Skip new and default

Helps with sourcefrog#25

.gitignore

@@ -2,3 +2,4 @@
 mutants.out
 mutants.out.old
 .cargo/config.toml
+wiki

NEWS.md

@@ -1,5 +1,11 @@
 # cargo-mutants changelog
 
+## Unreleased
+
+- Improved: Don't attempt to mutate functions called `new` or implementations of
+  `Default`. cargo-mutants can not yet generate good mutations for these so they
+  are generally false positives.
+
 ## 0.2.4
 
 Released 2022-03-26

src/visit.rs

@@ -43,7 +43,7 @@ impl DiscoveryVisitor {
     ) {
         self.in_namespace(&ident.to_string(), |v| {
             let function_name = Arc::new(v.namespace_stack.join("::"));
-            let return_type_str = Arc::new(format!("{}", return_type.to_token_stream()));
+            let return_type_str = Arc::new(return_type_to_string(return_type));
             for op in ops_for_return_type(return_type) {
                 v.mutants.push(Mutant::new(
                     v.source_file.clone(),
@@ -87,22 +87,36 @@ impl<'ast> Visit<'ast> for DiscoveryVisitor {
         if attrs_excluded(&i.attrs) {
             return;
         }
+        let type_name = type_name_string(&i.self_ty);
+        // The trait being implemented.
+        if let Some((_, trait_path, _)) = &i.trait_ {
+            let trait_name = &trait_path.segments.last().unwrap().ident;
+            if trait_name == "Default" {
+                // We don't know (yet) how to generate an interestingly-broken
+                // Default::default.
+                return;
+            }
+        }
         // Make an approximately-right namespace.
-        let name = type_name_string(&i.self_ty);
-        self.in_namespace(&name, |v| syn::visit::visit_item_impl(v, i));
+        // TODO: For `impl X for Y` get both X and Y onto the namespace
+        // stack so that we can show a more descriptive name.
+        self.in_namespace(&type_name, |v| syn::visit::visit_item_impl(v, i));
     }
 
     /// Visit `fn foo()` within an `impl`.
     fn visit_impl_item_method(&mut self, i: &'ast syn::ImplItemMethod) {
         if attrs_excluded(&i.attrs) {
             return;
+        } else if i.sig.ident == "new" {
+            return; // don't look inside constructors because there's often no good alternative
         }
         self.collect_fn_mutants(&i.sig.ident, &i.sig.output, &i.block.brace_token.span);
         self.in_namespace(&i.sig.ident.to_string(), |v| {
             syn::visit::visit_impl_item_method(v, i)
         });
     }
 
+    /// Visit `mod foo { ... }`.
     fn visit_item_mod(&mut self, node: &'ast syn::ItemMod) {
         if !attrs_excluded(&node.attrs) {
             self.in_namespace(&node.ident.to_string(), |v| {
@@ -154,6 +168,11 @@ fn type_name_string(ty: &syn::Type) -> String {
     }
 }
 
+fn return_type_to_string(return_type: &syn::ReturnType) -> String {
+    // TODO: Remove unnecessary spaces.
+    format!("{}", return_type.to_token_stream())
+}
+
 fn path_is_result(path: &syn::Path) -> bool {
     path.segments
         .last()

testdata/tree/well_tested/src/methods.rs

@@ -12,10 +12,9 @@ impl Foo {
     }
 }
 
-#[mutants::skip]
 impl Default for Foo {
     fn default() -> Self {
-        Foo { i: 1 }
+        Foo::new()
     }
 }
 
@@ -28,3 +27,15 @@ fn double() {
     foo.double();
     assert_eq!(foo.i, 128);
 }
+
+#[test]
+fn default() {
+    let foo = Foo::default();
+    assert_eq!(foo.i, 32);
+}
+
+#[test]
+fn new_foo() {
+    let foo = Foo::new();
+    assert_eq!(foo.i, 32);
+}

tests/snapshots/cli__list_mutants_json_well_tested.snap

@@ -1,7 +1,7 @@
 ---
 source: tests/cli.rs
+assertion_line: 41
 expression: "String::from_utf8_lossy(&output.stdout)"
-
 ---
 [
   {
@@ -11,13 +11,6 @@ expression: "String::from_utf8_lossy(&output.stdout)"
     "return_type": "-> & 'static str",
     "replacement": "Default::default()"
   },
-  {
-    "file": "src/methods.rs",
-    "line": 6,
-    "function": "Foo::new",
-    "return_type": "-> Foo",
-    "replacement": "Default::default()"
-  },
   {
     "file": "src/methods.rs",
     "line": 10,

tests/snapshots/cli__list_mutants_well_tested.snap

@@ -1,10 +1,9 @@
 ---
 source: tests/cli.rs
+assertion_line: 41
 expression: "String::from_utf8_lossy(&output.stdout)"
-
 ---
 src/inside_mod.rs:3: replace outer::inner::name -> & 'static str with Default::default()
-src/methods.rs:6: replace Foo::new -> Foo with Default::default()
 src/methods.rs:10: replace Foo::double with ()
 src/nested_function.rs:1: replace has_nested -> u32 with Default::default()
 src/nested_function.rs:2: replace has_nested::inner -> u32 with Default::default()

tests/snapshots/cli__well_tested_tree_check_only.snap

@@ -1,13 +1,13 @@
 ---
 source: tests/cli.rs
+assertion_line: 240
 expression: stdout
 ---
 Freshen source tree ... ok
 Copy source and build products to scratch directory ... done
 Unmutated baseline ... ok
-Found 13 mutants to test
+Found 12 mutants to test
 src/inside_mod.rs:3: replace outer::inner::name -> & 'static str with Default::default() ... check ok
-src/methods.rs:6: replace Foo::new -> Foo with Default::default() ... check ok
 src/methods.rs:10: replace Foo::double with () ... check ok
 src/nested_function.rs:1: replace has_nested -> u32 with Default::default() ... check ok
 src/nested_function.rs:2: replace has_nested::inner -> u32 with Default::default() ... check ok

tests/snapshots/cli__well_tested_tree_finds_no_problems.snap

@@ -1,13 +1,13 @@
 ---
 source: tests/cli.rs
+assertion_line: 221
 expression: stdout
 ---
 Freshen source tree ... ok
 Copy source and build products to scratch directory ... done
 Unmutated baseline ... ok
-Found 13 mutants to test
+Found 12 mutants to test
 src/inside_mod.rs:3: replace outer::inner::name -> & 'static str with Default::default() ... caught
-src/methods.rs:6: replace Foo::new -> Foo with Default::default() ... caught
 src/methods.rs:10: replace Foo::double with () ... caught
 src/nested_function.rs:1: replace has_nested -> u32 with Default::default() ... caught
 src/nested_function.rs:2: replace has_nested::inner -> u32 with Default::default() ... caught

tests/snapshots/cli__well_tested_tree_quiet.snap

@@ -1,9 +1,10 @@
 ---
 source: tests/cli.rs
+assertion_line: 199
 expression: stdout
 ---
 Freshen source tree ... ok
 Copy source and build products to scratch directory ... done
 Unmutated baseline ... ok
-Found 13 mutants to test
+Found 12 mutants to test