Add default_trait_access lint

clippy_lints/src/default_trait_access.rs

@@ -0,0 +1,63 @@
+use rustc::hir::*;
+use rustc::lint::*;
+
+use crate::utils::{match_def_path, opt_def_id, paths, span_lint_and_sugg};
+
+
+/// **What it does:** Checks for literal calls to `Default::default()`.
+///
+/// **Why is this bad?** It's more clear to the reader to use the name of the type whose default is
+/// being gotten than the generic `Default`.
+///
+/// **Known problems:** None.
+///
+/// **Example:**
+/// ```rust
+/// // Bad
+/// let s: String = Default::default();
+///
+/// // Good
+/// let s = String::default();
+/// ```
+declare_clippy_lint! {
+    pub DEFAULT_TRAIT_ACCESS,
+    style,
+    "checks for literal calls to Default::default()"
+}
+
+#[derive(Copy, Clone)]
+pub struct DefaultTraitAccess;
+
+impl LintPass for DefaultTraitAccess {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(DEFAULT_TRAIT_ACCESS)
+    }
+}
+
+impl<'a, 'tcx> LateLintPass<'a, 'tcx> for DefaultTraitAccess {
+    fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
+        if_chain! {
+            if let ExprCall(ref path, ..) = expr.node;
+            if let ExprPath(ref qpath) = path.node;
+            if let Some(def_id) = opt_def_id(cx.tables.qpath_def(qpath, path.hir_id));
+            if match_def_path(cx.tcx, def_id, &paths::DEFAULT_TRAIT_METHOD);
+            then {
+                match qpath {
+                    QPath::Resolved(..) => {
+                        // TODO: Work out a way to put "whatever the imported way of referencing
+                        // this type in this file" rather than a fully-qualified type.
+                        let replacement = format!("{}::default()", cx.tables.expr_ty(expr));
+                        span_lint_and_sugg(
+                            cx,
+                            DEFAULT_TRAIT_ACCESS,
+                            expr.span,
+                            &format!("Calling {} is more clear than this expression", replacement),
+                            "try",
+                            replacement);
+                    },
+                    QPath::TypeRelative(..) => {},
+                }
+            }
+         }
+    }
+}

clippy_lints/src/lib.rs

@@ -111,6 +111,7 @@ pub mod collapsible_if;
 pub mod const_static_lifetime;
 pub mod copies;
 pub mod cyclomatic_complexity;
+pub mod default_trait_access;
 pub mod derive;
 pub mod doc;
 pub mod double_comparison;
@@ -425,6 +426,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry) {
     reg.register_late_lint_pass(box neg_cmp_op_on_partial_ord::NoNegCompOpForPartialOrd);
     reg.register_late_lint_pass(box unwrap::Pass);
     reg.register_late_lint_pass(box duration_subsec::DurationSubsec);
+    reg.register_late_lint_pass(box default_trait_access::DefaultTraitAccess);
 
 
     reg.register_lint_group("clippy_restriction", vec![
@@ -512,6 +514,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry) {
         copies::IF_SAME_THEN_ELSE,
         copies::IFS_SAME_COND,
         cyclomatic_complexity::CYCLOMATIC_COMPLEXITY,
+        default_trait_access::DEFAULT_TRAIT_ACCESS,
         derive::DERIVE_HASH_XOR_EQ,
         double_comparison::DOUBLE_COMPARISONS,
         double_parens::DOUBLE_PARENS,
@@ -709,6 +712,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry) {
         block_in_if_condition::BLOCK_IN_IF_CONDITION_STMT,
         collapsible_if::COLLAPSIBLE_IF,
         const_static_lifetime::CONST_STATIC_LIFETIME,
+        default_trait_access::DEFAULT_TRAIT_ACCESS,
         enum_variants::ENUM_VARIANT_NAMES,
         enum_variants::MODULE_INCEPTION,
         eq_op::OP_REF,

clippy_lints/src/utils/paths.rs

@@ -24,6 +24,7 @@ pub const C_VOID: [&str; 4] = ["std", "os", "raw", "c_void"];
 pub const C_VOID_LIBC: [&str; 2] = ["libc", "c_void"];
 pub const DEBUG_FMT_METHOD: [&str; 4] = ["core", "fmt", "Debug", "fmt"];
 pub const DEFAULT_TRAIT: [&str; 3] = ["core", "default", "Default"];
+pub const DEFAULT_TRAIT_METHOD: [&str; 4] = ["core", "default", "Default", "default"];
 pub const DISPLAY_FMT_METHOD: [&str; 4] = ["core", "fmt", "Display", "fmt"];
 pub const DOUBLE_ENDED_ITERATOR: [&str; 4] = ["core", "iter", "traits", "DoubleEndedIterator"];
 pub const DROP: [&str; 3] = ["core", "mem", "drop"];

tests/ui/default_trait_access.rs

@@ -0,0 +1,39 @@
+#![warn(default_trait_access)]
+
+use std::default::Default as D2;
+use std::string;
+use std::default;
+
+fn main() {
+    let s1: String = Default::default();
+
+    let s2 = String::default();
+
+    let s3: String = D2::default();
+
+    let s4: String = std::default::Default::default();
+
+    let s5 = string::String::default();
+
+    let s6: String = default::Default::default();
+
+    let s7 = std::string::String::default();
+
+    let s8: String = DefaultFactory::make_t_badly();
+
+    let s9: String = DefaultFactory::make_t_nicely();
+
+    println!("[{}] [{}] [{}] [{}] [{}] [{}] [{}] [{}] [{}]", s1, s2, s3, s4, s5, s6, s7, s8, s9);
+}
+
+struct DefaultFactory;
+
+impl DefaultFactory {
+    pub fn make_t_badly<T: Default>() -> T {
+        Default::default()
+    }
+
+    pub fn make_t_nicely<T: Default>() -> T {
+        T::default()
+    }
+}

tests/ui/default_trait_access.stderr

@@ -0,0 +1,34 @@
+error: Calling std::string::String::default() is more clear than this expression
+ --> $DIR/default_trait_access.rs:8:22
+  |
+8 |     let s1: String = Default::default();
+  |                      ^^^^^^^^^^^^^^^^^^ help: try: `std::string::String::default()`
+  |
+  = note: `-D default-trait-access` implied by `-D warnings`
+
+error: Calling std::string::String::default() is more clear than this expression
+  --> $DIR/default_trait_access.rs:12:22
+   |
+12 |     let s3: String = D2::default();
+   |                      ^^^^^^^^^^^^^ help: try: `std::string::String::default()`
+
+error: Calling std::string::String::default() is more clear than this expression
+  --> $DIR/default_trait_access.rs:14:22
+   |
+14 |     let s4: String = std::default::Default::default();
+   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `std::string::String::default()`
+
+error: Calling std::string::String::default() is more clear than this expression
+  --> $DIR/default_trait_access.rs:18:22
+   |
+18 |     let s6: String = default::Default::default();
+   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `std::string::String::default()`
+
+error: Calling T::default() is more clear than this expression
+  --> $DIR/default_trait_access.rs:33:9
+   |
+33 |         Default::default()
+   |         ^^^^^^^^^^^^^^^^^^ help: try: `T::default()`
+
+error: aborting due to 5 previous errors
+