Improve error message when a recursive dependency between fixtures is detected

Fix #2909

changelog/2909.bugfix.rst

@@ -0,0 +1 @@
+Improve error message when a recursive dependency between fixtures is detected.

src/_pytest/fixtures.py

@@ -762,14 +762,19 @@ class FixtureLookupError(LookupError):
 
         if msg is None:
             fm = self.request._fixturemanager
-            available = []
+            available = set()
             parentid = self.request._pyfuncitem.parent.nodeid
             for name, fixturedefs in fm._arg2fixturedefs.items():
                 faclist = list(fm._matchfactories(fixturedefs, parentid))
-                if faclist and name not in available:
-                    available.append(name)
-            msg = "fixture %r not found" % (self.argname,)
-            msg += "\n available fixtures: %s" % (", ".join(sorted(available)),)
+                if faclist:
+                    available.add(name)
+            if self.argname in available:
+                msg = " recursive dependency involving fixture '{}' detected".format(
+                    self.argname
+                )
+            else:
+                msg = "fixture '{}' not found".format(self.argname)
+            msg += "\n available fixtures: {}".format(", ".join(sorted(available)))
             msg += "\n use 'pytest --fixtures [testpath]' for help on them."
 
         return FixtureLookupErrorRepr(fspath, lineno, tblines, msg, self.argname)

testing/example_scripts/fixtures/fill_fixtures/test_detect_recursive_dependency_error.py

@@ -0,0 +1,15 @@
+import pytest
+
+
+@pytest.fixture
+def fix1(fix2):
+    return 1
+
+
+@pytest.fixture
+def fix2(fix1):
+    return 1
+
+
+def test(fix1):
+    pass

testing/python/fixture.py

@@ -60,6 +60,13 @@ class TestFillFixtures(object):
             """
         )
 
+    def test_detect_recursive_dependency_error(self, testdir):
+        testdir.copy_example()
+        result = testdir.runpytest()
+        result.stdout.fnmatch_lines(
+            ["*recursive dependency involving fixture 'fix1' detected*"]
+        )
+
     def test_funcarg_basic(self, testdir):
         testdir.copy_example()
         item = testdir.getitem(Path("test_funcarg_basic.py"))