1 //! Polymorphization Analysis
2 //! =========================
4 //! This module implements an analysis of functions, methods and closures to determine which
5 //! generic parameters are unused (and eventually, in what ways generic parameters are used - only
6 //! for their size, offset of a field, etc.).
8 use rustc_hir::{def::DefKind, def_id::DefId};
9 use rustc_index::bit_set::FiniteBitSet;
10 use rustc_middle::mir::{
11 visit::{TyContext, Visitor},
12 Local, LocalDecl, Location,
14 use rustc_middle::ty::{
16 fold::{TypeFoldable, TypeVisitor},
20 use rustc_span::symbol::sym;
21 use std::convert::TryInto;
23 /// Provide implementations of queries relating to polymorphization analysis.
24 pub fn provide(providers: &mut Providers) {
25 providers.unused_generic_params = unused_generic_params;
28 /// Determine which generic parameters are used by the function/method/closure represented by
29 /// `def_id`. Returns a bitset where bits representing unused parameters are set (`is_empty`
30 /// indicates all parameters are used).
31 fn unused_generic_params(tcx: TyCtxt<'_>, def_id: DefId) -> FiniteBitSet<u32> {
32 debug!("unused_generic_params({:?})", def_id);
34 if !tcx.sess.opts.debugging_opts.polymorphize {
35 // If polymorphization disabled, then all parameters are used.
36 return FiniteBitSet::new_empty();
39 let generics = tcx.generics_of(def_id);
40 debug!("unused_generic_params: generics={:?}", generics);
42 // Exit early when there are no parameters to be unused.
43 if generics.count() == 0 {
44 return FiniteBitSet::new_empty();
47 // Exit early when there is no MIR available.
48 if !tcx.is_mir_available(def_id) {
49 debug!("unused_generic_params: (no mir available) def_id={:?}", def_id);
50 return FiniteBitSet::new_empty();
53 // Create a bitset with N rightmost ones for each parameter.
54 let generics_count: u32 =
55 generics.count().try_into().expect("more generic parameters than can fit into a `u32`");
56 let mut unused_parameters = FiniteBitSet::<u32>::new_empty();
57 unused_parameters.set_range(0..generics_count);
58 debug!("unused_generic_params: (start) unused_parameters={:?}", unused_parameters);
59 mark_used_by_default_parameters(tcx, def_id, generics, &mut unused_parameters);
60 debug!("unused_generic_params: (after default) unused_parameters={:?}", unused_parameters);
62 // Visit MIR and accumululate used generic parameters.
63 let body = tcx.optimized_mir(def_id);
65 UsedGenericParametersVisitor { tcx, def_id, unused_parameters: &mut unused_parameters };
67 debug!("unused_generic_params: (after visitor) unused_parameters={:?}", unused_parameters);
69 mark_used_by_predicates(tcx, def_id, &mut unused_parameters);
70 debug!("unused_generic_params: (end) unused_parameters={:?}", unused_parameters);
72 // Emit errors for debugging and testing if enabled.
73 if !unused_parameters.is_empty() {
74 emit_unused_generic_params_error(tcx, def_id, generics, &unused_parameters);
80 /// Some parameters are considered used-by-default, such as non-generic parameters and the dummy
81 /// generic parameters from closures, this function marks them as used. `leaf_is_closure` should
82 /// be `true` if the item that `unused_generic_params` was invoked on is a closure.
83 fn mark_used_by_default_parameters<'tcx>(
86 generics: &'tcx ty::Generics,
87 unused_parameters: &mut FiniteBitSet<u32>,
89 if !tcx.is_trait(def_id) && (tcx.is_closure(def_id) || tcx.type_of(def_id).is_generator()) {
90 for param in &generics.params {
91 debug!("mark_used_by_default_parameters: (closure/gen) param={:?}", param);
92 unused_parameters.clear(param.index);
95 for param in &generics.params {
96 debug!("mark_used_by_default_parameters: (other) param={:?}", param);
97 if let ty::GenericParamDefKind::Lifetime = param.kind {
98 unused_parameters.clear(param.index);
103 if let Some(parent) = generics.parent {
104 mark_used_by_default_parameters(tcx, parent, tcx.generics_of(parent), unused_parameters);
108 /// Search the predicates on used generic parameters for any unused generic parameters, and mark
110 fn mark_used_by_predicates<'tcx>(
113 unused_parameters: &mut FiniteBitSet<u32>,
115 let def_id = tcx.closure_base_def_id(def_id);
117 let is_self_ty_used = |unused_parameters: &mut FiniteBitSet<u32>, self_ty: Ty<'tcx>| {
118 debug!("unused_generic_params: self_ty={:?}", self_ty);
119 if let ty::Param(param) = self_ty.kind {
120 !unused_parameters.contains(param.index).unwrap_or(false)
126 let mark_ty = |unused_parameters: &mut FiniteBitSet<u32>, ty: Ty<'tcx>| {
127 let mut vis = UsedGenericParametersVisitor { tcx, def_id, unused_parameters };
128 ty.visit_with(&mut vis);
131 let predicates = tcx.explicit_predicates_of(def_id);
132 debug!("mark_parameters_used_in_predicates: predicates_of={:?}", predicates);
133 for (predicate, _) in predicates.predicates {
134 match predicate.skip_binders() {
135 ty::PredicateAtom::Trait(predicate, ..) => {
136 let trait_ref = predicate.trait_ref;
137 if is_self_ty_used(unused_parameters, trait_ref.self_ty()) {
138 for ty in trait_ref.substs.types() {
139 debug!("unused_generic_params: (trait) ty={:?}", ty);
140 mark_ty(unused_parameters, ty);
144 ty::PredicateAtom::Projection(proj, ..) => {
145 let self_ty = proj.projection_ty.self_ty();
146 if is_self_ty_used(unused_parameters, self_ty) {
147 debug!("unused_generic_params: (projection ty={:?}", proj.ty);
148 mark_ty(unused_parameters, proj.ty);
156 /// Emit errors for the function annotated by `#[rustc_polymorphize_error]`, labelling each generic
157 /// parameter which was unused.
158 fn emit_unused_generic_params_error<'tcx>(
161 generics: &'tcx ty::Generics,
162 unused_parameters: &FiniteBitSet<u32>,
164 debug!("emit_unused_generic_params_error: def_id={:?}", def_id);
165 let base_def_id = tcx.closure_base_def_id(def_id);
166 if !tcx.get_attrs(base_def_id).iter().any(|a| a.check_name(sym::rustc_polymorphize_error)) {
170 debug!("emit_unused_generic_params_error: unused_parameters={:?}", unused_parameters);
171 let fn_span = match tcx.opt_item_name(def_id) {
172 Some(ident) => ident.span,
173 _ => tcx.def_span(def_id),
176 let mut err = tcx.sess.struct_span_err(fn_span, "item has unused generic parameters");
178 let mut next_generics = Some(generics);
179 while let Some(generics) = next_generics {
180 for param in &generics.params {
181 if unused_parameters.contains(param.index).unwrap_or(false) {
182 debug!("emit_unused_generic_params_error: param={:?}", param);
183 let def_span = tcx.def_span(param.def_id);
184 err.span_label(def_span, &format!("generic parameter `{}` is unused", param.name));
188 next_generics = generics.parent.map(|did| tcx.generics_of(did));
194 /// Visitor used to aggregate generic parameter uses.
195 struct UsedGenericParametersVisitor<'a, 'tcx> {
198 unused_parameters: &'a mut FiniteBitSet<u32>,
201 impl<'a, 'tcx> Visitor<'tcx> for UsedGenericParametersVisitor<'a, 'tcx> {
202 fn visit_local_decl(&mut self, local: Local, local_decl: &LocalDecl<'tcx>) {
203 debug!("visit_local_decl: local_decl={:?}", local_decl);
204 if local == Local::from_usize(1) {
205 let def_kind = self.tcx.def_kind(self.def_id);
206 if matches!(def_kind, DefKind::Closure | DefKind::Generator) {
207 // Skip visiting the closure/generator that is currently being processed. This only
208 // happens because the first argument to the closure is a reference to itself and
209 // that will call `visit_substs`, resulting in each generic parameter captured being
210 // considered used by default.
211 debug!("visit_local_decl: skipping closure substs");
216 self.super_local_decl(local, local_decl);
219 fn visit_const(&mut self, c: &&'tcx Const<'tcx>, _: Location) {
223 fn visit_ty(&mut self, ty: Ty<'tcx>, _: TyContext) {
228 impl<'a, 'tcx> TypeVisitor<'tcx> for UsedGenericParametersVisitor<'a, 'tcx> {
229 fn visit_const(&mut self, c: &'tcx Const<'tcx>) -> bool {
230 debug!("visit_const: c={:?}", c);
231 if !c.has_param_types_or_consts() {
236 ty::ConstKind::Param(param) => {
237 debug!("visit_const: param={:?}", param);
238 self.unused_parameters.clear(param.index);
241 _ => c.super_visit_with(self),
245 fn visit_ty(&mut self, ty: Ty<'tcx>) -> bool {
246 debug!("visit_ty: ty={:?}", ty);
247 if !ty.has_param_types_or_consts() {
252 ty::Closure(def_id, substs) | ty::Generator(def_id, substs, ..) => {
253 debug!("visit_ty: def_id={:?}", def_id);
254 // Avoid cycle errors with generators.
255 if def_id == self.def_id {
259 // Consider any generic parameters used by any closures/generators as used in the
261 let unused = self.tcx.unused_generic_params(def_id);
263 "visit_ty: unused_parameters={:?} unused={:?}",
264 self.unused_parameters, unused
266 for (i, arg) in substs.iter().enumerate() {
267 let i = i.try_into().unwrap();
268 if !unused.contains(i).unwrap_or(false) {
269 arg.visit_with(self);
272 debug!("visit_ty: unused_parameters={:?}", self.unused_parameters);
276 ty::Param(param) => {
277 debug!("visit_ty: param={:?}", param);
278 self.unused_parameters.clear(param.index);
281 _ => ty.super_visit_with(self),