diff --git a/src/alejandra/tests/cases/apply/in b/src/alejandra/tests/cases/apply/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/apply/in
rename to src/alejandra/tests/cases/apply/in.nix
diff --git a/src/alejandra/tests/cases/apply/out b/src/alejandra/tests/cases/apply/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/apply/out
rename to src/alejandra/tests/cases/apply/out.nix
diff --git a/src/alejandra/tests/cases/assert/in b/src/alejandra/tests/cases/assert/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/assert/in
rename to src/alejandra/tests/cases/assert/in.nix
diff --git a/src/alejandra/tests/cases/assert/out b/src/alejandra/tests/cases/assert/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/assert/out
rename to src/alejandra/tests/cases/assert/out.nix
diff --git a/src/alejandra/tests/cases/attr_set/in b/src/alejandra/tests/cases/attr_set/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/attr_set/in
rename to src/alejandra/tests/cases/attr_set/in.nix
diff --git a/src/alejandra/tests/cases/attr_set/out b/src/alejandra/tests/cases/attr_set/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/attr_set/out
rename to src/alejandra/tests/cases/attr_set/out.nix
diff --git a/src/alejandra/tests/cases/bin_op/in b/src/alejandra/tests/cases/bin_op/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/bin_op/in
rename to src/alejandra/tests/cases/bin_op/in.nix
diff --git a/src/alejandra/tests/cases/bin_op/out b/src/alejandra/tests/cases/bin_op/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/bin_op/out
rename to src/alejandra/tests/cases/bin_op/out.nix
diff --git a/src/alejandra/tests/cases/comment/in b/src/alejandra/tests/cases/comment/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/comment/in
rename to src/alejandra/tests/cases/comment/in.nix
diff --git a/src/alejandra/tests/cases/comment/out b/src/alejandra/tests/cases/comment/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/comment/out
rename to src/alejandra/tests/cases/comment/out.nix
diff --git a/src/alejandra/tests/cases/dynamic/in b/src/alejandra/tests/cases/dynamic/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/dynamic/in
rename to src/alejandra/tests/cases/dynamic/in.nix
diff --git a/src/alejandra/tests/cases/dynamic/out b/src/alejandra/tests/cases/dynamic/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/dynamic/out
rename to src/alejandra/tests/cases/dynamic/out.nix
diff --git a/src/alejandra/tests/cases/error/in b/src/alejandra/tests/cases/error/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/error/in
rename to src/alejandra/tests/cases/error/in.nix
diff --git a/src/alejandra/tests/cases/error/out b/src/alejandra/tests/cases/error/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/error/out
rename to src/alejandra/tests/cases/error/out.nix
diff --git a/src/alejandra/tests/cases/idioms/in b/src/alejandra/tests/cases/idioms/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/idioms/in
rename to src/alejandra/tests/cases/idioms/in.nix
diff --git a/src/alejandra/tests/cases/idioms/out b/src/alejandra/tests/cases/idioms/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/idioms/out
rename to src/alejandra/tests/cases/idioms/out.nix
diff --git a/src/alejandra/tests/cases/idioms_lib_1/in b/src/alejandra/tests/cases/idioms_lib_1/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/idioms_lib_1/in
rename to src/alejandra/tests/cases/idioms_lib_1/in.nix
diff --git a/src/alejandra/tests/cases/idioms_lib_1/out b/src/alejandra/tests/cases/idioms_lib_1/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/idioms_lib_1/out
rename to src/alejandra/tests/cases/idioms_lib_1/out.nix
diff --git a/src/alejandra/tests/cases/idioms_pkgs_1/in b/src/alejandra/tests/cases/idioms_pkgs_1/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/idioms_pkgs_1/in
rename to src/alejandra/tests/cases/idioms_pkgs_1/in.nix
diff --git a/src/alejandra/tests/cases/idioms_pkgs_1/out b/src/alejandra/tests/cases/idioms_pkgs_1/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/idioms_pkgs_1/out
rename to src/alejandra/tests/cases/idioms_pkgs_1/out.nix
diff --git a/src/alejandra/tests/cases/if_else/in b/src/alejandra/tests/cases/if_else/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/if_else/in
rename to src/alejandra/tests/cases/if_else/in.nix
diff --git a/src/alejandra/tests/cases/if_else/out b/src/alejandra/tests/cases/if_else/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/if_else/out
rename to src/alejandra/tests/cases/if_else/out.nix
diff --git a/src/alejandra/tests/cases/inherit/in b/src/alejandra/tests/cases/inherit/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/inherit/in
rename to src/alejandra/tests/cases/inherit/in.nix
diff --git a/src/alejandra/tests/cases/inherit/out b/src/alejandra/tests/cases/inherit/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/inherit/out
rename to src/alejandra/tests/cases/inherit/out.nix
diff --git a/src/alejandra/tests/cases/inherit_blank_trailing/in b/src/alejandra/tests/cases/inherit_blank_trailing/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/inherit_blank_trailing/in
rename to src/alejandra/tests/cases/inherit_blank_trailing/in.nix
diff --git a/src/alejandra/tests/cases/inherit_blank_trailing/out b/src/alejandra/tests/cases/inherit_blank_trailing/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/inherit_blank_trailing/out
rename to src/alejandra/tests/cases/inherit_blank_trailing/out.nix
diff --git a/src/alejandra/tests/cases/inherit_comment/in b/src/alejandra/tests/cases/inherit_comment/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/inherit_comment/in
rename to src/alejandra/tests/cases/inherit_comment/in.nix
diff --git a/src/alejandra/tests/cases/inherit_comment/out b/src/alejandra/tests/cases/inherit_comment/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/inherit_comment/out
rename to src/alejandra/tests/cases/inherit_comment/out.nix
diff --git a/src/alejandra/tests/cases/inherit_from/in b/src/alejandra/tests/cases/inherit_from/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/inherit_from/in
rename to src/alejandra/tests/cases/inherit_from/in.nix
diff --git a/src/alejandra/tests/cases/inherit_from/out b/src/alejandra/tests/cases/inherit_from/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/inherit_from/out
rename to src/alejandra/tests/cases/inherit_from/out.nix
diff --git a/src/alejandra/tests/cases/key_value/in b/src/alejandra/tests/cases/key_value/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/key_value/in
rename to src/alejandra/tests/cases/key_value/in.nix
diff --git a/src/alejandra/tests/cases/key_value/out b/src/alejandra/tests/cases/key_value/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/key_value/out
rename to src/alejandra/tests/cases/key_value/out.nix
diff --git a/src/alejandra/tests/cases/lambda/in b/src/alejandra/tests/cases/lambda/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/lambda/in
rename to src/alejandra/tests/cases/lambda/in.nix
diff --git a/src/alejandra/tests/cases/lambda/out b/src/alejandra/tests/cases/lambda/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/lambda/out
rename to src/alejandra/tests/cases/lambda/out.nix
diff --git a/src/alejandra/tests/cases/let_in/in b/src/alejandra/tests/cases/let_in/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/let_in/in
rename to src/alejandra/tests/cases/let_in/in.nix
diff --git a/src/alejandra/tests/cases/let_in/out b/src/alejandra/tests/cases/let_in/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/let_in/out
rename to src/alejandra/tests/cases/let_in/out.nix
diff --git a/src/alejandra/tests/cases/lists/in b/src/alejandra/tests/cases/lists/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/lists/in
rename to src/alejandra/tests/cases/lists/in.nix
diff --git a/src/alejandra/tests/cases/lists/out b/src/alejandra/tests/cases/lists/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/lists/out
rename to src/alejandra/tests/cases/lists/out.nix
diff --git a/src/alejandra/tests/cases/or_default/in b/src/alejandra/tests/cases/or_default/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/or_default/in
rename to src/alejandra/tests/cases/or_default/in.nix
diff --git a/src/alejandra/tests/cases/or_default/out b/src/alejandra/tests/cases/or_default/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/or_default/out
rename to src/alejandra/tests/cases/or_default/out.nix
diff --git a/src/alejandra/tests/cases/paren/in b/src/alejandra/tests/cases/paren/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/paren/in
rename to src/alejandra/tests/cases/paren/in.nix
diff --git a/src/alejandra/tests/cases/paren/out b/src/alejandra/tests/cases/paren/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/paren/out
rename to src/alejandra/tests/cases/paren/out.nix
diff --git a/src/alejandra/tests/cases/pat_bind/in b/src/alejandra/tests/cases/pat_bind/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/pat_bind/in
rename to src/alejandra/tests/cases/pat_bind/in.nix
diff --git a/src/alejandra/tests/cases/pat_bind/out b/src/alejandra/tests/cases/pat_bind/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/pat_bind/out
rename to src/alejandra/tests/cases/pat_bind/out.nix
diff --git a/src/alejandra/tests/cases/pattern/in b/src/alejandra/tests/cases/pattern/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/pattern/in
rename to src/alejandra/tests/cases/pattern/in.nix
diff --git a/src/alejandra/tests/cases/pattern/out b/src/alejandra/tests/cases/pattern/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/pattern/out
rename to src/alejandra/tests/cases/pattern/out.nix
diff --git a/src/alejandra/tests/cases/root/in b/src/alejandra/tests/cases/root/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/root/in
rename to src/alejandra/tests/cases/root/in.nix
diff --git a/src/alejandra/tests/cases/root/out b/src/alejandra/tests/cases/root/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/root/out
rename to src/alejandra/tests/cases/root/out.nix
diff --git a/src/alejandra/tests/cases/select/in b/src/alejandra/tests/cases/select/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/select/in
rename to src/alejandra/tests/cases/select/in.nix
diff --git a/src/alejandra/tests/cases/select/out b/src/alejandra/tests/cases/select/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/select/out
rename to src/alejandra/tests/cases/select/out.nix
diff --git a/src/alejandra/tests/cases/string/in b/src/alejandra/tests/cases/string/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/string/in
rename to src/alejandra/tests/cases/string/in.nix
diff --git a/src/alejandra/tests/cases/string/out b/src/alejandra/tests/cases/string/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/string/out
rename to src/alejandra/tests/cases/string/out.nix
diff --git a/src/alejandra/tests/cases/string_interpol/in b/src/alejandra/tests/cases/string_interpol/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/string_interpol/in
rename to src/alejandra/tests/cases/string_interpol/in.nix
diff --git a/src/alejandra/tests/cases/string_interpol/out b/src/alejandra/tests/cases/string_interpol/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/string_interpol/out
rename to src/alejandra/tests/cases/string_interpol/out.nix
diff --git a/src/alejandra/tests/cases/with/in b/src/alejandra/tests/cases/with/in.nix
similarity index 100%
rename from src/alejandra/tests/cases/with/in
rename to src/alejandra/tests/cases/with/in.nix
diff --git a/src/alejandra/tests/cases/with/out b/src/alejandra/tests/cases/with/out.nix
similarity index 100%
rename from src/alejandra/tests/cases/with/out
rename to src/alejandra/tests/cases/with/out.nix
diff --git a/src/alejandra/tests/fmt.rs b/src/alejandra/tests/fmt.rs
index d1fbbd8..9248974 100644
--- a/src/alejandra/tests/fmt.rs
+++ b/src/alejandra/tests/fmt.rs
@@ -12,8 +12,8 @@ fn cases() {
             .collect();
 
     for case in cases {
-        let path_in = format!("tests/cases/{}/in", case);
-        let path_out = format!("tests/cases/{}/out", case);
+        let path_in = format!("tests/cases/{}/in.nix", case);
+        let path_out = format!("tests/cases/{}/out.nix", case);
         let content_in = std::fs::read_to_string(path_in.clone()).unwrap();
         let content_got =
             alejandra::format::in_memory(path_in, content_in.clone()).1;
diff --git a/treefmt.toml b/treefmt.toml
index 827c896..ed18ad2 100644
--- a/treefmt.toml
+++ b/treefmt.toml
@@ -3,6 +3,7 @@
 [formatter.nix]
 command = "cargo"
 includes = ["*.nix"]
+excludes = ["./src/alejandra/tests/cases/**"]
 options = ["run", "--"]
 
 [formatter.prettier]