Model var as a pattern operator (#4720)

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/check/full_pattern_stack.h

@@ -0,0 +1,82 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef CARBON_TOOLCHAIN_CHECK_FULL_PATTERN_STACK_H_
+#define CARBON_TOOLCHAIN_CHECK_FULL_PATTERN_STACK_H_
+
+#include "common/array_stack.h"
+#include "common/check.h"
+#include "toolchain/check/lexical_lookup.h"
+#include "toolchain/sem_ir/ids.h"
+
+namespace Carbon::Check {
+
+// Stack of full-patterns currently being checked. When a pattern
+// is followed by an explicit initializer, name bindings should not be used
+// within that initializer, although they are usable before it (within the
+// pattern) and after it. This class keeps track of those state transitions.
+// It is structured as a stack to handle situations like a pattern that
+// contains an initializer, or a pattern in a lambda in an expression pattern.
+//
+// TODO: Unify this with Context::pattern_block_stack, or differentiate them
+// more clearly (and consider unifying this with ScopeStack instead).
+class FullPatternStack {
+ public:
+  explicit FullPatternStack(LexicalLookup* lookup) : lookup_(lookup) {}
+
+  // Marks the possible start of a new full-pattern (i.e. a pattern which occurs
+  // in a non-pattern context).
+  auto PushFullPattern() -> void { bind_name_stack_.PushArray(); }
+
+  // Marks the start of the initializer for the full-pattern at the top of the
+  // stack.
+  auto StartPatternInitializer() -> void {
+    for (auto& [name_id, inst_id] : bind_name_stack_.PeekArray()) {
+      CARBON_CHECK(inst_id == SemIR::InstId::InitTombstone);
+      auto& lookup_result = lookup_->Get(name_id);
+      if (!lookup_result.empty()) {
+        // TODO: find a way to preserve location information, so that we can
+        // provide good diagnostics for a redeclaration of `name_id` in
+        // the initializer, if that becomes possible.
+        std::swap(lookup_result.back().inst_id, inst_id);
+      }
+    }
+  }
+
+  // Marks the end of the initializer for the full-pattern at the top of the
+  // stack.
+  auto EndPatternInitializer() -> void {
+    for (auto& [name_id, inst_id] : bind_name_stack_.PeekArray()) {
+      auto& lookup_result = lookup_->Get(name_id);
+      if (!lookup_result.empty()) {
+        std::swap(lookup_result.back().inst_id, inst_id);
+      }
+      CARBON_CHECK(inst_id == SemIR::InstId::InitTombstone);
+    }
+  }
+
+  // Marks the end of checking for the full-pattern at the top of the stack.
+  // This cannot be called while processing an initializer for the top
+  // pattern.
+  auto PopFullPattern() -> void { bind_name_stack_.PopArray(); }
+
+  // Records that `bind_inst_id` was introduced by the full-pattern at the
+  // top of the stack.
+  auto AddBindName(SemIR::NameId name_id) -> void {
+    bind_name_stack_.AppendToTop(
+        {.name_id = name_id, .inst_id = SemIR::InstId::InitTombstone});
+  }
+
+ private:
+  LexicalLookup* lookup_;
+  struct LookupEntry {
+    SemIR::NameId name_id;
+    SemIR::InstId inst_id;
+  };
+  ArrayStack<LookupEntry> bind_name_stack_;
+};
+
+}  // namespace Carbon::Check
+
+#endif  // CARBON_TOOLCHAIN_CHECK_FULL_PATTERN_STACK_H_