rust: Desugar IfLet* into MatchExpr

Replace the "regular" AST->HIR lowering for IfLet* with a desugaring
into a MatchExpr.

Desugar a simple if let:

   if let Some(y) = some_value {
     bar();
   }

into:

   match some_value {
     Some(y) => {bar();},
     _ => ()
   }

Same applies for IfLetExprConseqElse (if let with an else block).

Desugar:

   if let Some(y) = some_value {
     bar();
   } else {
     baz();
   }

into:

   match some_value {
     Some(y) => {bar();},
     _ => {baz();}
   }

Fixes #1177

gcc/rust/ChangeLog:

	* backend/rust-compile-block.h: Adjust after removal of
	HIR::IfLetExpr and HIR::IfLetExprConseqElse.
	* backend/rust-compile-expr.h: Likewise.
	* checks/errors/borrowck/rust-bir-builder-expr-stmt.cc
	(ExprStmtBuilder::visit): Likewise.
	* checks/errors/borrowck/rust-bir-builder-expr-stmt.h: Likewise.
	* checks/errors/borrowck/rust-bir-builder-lazyboolexpr.h:
	Likewise.
	* checks/errors/borrowck/rust-bir-builder-struct.h: Likewise.
	* checks/errors/borrowck/rust-function-collector.h: Likewise.
	* checks/errors/privacy/rust-privacy-reporter.cc
	(PrivacyReporter::visit): Likewise.
	* checks/errors/privacy/rust-privacy-reporter.h: Likewise.
	* checks/errors/rust-const-checker.cc (ConstChecker::visit):
	Likewise.
	* checks/errors/rust-const-checker.h: Likewise.
	* checks/errors/rust-unsafe-checker.cc (UnsafeChecker::visit):
	Likewise.
	* checks/errors/rust-unsafe-checker.h: Likewise.
	* hir/rust-ast-lower-block.h (ASTLoweringIfLetBlock::translate):
	Change return type.
	* hir/rust-ast-lower.cc (ASTLoweringIfLetBlock::desugar_iflet):
	New.
	(ASTLoweringIfLetBlock::visit(AST::IfLetExpr &)): Adjust and use
	desugar_iflet.
	* hir/rust-ast-lower.h: Add comment.
	* hir/rust-hir-dump.cc (Dump::do_ifletexpr): Remove.
	(Dump::visit(IfLetExpr&)): Remove.
	(Dump::visit(IfLetExprConseqElse&)): Remove.
	* hir/rust-hir-dump.h (Dump::do_ifletexpr): Remove.
	(Dump::visit(IfLetExpr&)): Remove.
	(Dump::visit(IfLetExprConseqElse&)): Remove.
	* hir/tree/rust-hir-expr.h (class IfLetExpr): Remove.
	(class IfLetExprConseqElse): Remove.
	* hir/tree/rust-hir-full-decls.h (class IfLetExpr): Remove.
	(class IfLetExprConseqElse): Remove.
	* hir/tree/rust-hir-visitor.h: Adjust after removal of
	HIR::IfLetExpr and HIR::IfLetExprConseqElse.
	* hir/tree/rust-hir.cc (IfLetExpr::as_string): Remove.
	(IfLetExprConseqElse::as_string): Remove.
	(IfLetExpr::accept_vis): Remove.
	(IfLetExprConseqElse::accept_vis): Remove.
	* hir/tree/rust-hir.h: Adjust after removal of HIR::IfLetExpr and
	HIR::IfLetExprConseqElse.
	* typecheck/rust-hir-type-check-expr.cc (TypeCheckExpr::visit):
	Likewise.
	* typecheck/rust-hir-type-check-expr.h: Likewise.

gcc/testsuite/ChangeLog:

	* rust/compile/if_let_expr.rs: Adjust.
	* rust/compile/if_let_expr_simple.rs: New test.
	* rust/compile/iflet.rs: New test.
	* rust/execute/torture/iflet.rs: New test.

Signed-off-by: Marc Poulhiès <dkm@kataplop.net>

gcc/rust/backend/rust-compile-block.h

@@ -96,8 +96,6 @@ class CompileConditionalBlocks : public HIRCompileBase,
   void visit (HIR::LoopExpr &) override {}
   void visit (HIR::WhileLoopExpr &) override {}
   void visit (HIR::WhileLetLoopExpr &) override {}
-  void visit (HIR::IfLetExpr &) override {}
-  void visit (HIR::IfLetExprConseqElse &) override {}
   void visit (HIR::MatchExpr &) override {}
   void visit (HIR::AwaitExpr &) override {}
   void visit (HIR::AsyncBlockExpr &) override {}
@@ -180,8 +178,6 @@ class CompileExprWithBlock : public HIRCompileBase,
   void visit (HIR::LoopExpr &) override {}
   void visit (HIR::WhileLoopExpr &) override {}
   void visit (HIR::WhileLetLoopExpr &) override {}
-  void visit (HIR::IfLetExpr &) override {}
-  void visit (HIR::IfLetExprConseqElse &) override {}
   void visit (HIR::MatchExpr &) override {}
   void visit (HIR::AwaitExpr &) override {}
   void visit (HIR::AsyncBlockExpr &) override {}

gcc/rust/backend/rust-compile-expr.h

@@ -77,8 +77,6 @@ class CompileExpr : private HIRCompileBase, protected HIR::HIRExpressionVisitor
   // TODO
   // these need to be sugared in the HIR to if statements and a match
   void visit (HIR::WhileLetLoopExpr &) override {}
-  void visit (HIR::IfLetExpr &) override {}
-  void visit (HIR::IfLetExprConseqElse &) override {}
 
   // lets not worry about async yet....
   void visit (HIR::AwaitExpr &) override {}

gcc/rust/checks/errors/borrowck/rust-bir-builder-expr-stmt.cc

@@ -611,18 +611,6 @@ ExprStmtBuilder::visit (HIR::IfExprConseqElse &expr)
     add_jump (else_end_bb, final_start_bb);
 }
 
-void
-ExprStmtBuilder::visit (HIR::IfLetExpr &expr)
-{
-  rust_sorry_at (expr.get_locus (), "if let expressions are not supported");
-}
-
-void
-ExprStmtBuilder::visit (HIR::IfLetExprConseqElse &expr)
-{
-  rust_sorry_at (expr.get_locus (), "if let expressions are not supported");
-}
-
 void
 ExprStmtBuilder::visit (HIR::MatchExpr &expr)
 {

gcc/rust/checks/errors/borrowck/rust-bir-builder-expr-stmt.h

@@ -101,8 +101,6 @@ class ExprStmtBuilder final : public AbstractExprBuilder,
   void visit (HIR::IfExprConseqElse &expr) override;
   void visit (HIR::InlineAsm &expr) override;
 
-  void visit (HIR::IfLetExpr &expr) override;
-  void visit (HIR::IfLetExprConseqElse &expr) override;
   void visit (HIR::MatchExpr &expr) override;
   void visit (HIR::AwaitExpr &expr) override;
   void visit (HIR::AsyncBlockExpr &expr) override;

gcc/rust/checks/errors/borrowck/rust-bir-builder-lazyboolexpr.h

@@ -193,14 +193,6 @@ class LazyBooleanExprBuilder : public AbstractExprBuilder
   {
     return_place (ExprStmtBuilder (ctx).build (expr), expr.get_locus ());
   }
-  void visit (HIR::IfLetExpr &expr) override
-  {
-    return_place (ExprStmtBuilder (ctx).build (expr), expr.get_locus ());
-  }
-  void visit (HIR::IfLetExprConseqElse &expr) override
-  {
-    return_place (ExprStmtBuilder (ctx).build (expr), expr.get_locus ());
-  }
   void visit (HIR::MatchExpr &expr) override
   {
     return_place (ExprStmtBuilder (ctx).build (expr), expr.get_locus ());

gcc/rust/checks/errors/borrowck/rust-bir-builder-struct.h

@@ -149,8 +149,6 @@ class StructBuilder : public AbstractBuilder, public HIR::HIRFullVisitor
   void visit (HIR::WhileLetLoopExpr &expr) override { rust_unreachable (); }
   void visit (HIR::IfExpr &expr) override { rust_unreachable (); }
   void visit (HIR::IfExprConseqElse &expr) override { rust_unreachable (); }
-  void visit (HIR::IfLetExpr &expr) override { rust_unreachable (); }
-  void visit (HIR::IfLetExprConseqElse &expr) override { rust_unreachable (); }
   void visit (HIR::MatchExpr &expr) override { rust_unreachable (); }
   void visit (HIR::AwaitExpr &expr) override { rust_unreachable (); }
   void visit (HIR::AsyncBlockExpr &expr) override { rust_unreachable (); }

gcc/rust/checks/errors/borrowck/rust-function-collector.h

@@ -119,8 +119,6 @@ class FunctionCollector : public HIR::HIRFullVisitor
   void visit (HIR::WhileLetLoopExpr &expr) override {}
   void visit (HIR::IfExpr &expr) override {}
   void visit (HIR::IfExprConseqElse &expr) override {}
-  void visit (HIR::IfLetExpr &expr) override {}
-  void visit (HIR::IfLetExprConseqElse &expr) override {}
   void visit (HIR::MatchExpr &expr) override {}
   void visit (HIR::AwaitExpr &expr) override {}
   void visit (HIR::AsyncBlockExpr &expr) override {}

gcc/rust/checks/errors/privacy/rust-privacy-reporter.cc

@@ -594,21 +594,6 @@ PrivacyReporter::visit (HIR::IfExprConseqElse &expr)
   expr.get_else_block ()->accept_vis (*this);
 }
 
-void
-PrivacyReporter::visit (HIR::IfLetExpr &)
-{
-  // TODO: We need to visit the if_let_expr
-  // TODO: We need to visit the block as well
-}
-
-void
-PrivacyReporter::visit (HIR::IfLetExprConseqElse &)
-{
-  // TODO: We need to visit the if_let_expr
-  // TODO: We need to visit the if_block as well
-  // TODO: We need to visit the else_block as well
-}
-
 void
 PrivacyReporter::visit (HIR::MatchExpr &expr)
 {

gcc/rust/checks/errors/privacy/rust-privacy-reporter.h

@@ -121,8 +121,6 @@ types
   virtual void visit (HIR::WhileLetLoopExpr &expr);
   virtual void visit (HIR::IfExpr &expr);
   virtual void visit (HIR::IfExprConseqElse &expr);
-  virtual void visit (HIR::IfLetExpr &expr);
-  virtual void visit (HIR::IfLetExprConseqElse &expr);
   virtual void visit (HIR::MatchExpr &expr);
   virtual void visit (HIR::AwaitExpr &expr);
   virtual void visit (HIR::AsyncBlockExpr &expr);

gcc/rust/checks/errors/rust-const-checker.cc

@@ -497,22 +497,6 @@ ConstChecker::visit (IfExprConseqElse &expr)
   expr.get_else_block ()->accept_vis (*this);
 }
 
-void
-ConstChecker::visit (IfLetExpr &expr)
-{
-  expr.get_scrutinee_expr ()->accept_vis (*this);
-  expr.get_if_block ()->accept_vis (*this);
-}
-
-void
-ConstChecker::visit (IfLetExprConseqElse &expr)
-{
-  expr.get_scrutinee_expr ()->accept_vis (*this);
-  expr.get_if_block ()->accept_vis (*this);
-
-  // TODO: Visit else expression
-}
-
 void
 ConstChecker::visit (MatchExpr &expr)
 {

gcc/rust/checks/errors/rust-const-checker.h

@@ -128,8 +128,6 @@ class ConstChecker : public HIRFullVisitor
   virtual void visit (WhileLetLoopExpr &expr) override;
   virtual void visit (IfExpr &expr) override;
   virtual void visit (IfExprConseqElse &expr) override;
-  virtual void visit (IfLetExpr &expr) override;
-  virtual void visit (IfLetExprConseqElse &expr) override;
   virtual void visit (MatchExpr &expr) override;
   virtual void visit (AwaitExpr &expr) override;
   virtual void visit (AsyncBlockExpr &expr) override;

gcc/rust/checks/errors/rust-unsafe-checker.cc

@@ -599,22 +599,6 @@ UnsafeChecker::visit (IfExprConseqElse &expr)
   expr.get_else_block ()->accept_vis (*this);
 }
 
-void
-UnsafeChecker::visit (IfLetExpr &expr)
-{
-  expr.get_scrutinee_expr ()->accept_vis (*this);
-  expr.get_if_block ()->accept_vis (*this);
-}
-
-void
-UnsafeChecker::visit (IfLetExprConseqElse &expr)
-{
-  expr.get_scrutinee_expr ()->accept_vis (*this);
-  expr.get_if_block ()->accept_vis (*this);
-
-  // TODO: Visit else expression
-}
-
 void
 UnsafeChecker::visit (MatchExpr &expr)
 {

gcc/rust/checks/errors/rust-unsafe-checker.h

@@ -110,8 +110,6 @@ class UnsafeChecker : public HIRFullVisitor
   virtual void visit (WhileLetLoopExpr &expr) override;
   virtual void visit (IfExpr &expr) override;
   virtual void visit (IfExprConseqElse &expr) override;
-  virtual void visit (IfLetExpr &expr) override;
-  virtual void visit (IfLetExprConseqElse &expr) override;
   virtual void visit (MatchExpr &expr) override;
   virtual void visit (AwaitExpr &expr) override;
   virtual void visit (AsyncBlockExpr &expr) override;

gcc/rust/hir/rust-ast-lower-block.h

@@ -115,7 +115,7 @@ class ASTLoweringIfLetBlock : public ASTLoweringBase
   using Rust::HIR::ASTLoweringBase::visit;
 
 public:
-  static HIR::IfLetExpr *translate (AST::IfLetExpr &expr)
+  static HIR::MatchExpr *translate (AST::IfLetExpr &expr)
   {
     ASTLoweringIfLetBlock resolver;
     expr.accept_vis (resolver);
@@ -135,7 +135,10 @@ class ASTLoweringIfLetBlock : public ASTLoweringBase
 private:
   ASTLoweringIfLetBlock () : ASTLoweringBase (), translated (nullptr) {}
 
-  HIR::IfLetExpr *translated;
+  void desugar_iflet (AST::IfLetExpr &, HIR::Expr **,
+		      std::vector<HIR::MatchCase> &);
+
+  HIR::MatchExpr *translated;
 };
 
 class ASTLoweringExprWithBlock : public ASTLoweringBase
@@ -149,9 +152,7 @@ class ASTLoweringExprWithBlock : public ASTLoweringBase
     ASTLoweringExprWithBlock resolver;
     expr.accept_vis (resolver);
     if (resolver.translated != nullptr)
-      {
-	resolver.mappings.insert_hir_expr (resolver.translated);
-      }
+      resolver.mappings.insert_hir_expr (resolver.translated);
 
     *terminated = resolver.terminated;
     return resolver.translated;

gcc/rust/hir/rust-ast-lower.cc

@@ -24,6 +24,8 @@
 #include "rust-ast-lower-type.h"
 #include "rust-ast-lower-pattern.h"
 #include "rust-ast-lower-struct-field-expr.h"
+#include "rust-expr.h"
+#include "rust-hir-expr.h"
 
 namespace Rust {
 namespace HIR {
@@ -198,62 +200,132 @@ ASTLoweringIfBlock::visit (AST::IfExprConseqElse &expr)
     std::unique_ptr<HIR::ExprWithBlock> (else_block), expr.get_locus ());
 }
 
+/**
+ *  Lowers the common part "if let 'pattern' = 'expr' { 'if_block' }" of
+ *  IfLetExpr[ConseqElse]:
+ *  - 'expr' is lowered into *BRANCH_VALUE
+ *  - 'pattern' + 'if_block' are lowered and resulting ARM pushed in MATCH_ARMS
+ *
+ *  Looks like:
+ *
+ *  match (expr) {
+ *    pattern => {if_block}
+ *  }
+ */
 void
-ASTLoweringIfLetBlock::visit (AST::IfLetExpr &expr)
+ASTLoweringIfLetBlock::desugar_iflet (AST::IfLetExpr &expr,
+				      HIR::Expr **branch_value,
+				      std::vector<HIR::MatchCase> &match_arms)
 {
-  std::vector<std::unique_ptr<HIR::Pattern>> patterns;
+  HIR::Expr *kase_expr;
+  std::vector<std::unique_ptr<HIR::Pattern>> match_arm_patterns;
+
+  *branch_value = ASTLoweringExpr::translate (expr.get_value_expr ());
+  kase_expr = ASTLoweringExpr::translate (expr.get_if_block ());
+
+  // FIXME: if let only accepts a single pattern. Why do we have a vector of
+  // patterns in the IfLet?
   for (auto &pattern : expr.get_patterns ())
     {
       HIR::Pattern *ptrn = ASTLoweringPattern::translate (*pattern);
-      patterns.push_back (std::unique_ptr<HIR::Pattern> (ptrn));
+      match_arm_patterns.push_back (std::unique_ptr<HIR::Pattern> (ptrn));
     }
-  HIR::Expr *value_ptr = ASTLoweringExpr::translate (expr.get_value_expr ());
 
-  bool ignored_terminated = false;
-  HIR::BlockExpr *block
-    = ASTLoweringBlock::translate (expr.get_if_block (), &ignored_terminated);
+  HIR::MatchArm arm (std::move (match_arm_patterns), expr.get_locus (), nullptr,
+		     {});
 
   auto crate_num = mappings.get_current_crate ();
   Analysis::NodeMapping mapping (crate_num, expr.get_node_id (),
 				 mappings.get_next_hir_id (crate_num),
 				 UNKNOWN_LOCAL_DEFID);
 
-  translated = new HIR::IfLetExpr (mapping, std::move (patterns),
-				   std::unique_ptr<HIR::Expr> (value_ptr),
-				   std::unique_ptr<HIR::BlockExpr> (block),
-				   expr.get_locus ());
+  HIR::MatchCase kase (std::move (mapping), std::move (arm),
+		       std::unique_ptr<HIR::Expr> (kase_expr));
+  match_arms.push_back (std::move (kase));
 }
 
 void
-ASTLoweringIfLetBlock::visit (AST::IfLetExprConseqElse &expr)
+ASTLoweringIfLetBlock::visit (AST::IfLetExpr &expr)
 {
-  std::vector<std::unique_ptr<HIR::Pattern>> patterns;
-  for (auto &pattern : expr.get_patterns ())
-    {
-      HIR::Pattern *ptrn = ASTLoweringPattern::translate (*pattern);
-      patterns.push_back (std::unique_ptr<HIR::Pattern> (ptrn));
-    }
-  HIR::Expr *value_ptr = ASTLoweringExpr::translate (expr.get_value_expr ());
+  // Desugar:
+  //   if let Some(y) = some_value {
+  //     bar();
+  //   }
+  //
+  //   into:
+  //
+  //   match some_value {
+  //     Some(y) => {bar();},
+  //     _ => ()
+  //   }
+
+  HIR::Expr *branch_value;
 
-  bool ignored_terminated = false;
-  HIR::BlockExpr *block
-    = ASTLoweringBlock::translate (expr.get_if_block (), &ignored_terminated);
+  std::vector<HIR::MatchCase> match_arms;
+  desugar_iflet (expr, &branch_value, match_arms);
 
-  HIR::ExprWithBlock *else_block
-    = ASTLoweringExprWithBlock::translate (expr.get_else_block (),
-					   &ignored_terminated);
+  auto crate_num = mappings.get_current_crate ();
+  Analysis::NodeMapping mapping (crate_num, expr.get_node_id (),
+				 mappings.get_next_hir_id (crate_num),
+				 UNKNOWN_LOCAL_DEFID);
+
+  translated
+    = new HIR::MatchExpr (mapping, std::unique_ptr<HIR::Expr> (branch_value),
+			  std::move (match_arms), {}, {}, expr.get_locus ());
+}
+
+void
+ASTLoweringIfLetBlock::visit (AST::IfLetExprConseqElse &expr)
+{
+  // desugar:
+  //   if let Some(y) = some_value {
+  //     bar();
+  //   } else {
+  //     baz();
+  //   }
+  //
+  //   into
+  //   match some_value {
+  //     Some(y) => {bar();},
+  //     _ => {baz();}
+  //   }
+  //
+
+  HIR::Expr *branch_value;
+  std::vector<HIR::MatchCase> match_arms;
 
-  rust_assert (else_block);
+  desugar_iflet (expr, &branch_value, match_arms);
 
   auto crate_num = mappings.get_current_crate ();
   Analysis::NodeMapping mapping (crate_num, expr.get_node_id (),
 				 mappings.get_next_hir_id (crate_num),
 				 UNKNOWN_LOCAL_DEFID);
 
-  translated = new HIR::IfLetExprConseqElse (
-    mapping, std::move (patterns), std::unique_ptr<HIR::Expr> (value_ptr),
-    std::unique_ptr<HIR::BlockExpr> (block),
-    std::unique_ptr<HIR::ExprWithBlock> (else_block), expr.get_locus ());
+  std::vector<std::unique_ptr<HIR::Pattern>> match_arm_patterns_wildcard;
+  Analysis::NodeMapping mapping_default (crate_num, expr.get_node_id (),
+					 mappings.get_next_hir_id (crate_num),
+					 UNKNOWN_LOCAL_DEFID);
+
+  std::unique_ptr<HIR::WildcardPattern> wc
+    = std::unique_ptr<HIR::WildcardPattern> (
+      new HIR::WildcardPattern (mapping_default, expr.get_locus ()));
+
+  match_arm_patterns_wildcard.push_back (std::move (wc));
+
+  HIR::MatchArm arm_default (std::move (match_arm_patterns_wildcard),
+			     expr.get_locus (), nullptr, {});
+
+  HIR::Expr *kase_else_expr
+    = ASTLoweringExpr::translate (expr.get_else_block ());
+
+  HIR::MatchCase kase_else (std::move (mapping_default),
+			    std::move (arm_default),
+			    std::unique_ptr<HIR::Expr> (kase_else_expr));
+  match_arms.push_back (std::move (kase_else));
+
+  translated
+    = new HIR::MatchExpr (mapping, std::unique_ptr<HIR::Expr> (branch_value),
+			  std::move (match_arms), {}, {}, expr.get_locus ());
 }
 
 // rust-ast-lower-struct-field-expr.h

gcc/rust/hir/rust-ast-lower.h

@@ -39,6 +39,11 @@ struct_field_name_exists (std::vector<HIR::StructField> &fields,
 Visibility
 translate_visibility (const AST::Visibility &vis);
 
+/**
+ * Main base class used for lowering AST to HIR.
+ *
+ * Every subclass should provide a translate() method that takes an AST node and
+ * lowers it to some HIR stored in the TRANSLATED member. */
 class ASTLowering
 {
 public: