Change `DeductionWorklist.context_` from reference to pointer (#5204)

Per [the style
guide](https://github.com/carbon-language/carbon-lang/blob/trunk/docs/project/cpp_style_guide.md#syntax-and-formatting):
* If it is captured and must outlive the call expression itself, use a
pointer and document that it must not be null (unless it is also
optional).
* When storing an object's address as a non-owned member, prefer storing
a pointer.

toolchain/check/deduce.cpp

@@ -26,7 +26,8 @@ namespace {
 // matter except when it influences which error is diagnosed.
 class DeductionWorklist {
  public:
-  explicit DeductionWorklist(Context& context) : context_(context) {}
+  // `context` must not be null.
+  explicit DeductionWorklist(Context* context) : context_(context) {}
 
   struct PendingDeduction {
     SemIR::InstId param;
@@ -44,7 +45,7 @@ class DeductionWorklist {
   // Adds a single (param, arg) type deduction.
   auto Add(SemIR::TypeId param, SemIR::TypeId arg, bool needs_substitution)
       -> void {
-    Add(context_.types().GetInstId(param), context_.types().GetInstId(arg),
+    Add(context_->types().GetInstId(param), context_->types().GetInstId(arg),
         needs_substitution);
   }
 
@@ -54,8 +55,8 @@ class DeductionWorklist {
     if (!param.has_value() || !arg.has_value()) {
       return;
     }
-    auto& param_specific = context_.specifics().Get(param);
-    auto& arg_specific = context_.specifics().Get(arg);
+    auto& param_specific = context_->specifics().Get(param);
+    auto& arg_specific = context_->specifics().Get(arg);
     if (param_specific.generic_id != arg_specific.generic_id) {
       // TODO: Decide whether to error on this or just treat the specific as
       // non-deduced. For now we treat it as non-deduced.
@@ -81,13 +82,13 @@ class DeductionWorklist {
 
   auto AddAll(SemIR::InstBlockId params, llvm::ArrayRef<SemIR::InstId> args,
               bool needs_substitution) -> void {
-    AddAll(context_.inst_blocks().Get(params), args, needs_substitution);
+    AddAll(context_->inst_blocks().Get(params), args, needs_substitution);
   }
 
   auto AddAll(SemIR::StructTypeFieldsId params, SemIR::StructTypeFieldsId args,
               bool needs_substitution) -> void {
-    const auto& param_fields = context_.struct_type_fields().Get(params);
-    const auto& arg_fields = context_.struct_type_fields().Get(args);
+    const auto& param_fields = context_->struct_type_fields().Get(params);
+    const auto& arg_fields = context_->struct_type_fields().Get(args);
     if (param_fields.size() != arg_fields.size()) {
       // TODO: Decide whether to error on this or just treat the parameter list
       // as non-deduced. For now we treat it as non-deduced.
@@ -108,21 +109,21 @@ class DeductionWorklist {
 
   auto AddAll(SemIR::InstBlockId params, SemIR::InstBlockId args,
               bool needs_substitution) -> void {
-    AddAll(context_.inst_blocks().Get(params), context_.inst_blocks().Get(args),
-           needs_substitution);
+    AddAll(context_->inst_blocks().Get(params),
+           context_->inst_blocks().Get(args), needs_substitution);
   }
 
   auto AddAll(SemIR::TypeBlockId params, SemIR::TypeBlockId args,
               bool needs_substitution) -> void {
-    AddAll(context_.type_blocks().Get(params), context_.type_blocks().Get(args),
-           needs_substitution);
+    AddAll(context_->type_blocks().Get(params),
+           context_->type_blocks().Get(args), needs_substitution);
   }
 
   auto AddAll(SemIR::FacetTypeId params, SemIR::FacetTypeId args,
               bool needs_substitution) -> void {
     const auto& param_impls =
-        context_.facet_types().Get(params).impls_constraints;
-    const auto& arg_impls = context_.facet_types().Get(args).impls_constraints;
+        context_->facet_types().Get(params).impls_constraints;
+    const auto& arg_impls = context_->facet_types().Get(args).impls_constraints;
     // TODO: Decide whether to error on these or just treat the parameter list
     // as non-deduced. For now we treat it as non-deduced.
     if (param_impls.size() != 1 || arg_impls.size() != 1) {
@@ -182,7 +183,7 @@ class DeductionWorklist {
   auto PopNext() -> PendingDeduction { return deductions_.pop_back_val(); }
 
  private:
-  Context& context_;
+  Context* context_;
   llvm::SmallVector<PendingDeduction> deductions_;
 };
 
@@ -191,8 +192,8 @@ class DeductionContext {
  public:
   // Preparse to perform deduction. If an enclosing specific or self type
   // are provided, adds the corresponding arguments as known arguments that will
-  // not be deduced.
-  DeductionContext(Context& context, SemIR::LocId loc_id,
+  // not be deduced. `context` must not be null.
+  DeductionContext(Context* context, SemIR::LocId loc_id,
                    SemIR::GenericId generic_id,
                    SemIR::SpecificId enclosing_specific_id,
                    SemIR::InstId self_type_id, bool diagnose);
@@ -262,11 +263,11 @@ static auto NoteGenericHere(Context& context, SemIR::GenericId generic_id,
   diag.Note(context.generics().Get(generic_id).decl_id, DeductionGenericHere);
 }
 
-DeductionContext::DeductionContext(Context& context, SemIR::LocId loc_id,
+DeductionContext::DeductionContext(Context* context, SemIR::LocId loc_id,
                                    SemIR::GenericId generic_id,
                                    SemIR::SpecificId enclosing_specific_id,
                                    SemIR::InstId self_type_id, bool diagnose)
-    : context_(&context),
+    : context_(context),
       loc_id_(loc_id),
       generic_id_(generic_id),
       diagnose_(diagnose),
@@ -277,16 +278,16 @@ DeductionContext::DeductionContext(Context& context, SemIR::LocId loc_id,
 
   // Initialize the deduced arguments to `None`.
   result_arg_ids_.resize(
-      context.inst_blocks()
-          .Get(context.generics().Get(generic_id_).bindings_id)
+      context->inst_blocks()
+          .Get(context->generics().Get(generic_id_).bindings_id)
           .size(),
       SemIR::InstId::None);
 
   if (enclosing_specific_id.has_value()) {
     // Copy any outer generic arguments from the specified instance and prepare
     // to substitute them into the function declaration.
-    auto args = context.inst_blocks().Get(
-        context.specifics().Get(enclosing_specific_id).args_id);
+    auto args = context->inst_blocks().Get(
+        context->specifics().Get(enclosing_specific_id).args_id);
     llvm::copy(args, result_arg_ids_.begin());
 
     // TODO: Subst is linear in the length of the substitutions list. Change
@@ -295,7 +296,7 @@ DeductionContext::DeductionContext(Context& context, SemIR::LocId loc_id,
     for (auto [i, subst_inst_id] : llvm::enumerate(args)) {
       substitutions_.push_back(
           {.bind_id = SemIR::CompileTimeBindIndex(i),
-           .replacement_id = context.constant_values().Get(subst_inst_id)});
+           .replacement_id = context->constant_values().Get(subst_inst_id)});
     }
     first_deduced_index_ = SemIR::CompileTimeBindIndex(args.size());
   }
@@ -306,7 +307,7 @@ DeductionContext::DeductionContext(Context& context, SemIR::LocId loc_id,
     result_arg_ids_[self_index.index] = self_type_id;
     substitutions_.push_back(
         {.bind_id = SemIR::CompileTimeBindIndex(self_index),
-         .replacement_id = context.constant_values().Get(self_type_id)});
+         .replacement_id = context->constant_values().Get(self_type_id)});
     first_deduced_index_ = SemIR::CompileTimeBindIndex(self_index.index + 1);
   }
 
@@ -626,8 +627,9 @@ auto DeduceGenericCallArguments(
     [[maybe_unused]] SemIR::InstBlockId implicit_params_id,
     SemIR::InstBlockId params_id, [[maybe_unused]] SemIR::InstId self_id,
     llvm::ArrayRef<SemIR::InstId> arg_ids) -> SemIR::SpecificId {
-  DeductionContext deduction(context, loc_id, generic_id, enclosing_specific_id,
-                             self_type_id, /*diagnose=*/true);
+  DeductionContext deduction(&context, loc_id, generic_id,
+                             enclosing_specific_id, self_type_id,
+                             /*diagnose=*/true);
 
   // Prepare to perform deduction of the explicit parameters against their
   // arguments.
@@ -645,7 +647,7 @@ auto DeduceImplArguments(Context& context, SemIR::LocId loc_id, DeduceImpl impl,
                          SemIR::ConstantId self_id,
                          SemIR::SpecificId constraint_specific_id)
     -> SemIR::SpecificId {
-  DeductionContext deduction(context, loc_id, impl.generic_id,
+  DeductionContext deduction(&context, loc_id, impl.generic_id,
                              /*enclosing_specific_id=*/SemIR::SpecificId::None,
                              /*self_type_id=*/SemIR::InstId::None,
                              /*diagnose=*/false);