Add duplicate trait bound lint

Co-authored-by: Esteban Kuber <estebank@users.noreply.github.com>

Author: pierwill

compiler/rustc_lint/src/lib.rs

@@ -209,6 +209,7 @@ macro_rules! late_lint_mod_passes {
                 UnstableFeatures: UnstableFeatures,
                 ArrayIntoIter: ArrayIntoIter::default(),
                 DropTraitConstraints: DropTraitConstraints,
+                DuplicateTraitBounds: DuplicateTraitBounds,
                 TemporaryCStringAsPtr: TemporaryCStringAsPtr,
                 NonPanicFmt: NonPanicFmt,
                 NoopMethodCall: NoopMethodCall,

compiler/rustc_lint/src/traits.rs

@@ -1,6 +1,8 @@
 use crate::LateContext;
 use crate::LateLintPass;
 use crate::LintContext;
+use hir::GenericBound;
+use rustc_data_structures::stable_set::FxHashSet;
 use rustc_errors::fluent;
 use rustc_hir as hir;
 use rustc_span::symbol::sym;
@@ -132,3 +134,44 @@ impl<'tcx> LateLintPass<'tcx> for DropTraitConstraints {
         }
     }
 }
+
+declare_lint! {
+    pub DUP_TRAIT_BOUNDS,
+    Deny,
+    "duplicate trait bounds"
+}
+
+declare_lint_pass!(
+    DuplicateTraitBounds => [DUP_TRAIT_BOUNDS]
+);
+
+impl<'tcx> LateLintPass<'tcx> for DuplicateTraitBounds {
+    fn check_item(&mut self, cx: &LateContext<'_>, item: &'tcx hir::Item<'tcx>) {
+        let bounds: &[hir::GenericBound<'_>] = match &item.kind {
+            hir::ItemKind::Trait(_, _, _, bounds, _) => bounds,
+            hir::ItemKind::TraitAlias(_, bounds) => bounds,
+            _ => return,
+        };
+
+        let mut set = FxHashSet::default();
+        for bound in bounds.into_iter() {
+            match bound {
+                GenericBound::Trait(polytraitref, _) => {
+                    let Some(did) = polytraitref.trait_ref.trait_def_id() else { continue; };
+                    // If inserting the trait bound into the set returns `false`,
+                    // there is a duplicate.
+                    if !set.insert(did) {
+                        let span = polytraitref.span;
+                        cx.struct_span_lint(DUP_TRAIT_BOUNDS, span, |lint| {
+                            let msg = format!("Duplicate trait bound");
+                            lint.build(&msg)
+                                .span_help(span, "Remove this duplicate trait bound")
+                                .emit();
+                        });
+                    };
+                }
+                _ => continue,
+            }
+        }
+    }
+}

src/test/ui/traits/duplicate-trait-bounds.rs

@@ -0,0 +1,12 @@
+pub trait DirectedGraph {}
+pub trait WithStartNode {}
+pub trait WithPredecessors {}
+pub trait WithSuccessors {}
+pub trait WithNumNodes {}
+
+pub trait ControlFlowGraph:
+    DirectedGraph + WithStartNode + WithPredecessors + WithStartNode + WithSuccessors + WithNumNodes
+                                                       //~ ERROR duplicate trait bound
+{}
+
+fn main() {}

src/test/ui/traits/duplicate-trait-bounds.stderr

@@ -0,0 +1,15 @@
+error: Duplicate trait bound
+  --> $DIR/duplicate-trait-bounds.rs:8:56
+   |
+LL |     DirectedGraph + WithStartNode + WithPredecessors + WithStartNode + WithSuccessors + WithNumNodes
+   |                                                        ^^^^^^^^^^^^^
+   |
+   = note: `#[deny(dup_trait_bounds)]` on by default
+help: Remove this duplicate trait bound
+  --> $DIR/duplicate-trait-bounds.rs:8:56
+   |
+LL |     DirectedGraph + WithStartNode + WithPredecessors + WithStartNode + WithSuccessors + WithNumNodes
+   |                                                        ^^^^^^^^^^^^^
+
+error: aborting due to previous error
+