(conanfile, "config_options"):
         with conanfile_exception_formatter(conanfile, "config_options"):
             conanfile.config_options()
@@ -23,6 +25,9 @@ def run_configure_method(conanfile, down_options, profile_options, ref):
     elif "auto_shared_fpic" in conanfile.implements:
         auto_shared_fpic_configure(conanfile)
 
+    if initial_requires_count != len(conanfile.requires):
+        conanfile.output.warning("Requirements should only be added in the requirements()/build_requirements() methods, not configure()/config_options(), which might raise errors in the future.", warn_tag="deprecated")
+
     result = conanfile.options.get_upstream_options(down_options, ref, is_consumer)
     self_options, up_options, private_up_options = result
     # self_options are the minimum to reproduce state, as defined from downstream (not profile)

conans/model/requires.py

@@ -574,3 +574,6 @@ def __repr__(self):
 
     def serialize(self):
         return [v.serialize() for v in self._requires.values()]
+
+    def __len__(self):
+        return len(self._requires)

conans/test/integration/build_requires/build_requires_test.py

@@ -581,3 +581,17 @@ def test_build_missing_build_requires():
     c.run("install app --build=missing")
     assert "- Build" not in c.out
     assert re.search(r"Skipped binaries(\s*)tool/0.1, tooldep/0.1", c.out)
+
+
+def test_requirement_in_wrong_method():
+    tc = TestClient(light=True)
+    tc.save({"conanfile.py": textwrap.dedent("""
+        from conan import ConanFile
+        class Pkg(ConanFile):
+            name = "pkg"
+            version = "0.1"
+            def configure(self):
+                self.requires("foo/1.0")
+        """)})
+    tc.run('create . -cc="core:warnings_as_errors=[\'*\']"', assert_error=True)
+    assert "ERROR: deprecated: Requirements should only be added in the requirements()/build_requirements() methods, not configure()/config_options(), which might raise errors in the future." in tc.out