Lint duplicate methods of trait bounds

Fixes rust-lang#5777

Author: wiomoc

clippy_lints/src/lib.rs

@@ -787,6 +787,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &temporary_assignment::TEMPORARY_ASSIGNMENT,
         &to_digit_is_some::TO_DIGIT_IS_SOME,
         &trait_bounds::TYPE_REPETITION_IN_BOUNDS,
+        &trait_bounds::TRAIT_DUPLICATION_IN_BOUNDS,
         &transmute::CROSSPOINTER_TRANSMUTE,
         &transmute::TRANSMUTE_BYTES_TO_STR,
         &transmute::TRANSMUTE_FLOAT_TO_INT,
@@ -1175,6 +1176,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&shadow::SHADOW_UNRELATED),
         LintId::of(&strings::STRING_ADD_ASSIGN),
         LintId::of(&trait_bounds::TYPE_REPETITION_IN_BOUNDS),
+        LintId::of(&trait_bounds::TRAIT_DUPLICATION_IN_BOUNDS),
         LintId::of(&trivially_copy_pass_by_ref::TRIVIALLY_COPY_PASS_BY_REF),
         LintId::of(&types::CAST_LOSSLESS),
         LintId::of(&types::CAST_POSSIBLE_TRUNCATION),

clippy_lints/src/trait_bounds.rs

@@ -2,9 +2,10 @@ use crate::utils::{in_macro, snippet, snippet_with_applicability, span_lint_and_
 use if_chain::if_chain;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_errors::Applicability;
-use rustc_hir::{GenericBound, Generics, WherePredicate};
+use rustc_hir::{def::Res, GenericBound, Generics, ParamName, Path, QPath, TyKind, WherePredicate};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::{declare_tool_lint, impl_lint_pass};
+use rustc_span::Span;
 
 declare_clippy_lint! {
     /// **What it does:** This lint warns about unnecessary type repetitions in trait bounds
@@ -29,6 +30,35 @@ declare_clippy_lint! {
     "Types are repeated unnecessary in trait bounds use `+` instead of using `T: _, T: _`"
 }
 
+declare_clippy_lint! {
+    /// **What it does:** Checks for cases where generics are being used and multiple
+    /// syntax specifications for trait bounds are used simultaneously.
+    ///
+    /// **Why is this bad?** Duplicate bounds makes the code
+    /// less readable than specifing them only once.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// fn func<T: Clone + Default>(arg: T) where T: Clone + Default {}
+    /// ```
+    ///
+    /// Could be written as:
+    ///
+    /// ```rust
+    /// fn func<T: Clone + Default>(arg: T) {}
+    /// ```
+    /// or
+    ///  ///
+    /// ```rust
+    /// fn func<T>(arg: T) where T: Clone + Default {}
+    /// ```
+    pub TRAIT_DUPLICATION_IN_BOUNDS,
+    pedantic,
+    "Check if the same trait bounds are specifed twice during a function declaration"
+}
+
 #[derive(Copy, Clone)]
 pub struct TraitBounds {
     max_trait_bounds: u64,
@@ -41,10 +71,25 @@ impl TraitBounds {
     }
 }
 
-impl_lint_pass!(TraitBounds => [TYPE_REPETITION_IN_BOUNDS]);
+impl_lint_pass!(TraitBounds => [TYPE_REPETITION_IN_BOUNDS, TRAIT_DUPLICATION_IN_BOUNDS]);
 
 impl<'tcx> LateLintPass<'tcx> for TraitBounds {
     fn check_generics(&mut self, cx: &LateContext<'tcx>, gen: &'tcx Generics<'_>) {
+        self.check_type_repetition(cx, gen);
+        check_trait_bound_duplication(cx, gen);
+    }
+}
+
+fn get_trait_res_span_from_bound(bound: &GenericBound<'_>) -> Option<(Res, Span)> {
+    if let GenericBound::Trait(t, _) = bound {
+        Some((t.trait_ref.path.res, t.span))
+    } else {
+        None
+    }
+}
+
+impl TraitBounds {
+    fn check_type_repetition(self, cx: &LateContext<'_>, gen: &'_ Generics<'_>) {
         if in_macro(gen.span) {
             return;
         }
@@ -101,3 +146,46 @@ impl<'tcx> LateLintPass<'tcx> for TraitBounds {
         }
     }
 }
+
+fn check_trait_bound_duplication(cx: &LateContext<'_>, gen: &'_ Generics<'_>) {
+    if in_macro(gen.span) {
+        return;
+    }
+
+    let mut map = FxHashMap::default();
+    for param in gen.params {
+        if let ParamName::Plain(ref ident) = param.name {
+            let res = param
+                .bounds
+                .iter()
+                .filter_map(get_trait_res_span_from_bound)
+                .collect::<Vec<_>>();
+            map.insert(*ident, res);
+        }
+    }
+
+    for predicate in gen.where_clause.predicates {
+        if_chain! {
+            if let WherePredicate::BoundPredicate(ref bound_predicate) = predicate;
+            if !in_macro(bound_predicate.span);
+            if let TyKind::Path(ref path) = bound_predicate.bounded_ty.kind;
+            if let QPath::Resolved(_, Path { ref segments, .. }) = path;
+            if let Some(segment) = segments.first();
+            if let Some(trait_resolutions_direct) = map.get(&segment.ident);
+            then {
+                for (res_where, _) in bound_predicate.bounds.iter().filter_map(get_trait_res_span_from_bound) {
+                    if let Some((_, span_direct)) = trait_resolutions_direct.iter().find(|(res_direct, _)| *res_direct == res_where) {
+                        span_lint_and_help(
+                            cx,
+                            TRAIT_DUPLICATION_IN_BOUNDS,
+                            *span_direct,
+                            "this trait bound is already specified in the where clause",
+                            None,
+                            "consider removing this trait bound",
+                        );
+                    }
+                }
+            }
+        }
+    }
+}

src/lintlist/mod.rs

@@ -2257,6 +2257,13 @@ pub static ref ALL_LINTS: Vec<Lint> = vec![
         deprecation: None,
         module: "trait_bounds",
     },
+    Lint {
+        name: "trait_duplication_in_bounds",
+        group: "pedantic",
+        desc: "trait bound is defined twice, firstly directly and secondly in the where clause",
+        deprecation: None,
+        module: "trait_bounds",
+    },
     Lint {
         name: "unicode_not_nfc",
         group: "pedantic",

tests/ui/trait_duplication_in_bounds.rs

@@ -0,0 +1,31 @@
+#![deny(clippy::trait_duplication_in_bounds)]
+
+use std::ops::{Add, AddAssign, Div, DivAssign, Mul, MulAssign, Sub, SubAssign};
+
+fn bad_foo<T: Clone + Default, Z: Copy>(arg0: T, arg1: Z)
+where
+    T: Clone,
+    T: Default,
+{
+    unimplemented!();
+}
+
+fn good_bar<T: Clone + Default>(arg: T) {
+    unimplemented!();
+}
+
+fn good_foo<T>(arg: T)
+where
+    T: Clone + Default,
+{
+    unimplemented!();
+}
+
+fn good_foobar<T: Default>(arg: T)
+where
+    T: Clone,
+{
+    unimplemented!();
+}
+
+fn main() {}

tests/ui/trait_duplication_in_bounds.stderr

@@ -0,0 +1,23 @@
+error: this trait bound is already specified in the where clause
+  --> $DIR/trait_duplication_in_bounds.rs:5:15
+   |
+LL | fn bad_foo<T: Clone + Default, Z: Copy>(arg0: T, arg1: Z)
+   |               ^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/trait_duplication_in_bounds.rs:1:9
+   |
+LL | #![deny(clippy::trait_duplication_in_bounds)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = help: consider removing this trait bound
+
+error: this trait bound is already specified in the where clause
+  --> $DIR/trait_duplication_in_bounds.rs:5:23
+   |
+LL | fn bad_foo<T: Clone + Default, Z: Copy>(arg0: T, arg1: Z)
+   |                       ^^^^^^^
+   |
+   = help: consider removing this trait bound
+
+error: aborting due to 2 previous errors
+