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Diffstat (limited to 'src/tools/clippy/clippy_lints/src/extra_unused_type_parameters.rs')
| -rw-r--r-- | src/tools/clippy/clippy_lints/src/extra_unused_type_parameters.rs | 229 |
1 files changed, 229 insertions, 0 deletions
diff --git a/src/tools/clippy/clippy_lints/src/extra_unused_type_parameters.rs b/src/tools/clippy/clippy_lints/src/extra_unused_type_parameters.rs new file mode 100644 index 000000000..20565e1d2 --- /dev/null +++ b/src/tools/clippy/clippy_lints/src/extra_unused_type_parameters.rs @@ -0,0 +1,229 @@ +use clippy_utils::diagnostics::span_lint_and_help; +use clippy_utils::trait_ref_of_method; +use rustc_data_structures::fx::FxHashMap; +use rustc_errors::MultiSpan; +use rustc_hir::intravisit::{walk_impl_item, walk_item, walk_param_bound, walk_ty, Visitor}; +use rustc_hir::{ + BodyId, ExprKind, GenericBound, GenericParamKind, Generics, ImplItem, ImplItemKind, Item, ItemKind, + PredicateOrigin, Ty, TyKind, WherePredicate, +}; +use rustc_lint::{LateContext, LateLintPass, LintContext}; +use rustc_middle::hir::nested_filter; +use rustc_middle::lint::in_external_macro; +use rustc_session::{declare_tool_lint, impl_lint_pass}; +use rustc_span::{ + def_id::{DefId, LocalDefId}, + Span, +}; + +declare_clippy_lint! { + /// ### What it does + /// Checks for type parameters in generics that are never used anywhere else. + /// + /// ### Why is this bad? + /// Functions cannot infer the value of unused type parameters; therefore, calling them + /// requires using a turbofish, which serves no purpose but to satisfy the compiler. + /// + /// ### Example + /// ```rust + /// fn unused_ty<T>(x: u8) { + /// // .. + /// } + /// ``` + /// Use instead: + /// ```rust + /// fn no_unused_ty(x: u8) { + /// // .. + /// } + /// ``` + #[clippy::version = "1.69.0"] + pub EXTRA_UNUSED_TYPE_PARAMETERS, + complexity, + "unused type parameters in function definitions" +} + +pub struct ExtraUnusedTypeParameters { + avoid_breaking_exported_api: bool, +} + +impl ExtraUnusedTypeParameters { + pub fn new(avoid_breaking_exported_api: bool) -> Self { + Self { + avoid_breaking_exported_api, + } + } + + /// Don't lint external macros or functions with empty bodies. Also, don't lint public items if + /// the `avoid_breaking_exported_api` config option is set. + fn check_false_positive(&self, cx: &LateContext<'_>, span: Span, def_id: LocalDefId, body_id: BodyId) -> bool { + let body = cx.tcx.hir().body(body_id).value; + let fn_empty = matches!(&body.kind, ExprKind::Block(blk, None) if blk.stmts.is_empty() && blk.expr.is_none()); + let is_exported = cx.effective_visibilities.is_exported(def_id); + in_external_macro(cx.sess(), span) || (self.avoid_breaking_exported_api && is_exported) || fn_empty + } +} + +impl_lint_pass!(ExtraUnusedTypeParameters => [EXTRA_UNUSED_TYPE_PARAMETERS]); + +/// A visitor struct that walks a given function and gathers generic type parameters, plus any +/// trait bounds those parameters have. +struct TypeWalker<'cx, 'tcx> { + cx: &'cx LateContext<'tcx>, + /// Collection of all the function's type parameters. + ty_params: FxHashMap<DefId, Span>, + /// Collection of any (inline) trait bounds corresponding to each type parameter. + bounds: FxHashMap<DefId, Span>, + /// The entire `Generics` object of the function, useful for querying purposes. + generics: &'tcx Generics<'tcx>, + /// The value of this will remain `true` if *every* parameter: + /// 1. Is a type parameter, and + /// 2. Goes unused in the function. + /// Otherwise, if any type parameters end up being used, or if any lifetime or const-generic + /// parameters are present, this will be set to `false`. + all_params_unused: bool, +} + +impl<'cx, 'tcx> TypeWalker<'cx, 'tcx> { + fn new(cx: &'cx LateContext<'tcx>, generics: &'tcx Generics<'tcx>) -> Self { + let mut all_params_unused = true; + let ty_params = generics + .params + .iter() + .filter_map(|param| { + if let GenericParamKind::Type { synthetic, .. } = param.kind { + (!synthetic).then_some((param.def_id.into(), param.span)) + } else { + if !param.is_elided_lifetime() { + all_params_unused = false; + } + None + } + }) + .collect(); + + Self { + cx, + ty_params, + bounds: FxHashMap::default(), + generics, + all_params_unused, + } + } + + fn mark_param_used(&mut self, def_id: DefId) { + if self.ty_params.remove(&def_id).is_some() { + self.all_params_unused = false; + } + } + + fn emit_lint(&self) { + let (msg, help) = match self.ty_params.len() { + 0 => return, + 1 => ( + "type parameter goes unused in function definition", + "consider removing the parameter", + ), + _ => ( + "type parameters go unused in function definition", + "consider removing the parameters", + ), + }; + + let source_map = self.cx.sess().source_map(); + let span = if self.all_params_unused { + self.generics.span.into() // Remove the entire list of generics + } else { + MultiSpan::from_spans( + self.ty_params + .iter() + .map(|(def_id, &span)| { + // Extend the span past any trait bounds, and include the comma at the end. + let span_to_extend = self.bounds.get(def_id).copied().map_or(span, Span::shrink_to_hi); + let comma_range = source_map.span_extend_to_next_char(span_to_extend, '>', false); + let comma_span = source_map.span_through_char(comma_range, ','); + span.with_hi(comma_span.hi()) + }) + .collect(), + ) + }; + + span_lint_and_help(self.cx, EXTRA_UNUSED_TYPE_PARAMETERS, span, msg, None, help); + } +} + +/// Given a generic bound, if the bound is for a trait that's not a `LangItem`, return the +/// `LocalDefId` for that trait. +fn bound_to_trait_def_id(bound: &GenericBound<'_>) -> Option<LocalDefId> { + bound.trait_ref()?.trait_def_id()?.as_local() +} + +impl<'cx, 'tcx> Visitor<'tcx> for TypeWalker<'cx, 'tcx> { + type NestedFilter = nested_filter::OnlyBodies; + + fn visit_ty(&mut self, t: &'tcx Ty<'tcx>) { + if let Some((def_id, _)) = t.peel_refs().as_generic_param() { + self.mark_param_used(def_id); + } else if let TyKind::OpaqueDef(id, _, _) = t.kind { + // Explicitly walk OpaqueDef. Normally `walk_ty` would do the job, but it calls + // `visit_nested_item`, which checks that `Self::NestedFilter::INTER` is set. We're + // using `OnlyBodies`, so the check ends up failing and the type isn't fully walked. + let item = self.nested_visit_map().item(id); + walk_item(self, item); + } else { + walk_ty(self, t); + } + } + + fn visit_where_predicate(&mut self, predicate: &'tcx WherePredicate<'tcx>) { + if let WherePredicate::BoundPredicate(predicate) = predicate { + // Collect spans for any bounds on type parameters. We only keep bounds that appear in + // the list of generics (not in a where-clause). + if let Some((def_id, _)) = predicate.bounded_ty.peel_refs().as_generic_param() { + // If the bound contains non-public traits, err on the safe side and don't lint the + // corresponding parameter. + if !predicate + .bounds + .iter() + .filter_map(bound_to_trait_def_id) + .all(|id| self.cx.effective_visibilities.is_exported(id)) + { + self.mark_param_used(def_id); + } else if let PredicateOrigin::GenericParam = predicate.origin { + self.bounds.insert(def_id, predicate.span); + } + } + // Only walk the right-hand side of where-bounds + for bound in predicate.bounds { + walk_param_bound(self, bound); + } + } + } + + fn nested_visit_map(&mut self) -> Self::Map { + self.cx.tcx.hir() + } +} + +impl<'tcx> LateLintPass<'tcx> for ExtraUnusedTypeParameters { + fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'tcx>) { + if let ItemKind::Fn(_, generics, body_id) = item.kind + && !self.check_false_positive(cx, item.span, item.owner_id.def_id, body_id) + { + let mut walker = TypeWalker::new(cx, generics); + walk_item(&mut walker, item); + walker.emit_lint(); + } + } + + fn check_impl_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx ImplItem<'tcx>) { + // Only lint on inherent methods, not trait methods. + if let ImplItemKind::Fn(.., body_id) = item.kind + && trait_ref_of_method(cx, item.owner_id.def_id).is_none() + && !self.check_false_positive(cx, item.span, item.owner_id.def_id, body_id) + { + let mut walker = TypeWalker::new(cx, item.generics); + walk_impl_item(&mut walker, item); + walker.emit_lint(); + } + } +} |