new lint trivial_default_constructed_types

rust-lang · Jun 18, 2023 · fa1c3c8 · fa1c3c8
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diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -5226,6 +5226,7 @@ Released 2018-09-13
 [`transmutes_expressible_as_ptr_casts`]: https://rust-lang.github.io/rust-clippy/master/index.html#transmutes_expressible_as_ptr_casts
 [`transmuting_null`]: https://rust-lang.github.io/rust-clippy/master/index.html#transmuting_null
 [`trim_split_whitespace`]: https://rust-lang.github.io/rust-clippy/master/index.html#trim_split_whitespace
+[`trivial_default_constructed_types`]: https://rust-lang.github.io/rust-clippy/master/index.html#trivial_default_constructed_types
 [`trivial_regex`]: https://rust-lang.github.io/rust-clippy/master/index.html#trivial_regex
 [`trivially_copy_pass_by_ref`]: https://rust-lang.github.io/rust-clippy/master/index.html#trivially_copy_pass_by_ref
 [`try_err`]: https://rust-lang.github.io/rust-clippy/master/index.html#try_err

diff --git a/clippy_lints/src/declared_lints.rs b/clippy_lints/src/declared_lints.rs
@@ -620,6 +620,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::transmute::UNSOUND_COLLECTION_TRANSMUTE_INFO,
     crate::transmute::USELESS_TRANSMUTE_INFO,
     crate::transmute::WRONG_TRANSMUTE_INFO,
+    crate::trivial_default_constructed_types::TRIVIAL_DEFAULT_CONSTRUCTED_TYPES_INFO,
     crate::types::BORROWED_BOX_INFO,
     crate::types::BOX_COLLECTION_INFO,
     crate::types::LINKEDLIST_INFO,

diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
@@ -309,6 +309,7 @@ mod to_digit_is_some;
 mod trailing_empty_array;
 mod trait_bounds;
 mod transmute;
+mod trivial_default_constructed_types;
 mod types;
 mod undocumented_unsafe_blocks;
 mod unicode;
@@ -1055,6 +1056,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(move |_| Box::new(large_stack_frames::LargeStackFrames::new(stack_size_threshold)));
     store.register_late_pass(|_| Box::new(single_range_in_vec_init::SingleRangeInVecInit));
     store.register_late_pass(|_| Box::new(incorrect_impls::IncorrectImpls));
+    store.register_late_pass(|_| Box::new(trivial_default_constructed_types::TrivialDefaultConstructedTypes));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 

diff --git a/clippy_lints/src/trivial_default_constructed_types.rs b/clippy_lints/src/trivial_default_constructed_types.rs
@@ -0,0 +1,120 @@
+use clippy_utils::{diagnostics::span_lint_and_sugg, is_from_proc_macro, is_lang_item_or_ctor, last_path_segment};
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind, LangItem, QPath};
+use rustc_lint::{LateContext, LateLintPass, LintContext};
+use rustc_middle::{
+    lint::in_external_macro,
+    ty::{self, Ty},
+};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::symbol::kw;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for types constructed by `default` that really don't need to be.
+    ///
+    /// ### Why is this bad?
+    /// It's harder for the reader to know what the value is, and it's an unnecessary function call.
+    ///
+    /// ### Example
+    /// ```rust,ignore
+    /// let a = A(Option::default());
+    /// ```
+    /// Use instead:
+    /// ```rust,ignore
+    /// let a = A(None);
+    /// ```
+    #[clippy::version = "1.72.0"]
+    pub TRIVIAL_DEFAULT_CONSTRUCTED_TYPES,
+    pedantic,
+    "checks for usage of `Default::default` to construct trivial types"
+}
+declare_lint_pass!(TrivialDefaultConstructedTypes => [TRIVIAL_DEFAULT_CONSTRUCTED_TYPES]);
+
+impl<'tcx> LateLintPass<'tcx> for TrivialDefaultConstructedTypes {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &Expr<'tcx>) {
+        if !in_external_macro(cx.sess(), expr.span)
+            && let ExprKind::Call(call, _) = expr.kind
+            && let ExprKind::Path(qpath) = call.kind
+            // `last_path_segment` ICEs if we give it a `LangItem`.
+            && !matches!(qpath, QPath::LangItem(..))
+            && last_path_segment(&qpath).ident.name == kw::Default
+        {
+            let ret_ty = cx
+                .typeck_results()
+                .expr_ty(call)
+                .fn_sig(cx.tcx)
+                .output()
+                .skip_binder()
+                .peel_refs();
+            if let Some(default) = default_value(cx, ret_ty) && !is_from_proc_macro(cx, expr) {
+                span_lint_and_sugg(
+                    cx,
+                    TRIVIAL_DEFAULT_CONSTRUCTED_TYPES,
+                    expr.span,
+                    "constructing a trivial type using `default`",
+                    "try",
+                    default.to_string(),
+                    Applicability::MachineApplicable,
+                );
+            } else if let ty::Tuple(fields) = ret_ty.kind()
+                && let Some(fields_default) = fields.iter()
+                    .map(|field| default_value(cx, field))
+                    .collect::<Option<Vec<&'static str>>>()
+                && !is_from_proc_macro(cx, expr)
+            {
+                let default = if fields.len() == 1 {
+                    // Needs trailing comma to be a single-element tuple
+                    fields_default[0].to_owned() + ","
+                } else {
+                    fields_default.join(", ")
+                };
+
+                span_lint_and_sugg(
+                    cx,
+                    TRIVIAL_DEFAULT_CONSTRUCTED_TYPES,
+                    expr.span,
+                    &format!(
+                        "constructing a {} using `default`",
+                        if fields.is_empty() { "unit" } else { "trivial tuple" },
+                    ),
+                    "try",
+                    format!("({default})"),
+                    Applicability::MachineApplicable,
+                );
+            } else if let ty::Array(ty, len) = ret_ty.kind()
+                && let Some(default) = default_value(cx, *ty)
+                && !is_from_proc_macro(cx, expr)
+            {
+                span_lint_and_sugg(
+                    cx,
+                    TRIVIAL_DEFAULT_CONSTRUCTED_TYPES,
+                    expr.span,
+                    "constructing a trivial array using `default`",
+                    "try",
+                    format!("[{default}; {len}]"),
+                    Applicability::MachineApplicable,
+                );
+            }
+        }
+    }
+}
+
+/// Gets the default value of `ty`.
+fn default_value(cx: &LateContext<'_>, ty: Ty<'_>) -> Option<&'static str> {
+    match ty.kind() {
+        ty::Adt(def, _) => {
+            if is_lang_item_or_ctor(cx, def.did(), LangItem::Option) {
+                return Some("None");
+            }
+
+            None
+        },
+        ty::Bool => Some("false"),
+        ty::Str => Some(r#""""#),
+        ty::Int(_) | ty::Uint(_) => Some("0"),
+        ty::Float(_) => Some("0.0"),
+        // Do not handle `ty::Char`, it's a lot less readable
+        _ => None,
+    }
+}
diff --git a/tests/ui/trivial_default_constructed_types.fixed b/tests/ui/trivial_default_constructed_types.fixed
@@ -0,0 +1,44 @@
+//@run-rustfix
+//@aux-build:proc_macros.rs
+#![allow(clippy::no_effect, unused)]
+#![warn(clippy::trivial_default_constructed_types)]
+
+#[macro_use]
+extern crate proc_macros;
+
+fn main() {
+    0;
+    let x: Option<u32> = None;
+    let y: (usize,) = (0,);
+    ();
+    let x: [u32; 10] = [0; 10];
+    let x: [f32; 1000] = [0.0; 1000];
+    let x = "";
+    let x = false;
+    // Do not lint
+    let x = char::default();
+
+    external! {
+        u32::default();
+        let x: Option<u32> = Option::default();
+        let y: (usize,) = Default::default();
+        <()>::default();
+        let x: [u32; 10] = Default::default();
+        let x: [f32; 1000] = [Default::default(); 1000];
+        let x = <&str>::default();
+        let x = bool::default();
+        let x = char::default();
+    }
+    with_span! {
+        span
+        u32::default();
+        let x: Option<u32> = Option::default();
+        let y: (usize,) = Default::default();
+        <()>::default();
+        let x: [u32; 10] = Default::default();
+        let x: [f32; 1000] = [Default::default(); 1000];
+        let x = <&str>::default();
+        let x = bool::default();
+        let x = char::default();
+    }
+}
diff --git a/tests/ui/trivial_default_constructed_types.rs b/tests/ui/trivial_default_constructed_types.rs
@@ -0,0 +1,44 @@
+//@run-rustfix
+//@aux-build:proc_macros.rs
+#![allow(clippy::no_effect, unused)]
+#![warn(clippy::trivial_default_constructed_types)]
+
+#[macro_use]
+extern crate proc_macros;
+
+fn main() {
+    u32::default();
+    let x: Option<u32> = Option::default();
+    let y: (usize,) = Default::default();
+    <()>::default();
+    let x: [u32; 10] = Default::default();
+    let x: [f32; 1000] = [Default::default(); 1000];
+    let x = <&str>::default();
+    let x = bool::default();
+    // Do not lint
+    let x = char::default();
+
+    external! {
+        u32::default();
+        let x: Option<u32> = Option::default();
+        let y: (usize,) = Default::default();
+        <()>::default();
+        let x: [u32; 10] = Default::default();
+        let x: [f32; 1000] = [Default::default(); 1000];
+        let x = <&str>::default();
+        let x = bool::default();
+        let x = char::default();
+    }
+    with_span! {
+        span
+        u32::default();
+        let x: Option<u32> = Option::default();
+        let y: (usize,) = Default::default();
+        <()>::default();
+        let x: [u32; 10] = Default::default();
+        let x: [f32; 1000] = [Default::default(); 1000];
+        let x = <&str>::default();
+        let x = bool::default();
+        let x = char::default();
+    }
+}
diff --git a/tests/ui/trivial_default_constructed_types.stderr b/tests/ui/trivial_default_constructed_types.stderr
@@ -0,0 +1,52 @@
+error: constructing a trivial type using `default`
+  --> $DIR/trivial_default_constructed_types.rs:10:5
+   |
+LL |     u32::default();
+   |     ^^^^^^^^^^^^^^ help: try: `0`
+   |
+   = note: `-D clippy::trivial-default-constructed-types` implied by `-D warnings`
+
+error: constructing a trivial type using `default`
+  --> $DIR/trivial_default_constructed_types.rs:11:26
+   |
+LL |     let x: Option<u32> = Option::default();
+   |                          ^^^^^^^^^^^^^^^^^ help: try: `None`
+
+error: constructing a trivial tuple using `default`
+  --> $DIR/trivial_default_constructed_types.rs:12:23
+   |
+LL |     let y: (usize,) = Default::default();
+   |                       ^^^^^^^^^^^^^^^^^^ help: try: `(0,)`
+
+error: constructing a unit using `default`
+  --> $DIR/trivial_default_constructed_types.rs:13:5
+   |
+LL |     <()>::default();
+   |     ^^^^^^^^^^^^^^^ help: try: `()`
+
+error: constructing a trivial array using `default`
+  --> $DIR/trivial_default_constructed_types.rs:14:24
+   |
+LL |     let x: [u32; 10] = Default::default();
+   |                        ^^^^^^^^^^^^^^^^^^ help: try: `[0; 10]`
+
+error: constructing a trivial type using `default`
+  --> $DIR/trivial_default_constructed_types.rs:15:27
+   |
+LL |     let x: [f32; 1000] = [Default::default(); 1000];
+   |                           ^^^^^^^^^^^^^^^^^^ help: try: `0.0`
+
+error: constructing a trivial type using `default`
+  --> $DIR/trivial_default_constructed_types.rs:16:13
+   |
+LL |     let x = <&str>::default();
+   |             ^^^^^^^^^^^^^^^^^ help: try: `""`
+
+error: constructing a trivial type using `default`
+  --> $DIR/trivial_default_constructed_types.rs:17:13
+   |
+LL |     let x = bool::default();
+   |             ^^^^^^^^^^^^^^^ help: try: `false`
+
+error: aborting due to 8 previous errors
+