Unconstrained parameter fix

compiler/rustc_hir/src/hir.rs

@@ -1066,6 +1066,61 @@ impl<'hir> Generics<'hir> {
             bound_span.with_lo(bounds[bound_pos - 1].span().hi())
         }
     }
+
+    pub fn span_for_param_removal(&self, param_index: usize) -> Span {
+        if param_index >= self.params.len() {
+            return self.span.shrink_to_hi();
+        }
+
+        let is_impl_generic = |par: &&GenericParam<'_>| match par.kind {
+            GenericParamKind::Type { .. }
+            | GenericParamKind::Const { .. }
+            | GenericParamKind::Lifetime { kind: LifetimeParamKind::Explicit } => true,
+            _ => false,
+        };
+
+        // Find the span of the type parameter.
+        if let Some(next) = self.params[param_index + 1..].iter().find(is_impl_generic) {
+            self.params[param_index].span.until(next.span)
+        } else if let Some(prev) = self.params[..param_index].iter().rfind(is_impl_generic) {
+            let mut prev_span = prev.span;
+            // Consider the span of the bounds with the previous generic parameter when there is.
+            if let Some(prev_bounds_span) = self.span_for_param_bounds(prev) {
+                prev_span = prev_span.to(prev_bounds_span);
+            }
+
+            // Consider the span of the bounds with the current generic parameter when there is.
+            prev_span.shrink_to_hi().to(
+                if let Some(cur_bounds_span) = self.span_for_param_bounds(&self.params[param_index])
+                {
+                    cur_bounds_span
+                } else {
+                    self.params[param_index].span
+                },
+            )
+        } else {
+            // Remove also angle brackets <> when there is just ONE generic parameter.
+            self.span
+        }
+    }
+
+    fn span_for_param_bounds(&self, param: &GenericParam<'hir>) -> Option<Span> {
+        self.predicates
+            .iter()
+            .find(|pred| {
+                if let WherePredicateKind::BoundPredicate(WhereBoundPredicate {
+                    origin: PredicateOrigin::GenericParam,
+                    bounded_ty,
+                    ..
+                }) = pred.kind
+                {
+                    bounded_ty.span == param.span
+                } else {
+                    false
+                }
+            })
+            .map(|pred| pred.span)
+    }
 }
 
 /// A single predicate in a where-clause.

compiler/rustc_hir_analysis/src/impl_wf_check.rs

@@ -9,14 +9,18 @@
 //! fixed, but for the moment it's easier to do these checks early.
 
 use std::assert_matches::debug_assert_matches;
+use std::ops::ControlFlow;
 
 use min_specialization::check_min_specialization;
 use rustc_data_structures::fx::FxHashSet;
-use rustc_errors::Applicability;
 use rustc_errors::codes::*;
+use rustc_errors::{Applicability, Diag};
 use rustc_hir::def::DefKind;
-use rustc_hir::def_id::LocalDefId;
-use rustc_middle::ty::{self, TyCtxt, TypeVisitableExt};
+use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_hir::intravisit::{self, Visitor, walk_lifetime};
+use rustc_hir::{HirId, LifetimeKind, Path, QPath, Ty, TyKind};
+use rustc_middle::hir::nested_filter::All;
+use rustc_middle::ty::{self, GenericParamDef, TyCtxt, TypeVisitableExt};
 use rustc_span::{ErrorGuaranteed, kw};
 
 use crate::constrained_generic_params as cgp;
@@ -172,6 +176,13 @@ pub(crate) fn enforce_impl_lifetime_params_are_constrained(
                             );
                         }
                     }
+                    suggest_to_remove_or_use_generic(
+                        tcx,
+                        &mut diag,
+                        impl_def_id,
+                        param,
+                        "lifetime",
+                    );
                     res = Err(diag.emit());
                 }
             }
@@ -235,8 +246,130 @@ pub(crate) fn enforce_impl_non_lifetime_params_are_constrained(
                 const_param_note2: const_param_note,
             });
             diag.code(E0207);
+            suggest_to_remove_or_use_generic(tcx, &mut diag, impl_def_id, &param, "type");
             res = Err(diag.emit());
         }
     }
     res
 }
+
+/// A HIR visitor that checks if a specific generic parameter (by its `DefId`)
+/// is used within a given HIR tree.
+struct ParamUsageVisitor<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    /// The `DefId` of the generic parameter we are looking for.
+    param_def_id: DefId,
+    found: bool,
+}
+
+// todo: maybe this can be done more efficiently by only searching for generics OR lifetimes and searching more effectively
+impl<'tcx> Visitor<'tcx> for ParamUsageVisitor<'tcx> {
+    type NestedFilter = All;
+
+    fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
+        self.tcx
+    }
+
+    /// We use `ControlFlow` to stop visiting as soon as we find what we're looking for.
+    type Result = ControlFlow<()>;
+
+    /// This is the primary method for finding usages of type or const parameters.
+    fn visit_path(&mut self, path: &Path<'tcx>, _id: HirId) -> Self::Result {
+        if let Some(res_def_id) = path.res.opt_def_id() {
+            if res_def_id == self.param_def_id {
+                self.found = true;
+                return ControlFlow::Break(()); // Found it, stop visiting.
+            }
+        }
+        // If not found, continue walking down the HIR tree.
+        intravisit::walk_path(self, path)
+    }
+
+    fn visit_lifetime(&mut self, lifetime: &'tcx rustc_hir::Lifetime) -> Self::Result {
+        if let LifetimeKind::Param(id) = lifetime.kind {
+            if let Some(local_def_id) = self.param_def_id.as_local() {
+                if id == local_def_id {
+                    self.found = true;
+                    return ControlFlow::Break(()); // Found it, stop visiting.
+                }
+            }
+        }
+        walk_lifetime(self, lifetime)
+    }
+}
+
+fn suggest_to_remove_or_use_generic(
+    tcx: TyCtxt<'_>,
+    diag: &mut Diag<'_>,
+    impl_def_id: LocalDefId,
+    param: &GenericParamDef,
+    parameter_type: &str,
+) {
+    let node = tcx.hir_node_by_def_id(impl_def_id);
+    let hir_impl = node.expect_item().expect_impl();
+
+    let Some((index, _)) = hir_impl
+        .generics
+        .params
+        .iter()
+        .enumerate()
+        .find(|(_, par)| par.def_id.to_def_id() == param.def_id)
+    else {
+        return;
+    };
+
+    // search if the parameter is used in the impl body
+    let mut visitor = ParamUsageVisitor {
+        tcx, // Pass the TyCtxt
+        param_def_id: param.def_id,
+        found: false,
+    };
+
+    for item_ref in hir_impl.items {
+        let _ = visitor.visit_impl_item_ref(item_ref);
+        if visitor.found {
+            break;
+        }
+    }
+
+    let is_param_used = visitor.found;
+
+    // Suggestion for removing the type parameter.
+    let mut suggestions = vec![];
+    if !is_param_used {
+        // Only suggest removing it if it's not used anywhere.
+        suggestions.push(vec![(hir_impl.generics.span_for_param_removal(index), String::new())]);
+    }
+
+    // Suggestion for making use of the type parameter.
+    if let Some(path) = extract_ty_as_path(hir_impl.self_ty) {
+        let seg = path.segments.last().unwrap();
+        if let Some(args) = seg.args {
+            suggestions
+                .push(vec![(args.span().unwrap().shrink_to_hi(), format!(", {}", param.name))]);
+        } else {
+            suggestions.push(vec![(seg.ident.span.shrink_to_hi(), format!("<{}>", param.name))]);
+        }
+    }
+
+    let msg = if is_param_used {
+        // If it's used, the only valid fix is to constrain it.
+        format!("make use of the {} parameter `{}` in the `self` type", parameter_type, param.name)
+    } else {
+        format!(
+            "either remove the unused {} parameter `{}`, or make use of it",
+            parameter_type, param.name
+        )
+    };
+
+    diag.multipart_suggestions(msg, suggestions, Applicability::MaybeIncorrect);
+}
+
+fn extract_ty_as_path<'hir>(ty: &Ty<'hir>) -> Option<&'hir Path<'hir>> {
+    match ty.kind {
+        TyKind::Path(QPath::Resolved(_, path)) => Some(path),
+        TyKind::Slice(ty) | TyKind::Array(ty, _) => extract_ty_as_path(ty),
+        TyKind::Ptr(ty) | TyKind::Ref(_, ty) => extract_ty_as_path(ty.ty),
+        _ => None,
+    }
+}

tests/rustdoc-ui/not-wf-ambiguous-normalization.stderr

@@ -3,6 +3,14 @@ error[E0207]: the type parameter `T` is not constrained by the impl trait, self
    |
 LL | impl<T> Allocator for DefaultAllocator {
    |      ^ unconstrained type parameter
+   |
+help: either remove the unused type parameter `T`, or make use of it
+   |
+LL - impl<T> Allocator for DefaultAllocator {
+LL + impl Allocator for DefaultAllocator {
+   |
+LL | impl<T> Allocator for DefaultAllocator<T> {
+   |                                       +++
 
 error: aborting due to 1 previous error
 

tests/rustdoc-ui/synthetic-auto-trait-impls/unconstrained-param-in-impl-ambiguity.stderr

@@ -3,6 +3,14 @@ error[E0207]: the type parameter `Q` is not constrained by the impl trait, self
    |
 LL | unsafe impl<Q: Trait> Send for Inner {}
    |             ^ unconstrained type parameter
+   |
+help: either remove the unused type parameter `Q`, or make use of it
+   |
+LL - unsafe impl<Q: Trait> Send for Inner {}
+LL + unsafe impl Send for Inner {}
+   |
+LL | unsafe impl<Q: Trait> Send for Inner<Q> {}
+   |                                     +++
 
 error: aborting due to 1 previous error
 

tests/ui/associated-inherent-types/hr-do-not-blame-outlives-static-ice.stderr

@@ -12,6 +12,11 @@ error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait,
    |
 LL | impl<'a> Foo<fn(&())> {
    |      ^^ unconstrained lifetime parameter
+   |
+help: make use of the lifetime parameter `'a` in the `self` type
+   |
+LL | impl<'a> Foo<fn(&()), 'a> {
+   |                     ++++
 
 error[E0308]: mismatched types
   --> $DIR/hr-do-not-blame-outlives-static-ice.rs:14:11

tests/ui/associated-types/issue-26262.stderr

@@ -3,12 +3,22 @@ error[E0207]: the type parameter `T` is not constrained by the impl trait, self
    |
 LL | impl<T: Tr> S<T::Assoc> {
    |      ^ unconstrained type parameter
+   |
+help: make use of the type parameter `T` in the `self` type
+   |
+LL | impl<T: Tr> S<T::Assoc, T> {
+   |                       +++
 
 error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait, self type, or predicates
   --> $DIR/issue-26262.rs:17:6
    |
 LL | impl<'a,T: Trait2<'a>> Trait1<<T as Trait2<'a>>::Foo> for T {
    |      ^^ unconstrained lifetime parameter
+   |
+help: make use of the lifetime parameter `'a` in the `self` type
+   |
+LL | impl<'a,T: Trait2<'a>> Trait1<<T as Trait2<'a>>::Foo> for T<'a> {
+   |                                                            ++++
 
 error: aborting due to 2 previous errors
 

tests/ui/associated-types/projection-dyn-associated-type.stderr

@@ -28,6 +28,7 @@ error[E0207]: the type parameter `T` is not constrained by the impl trait, self
    |
 LL | impl<T: ?Sized> Mirror for A {
    |      ^ unconstrained type parameter
+   |
 
 error[E0277]: the trait bound `(dyn B + 'static): Mirror` is not satisfied
   --> $DIR/projection-dyn-associated-type.rs:22:6

tests/ui/associated-types/unconstrained-lifetime-assoc-type.stderr

@@ -3,6 +3,11 @@ error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait,
    |
 LL | impl<'a> Fun for Holder {
    |      ^^ unconstrained lifetime parameter
+   |
+help: make use of the lifetime parameter `'a` in the `self` type
+   |
+LL | impl<'a> Fun for Holder<'a> {
+   |                        ++++
 
 error: aborting due to 1 previous error
 

tests/ui/async-await/in-trait/unconstrained-impl-region.stderr

@@ -3,6 +3,7 @@ error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait,
    |
 LL | impl<'a> Actor for () {
    |      ^^ unconstrained lifetime parameter
+   |
 
 error: aborting due to 1 previous error
 

tests/ui/async-await/issues/issue-78654.full.stderr

@@ -12,6 +12,13 @@ LL | impl<const H: feature> Foo {
    |
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
+help: either remove the unused type parameter `H`, or make use of it
+   |
+LL - impl<const H: feature> Foo {
+LL + impl Foo {
+   |
+LL | impl<const H: feature> Foo<H> {
+   |                           +++
 
 error: aborting due to 2 previous errors
 

tests/ui/async-await/issues/issue-78654.min.stderr

@@ -12,6 +12,13 @@ LL | impl<const H: feature> Foo {
    |
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
+help: either remove the unused type parameter `H`, or make use of it
+   |
+LL - impl<const H: feature> Foo {
+LL + impl Foo {
+   |
+LL | impl<const H: feature> Foo<H> {
+   |                           +++
 
 error: aborting due to 2 previous errors
 

tests/ui/const-generics/associated_const_equality/unconstrained_impl_param.stderr

@@ -13,6 +13,12 @@ error[E0207]: the type parameter `U` is not constrained by the impl trait, self
    |
 LL | impl<U> Trait for () where (U,): AssocConst<A = { 0 }> {}
    |      ^ unconstrained type parameter
+   |
+help: either remove the unused type parameter `U`, or make use of it
+   |
+LL - impl<U> Trait for () where (U,): AssocConst<A = { 0 }> {}
+LL + impl Trait for () where (U,): AssocConst<A = { 0 }> {}
+   |
 
 error[E0282]: type annotations needed
   --> $DIR/unconstrained_impl_param.rs:21:5

tests/ui/const-generics/generic_const_exprs/post-analysis-user-facing-param-env.stderr

@@ -37,6 +37,13 @@ LL | impl<'a, const NUM: usize> std::ops::Add<&'a Foo> for Foo
    |
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
+help: either remove the unused type parameter `NUM`, or make use of it
+   |
+LL - impl<'a, const NUM: usize> std::ops::Add<&'a Foo> for Foo
+LL + impl<'a> std::ops::Add<&'a Foo> for Foo
+   |
+LL | impl<'a, const NUM: usize> std::ops::Add<&'a Foo> for Foo<NUM>
+   |                                                          +++++
 
 error: aborting due to 3 previous errors; 1 warning emitted
 

tests/ui/const-generics/ice-unexpected-inference-var-122549.stderr

@@ -71,6 +71,10 @@ LL | impl<'a, T, const N: usize> Iterator for ConstChunksExact<'a, T, {}> {
    |
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
+help: make use of the type parameter `N` in the `self` type
+   |
+LL | impl<'a, T, const N: usize> Iterator for ConstChunksExact<'a, T, {}, N> {
+   |                                                                    +++
 
 error: aborting due to 8 previous errors