Unconstrained parameter fix

compiler/rustc_hir/src/hir.rs

@@ -1095,6 +1095,93 @@ impl<'hir> Generics<'hir> {
             bound_span.with_lo(bounds[bound_pos - 1].span().hi())
         }
     }
+
+    /// Computes the span representing the removal of a generic parameter at `param_index`.
+    ///
+    /// This function identifies the correct slice of source code to delete so that the
+    /// remaining generic list remains syntactically valid (handling commas and brackets).
+    ///
+    /// ### Examples
+    ///
+    /// 1. **With a following parameter:** (Includes the trailing comma)
+    ///    - Input: `<T, U>` (index 0)
+    ///    - Produces span for: `T, `
+    ///
+    /// 2. **With a previous parameter:** (Includes the leading comma and bounds)
+    ///    - Input: `<T: Clone, U>` (index 1)
+    ///    - Produces span for: `, U`
+    ///
+    /// 3. **The only parameter:** (Includes the angle brackets)
+    ///    - Input: `<T>` (index 0)
+    ///    - Produces span for: `<T>`
+    ///
+    /// 4. **Parameter with where-clause bounds:**
+    ///    - Input: `fn foo<T, U>() where T: Copy` (index 0)
+    ///    - Produces span for: `T, ` (The where-clause remains for other logic to handle).
+    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
+        }
+    }
+
+    /// Returns the span of the `WherePredicate` associated with the given `GenericParam`, if any.
+    ///
+    /// This looks specifically for predicates in the `where` clause that were generated
+    /// from the parameter definition (e.g., `T` in `where T: Bound`).
+    ///
+    /// ### Example
+    ///
+    /// - Input: `param` representing `T`
+    /// - Context: `where T: Clone + Default, U: Copy`
+    /// - Returns: Span of `T: Clone + Default`
+    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/errors.rs

@@ -15,6 +15,8 @@ pub(crate) mod wrong_number_of_generic_args;
 mod precise_captures;
 pub(crate) use precise_captures::*;
 
+pub(crate) mod remove_or_use_generic;
+
 #[derive(Diagnostic)]
 #[diag("ambiguous associated {$assoc_kind} `{$assoc_ident}` in bounds of `{$qself}`")]
 pub(crate) struct AmbiguousAssocItem<'a> {

compiler/rustc_hir_analysis/src/errors/remove_or_use_generic.rs

@@ -0,0 +1,217 @@
+use std::ops::ControlFlow;
+
+use rustc_errors::{Applicability, Diag};
+use rustc_hir::def::DefKind;
+use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_hir::intravisit::{self, Visitor, walk_lifetime};
+use rustc_hir::{GenericArg, HirId, LifetimeKind, Path, QPath, TyKind};
+use rustc_middle::hir::nested_filter::All;
+use rustc_middle::ty::{GenericParamDef, GenericParamDefKind, TyCtxt};
+
+use crate::hir::def::Res;
+
+/// Use a Visitor to find usages of the type or lifetime parameter
+struct ParamUsageVisitor<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    /// The `DefId` of the generic parameter we are looking for.
+    param_def_id: DefId,
+    found: bool,
+}
+
+impl<'tcx> Visitor<'tcx> for ParamUsageVisitor<'tcx> {
+    type NestedFilter = All;
+
+    fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
+        self.tcx
+    }
+
+    type Result = ControlFlow<()>;
+
+    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(());
+            }
+        }
+        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(());
+                }
+            }
+        }
+        walk_lifetime(self, lifetime)
+    }
+}
+
+/// Adds a suggestion to a diagnostic to either remove an unused generic parameter, or use it.
+///
+/// # Examples
+///
+/// - `impl<T> Struct { ... }` where `T` is unused -> suggests removing `T` or using it.
+/// - `impl<'a> Struct { ... }` where `'a` is unused -> suggests removing `'a`.
+/// - `impl<T> Struct { // T used in here }` where `T` is used in the body but not in the self type -> suggests adding `T` to the self type and struct definition.
+/// - `impl<T> Struct { ... }` where the struct has a generic parameter with a default -> suggests adding `T` to the self type.
+pub(crate) fn suggest_to_remove_or_use_generic(
+    tcx: TyCtxt<'_>,
+    diag: &mut Diag<'_>,
+    impl_def_id: LocalDefId,
+    param: &GenericParamDef,
+    is_lifetime: bool,
+) {
+    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;
+    };
+
+    // Get the Struct/ADT definition ID from the self type
+    let struct_def_id = if let TyKind::Path(QPath::Resolved(_, path)) = hir_impl.self_ty.kind
+        && let Res::Def(DefKind::Struct | DefKind::Enum | DefKind::Union, def_id) = path.res
+    {
+        def_id
+    } else {
+        return;
+    };
+
+    // Count how many generic parameters are defined in the struct definition
+    let generics = tcx.generics_of(struct_def_id);
+    let total_params = generics
+        .own_params
+        .iter()
+        .filter(|p| {
+            if is_lifetime {
+                matches!(p.kind, GenericParamDefKind::Lifetime)
+            } else {
+                matches!(p.kind, GenericParamDefKind::Type { .. })
+            }
+        })
+        .count();
+
+    // Count how many arguments are currently provided in the impl
+    let mut provided_params = 0;
+    let mut last_segment_args = None;
+
+    if let TyKind::Path(QPath::Resolved(_, path)) = hir_impl.self_ty.kind
+        && let Some(seg) = path.segments.last()
+        && let Some(args) = seg.args
+    {
+        last_segment_args = Some(args);
+        provided_params = args
+            .args
+            .iter()
+            .filter(|arg| match arg {
+                GenericArg::Lifetime(_) => is_lifetime,
+                GenericArg::Type(_) => !is_lifetime,
+                _ => false,
+            })
+            .count();
+    }
+
+    let mut visitor = ParamUsageVisitor { tcx, 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;
+
+    let mut suggestions = vec![];
+
+    // Option A: Remove (Only if not used in body)
+    if !is_param_used {
+        suggestions.push((hir_impl.generics.span_for_param_removal(index), String::new()));
+    }
+
+    // Option B: Suggest adding only if there's an available parameter in the struct definition
+    // or the parameter is already used somewhere, then we suggest adding to the impl struct and the struct definition
+    if provided_params < total_params || is_param_used {
+        if let Some(args) = last_segment_args {
+            // Struct already has <...>, append to it
+            suggestions.push((args.span().unwrap().shrink_to_hi(), format!(", {}", param.name)));
+        } else if let TyKind::Path(QPath::Resolved(_, path)) = hir_impl.self_ty.kind {
+            // Struct has no <...> yet, add it
+            let seg = path.segments.last().unwrap();
+            suggestions.push((seg.ident.span.shrink_to_hi(), format!("<{}>", param.name)));
+        }
+        if is_param_used {
+            // If the parameter is used in the body, we also want to suggest adding it to the struct definition if it's not already there
+            let struct_span = tcx.def_span(struct_def_id);
+            let struct_generic_params = tcx
+                .hir_node_by_def_id(struct_def_id.expect_local())
+                .expect_item()
+                .expect_struct()
+                .1
+                .params;
+            if struct_generic_params.len() > 0 {
+                let last_param = struct_generic_params.last().unwrap();
+                suggestions.push((last_param.span.shrink_to_hi(), format!(", {}", param.name)));
+            } else {
+                suggestions.push((struct_span.shrink_to_hi(), format!("<{}>", param.name)));
+            }
+        }
+    }
+
+    if suggestions.is_empty() {
+        return;
+    }
+
+    let parameter_type = if is_lifetime { "lifetime" } else { "type" };
+    if is_param_used {
+        let msg = format!(
+            "make use of the {} parameter `{}` in the `self` type",
+            parameter_type, param.name
+        );
+        diag.span_suggestion(
+            suggestions[0].0,
+            msg,
+            suggestions[0].1.clone(),
+            Applicability::MaybeIncorrect,
+        );
+        let msg2 = format!(
+            "and add it to the struct definition of {} as well since it's used in the body of the impl",
+            tcx.def_path_str(struct_def_id)
+        );
+        diag.span_suggestion(
+            suggestions[1].0,
+            msg2,
+            suggestions[1].1.clone(),
+            Applicability::MaybeIncorrect,
+        );
+    } else {
+        let msg = if suggestions.len() == 2 {
+            format!("either remove the unused {} parameter `{}`", parameter_type, param.name)
+        } else {
+            format!("remove the unused {} parameter `{}`", parameter_type, param.name)
+        };
+        diag.span_suggestion(
+            suggestions[0].0,
+            msg,
+            suggestions[0].1.clone(),
+            Applicability::MaybeIncorrect,
+        );
+        if suggestions.len() == 2 {
+            let msg = format!("or make use of it");
+            diag.span_suggestion(
+                suggestions[1].0,
+                msg,
+                suggestions[1].1.clone(),
+                Applicability::MaybeIncorrect,
+            );
+        }
+    };
+}

compiler/rustc_hir_analysis/src/impl_wf_check.rs

@@ -21,6 +21,7 @@ use rustc_span::{ErrorGuaranteed, kw};
 
 use crate::constrained_generic_params as cgp;
 use crate::errors::UnconstrainedGenericParameter;
+use crate::errors::remove_or_use_generic::suggest_to_remove_or_use_generic;
 
 mod min_specialization;
 
@@ -172,6 +173,7 @@ pub(crate) fn enforce_impl_lifetime_params_are_constrained(
                             );
                         }
                     }
+                    suggest_to_remove_or_use_generic(tcx, &mut diag, impl_def_id, param, true);
                     res = Err(diag.emit());
                 }
             }
@@ -235,6 +237,7 @@ 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, false);
             res = Err(diag.emit());
         }
     }

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

@@ -2,7 +2,10 @@ error[E0207]: the type parameter `T` is not constrained by the impl trait, self
   --> $DIR/not-wf-ambiguous-normalization.rs:14:6
    |
 LL | impl<T> Allocator for DefaultAllocator {
-   |      ^ unconstrained type parameter
+   |     -^-
+   |     ||
+   |     |unconstrained type parameter
+   |     help: remove the unused type parameter `T`
 
 error: aborting due to 1 previous error
 

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

@@ -2,7 +2,10 @@ error[E0207]: the type parameter `Q` is not constrained by the impl trait, self
   --> $DIR/unconstrained-param-in-impl-ambiguity.rs:7:13
    |
 LL | unsafe impl<Q: Trait> Send for Inner {}
-   |             ^ unconstrained type parameter
+   |            -^--------
+   |            ||
+   |            |unconstrained type parameter
+   |            help: remove the unused type parameter `Q`
 
 error: aborting due to 1 previous error
 

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

@@ -12,6 +12,15 @@ 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> {
+   |                     ++++
+help: and add it to the struct definition of Foo as well since it's used in the body of the impl
+   |
+LL | struct Foo<T, 'a>(T);
+   |             ++++
 
 error[E0308]: mismatched types
   --> $DIR/hr-do-not-blame-outlives-static-ice.rs:14:11

tests/ui/associated-inherent-types/inherent-assoc-ty-mismatch-issue-153539.stderr

@@ -2,7 +2,10 @@ error[E0207]: the type parameter `X` is not constrained by the impl trait, self
   --> $DIR/inherent-assoc-ty-mismatch-issue-153539.rs:9:6
    |
 LL | impl<X> S<'_> {
-   |      ^ unconstrained type parameter
+   |     -^-
+   |     ||
+   |     |unconstrained type parameter
+   |     help: remove the unused type parameter `X`
 
 error[E0308]: mismatched types
   --> $DIR/inherent-assoc-ty-mismatch-issue-153539.rs:17:5