Debugging quality-of-life improvements (#810)

* Debugging quality-of-life improvements

- Use std::abort for `CHECK`/`FATAL` failures, which acts as a debugger breakpoint as well as automatically printing a stack trace.
- Re-enable printing continuations in `--trace` mode.
- Log the source location of each step in `--trace` mode.

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

executable_semantics/interpreter/BUILD

@@ -4,18 +4,31 @@
 
 package(default_visibility = ["//executable_semantics:__pkg__"])
 
+# These currently have to be a single build rule because of a dependency cycle
+# in printing.
 cc_library(
-    name = "action",
-    srcs = ["action.cpp"],
-    hdrs = ["action.h"],
+    name = "action_frame_and_value",
+    srcs = [
+        "action.cpp",
+        "frame.cpp",
+        "value.cpp",
+    ],
+    hdrs = [
+        "action.h",
+        "frame.h",
+        "value.h",
+    ],
     deps = [
+        ":address",
+        ":dictionary",
+        ":field_path",
         ":stack",
-        ":value",
         "//common:ostream",
         "//executable_semantics/ast:expression",
         "//executable_semantics/ast:function_definition",
         "//executable_semantics/ast:statement",
         "//executable_semantics/common:arena",
+        "//executable_semantics/common:error",
         "@llvm-project//llvm:Support",
     ],
 )
@@ -56,27 +69,13 @@ cc_library(
     ],
 )
 
-cc_library(
-    name = "frame",
-    srcs = ["frame.cpp"],
-    hdrs = ["frame.h"],
-    deps = [
-        ":action",
-        ":address",
-        ":dictionary",
-        ":stack",
-        "//common:ostream",
-        "@llvm-project//llvm:Support",
-    ],
-)
-
 cc_library(
     name = "heap",
     srcs = ["heap.cpp"],
     hdrs = ["heap.h"],
     deps = [
+        ":action_frame_and_value",
         ":address",
-        ":value",
         "//common:ostream",
         "@llvm-project//llvm:Support",
     ],
@@ -91,11 +90,9 @@ cc_library(
         "interpreter.h",
     ],
     deps = [
-        ":action",
+        ":action_frame_and_value",
         ":address",
-        ":frame",
         ":heap",
-        ":value",
         "//common:check",
         "//common:ostream",
         "//executable_semantics/ast:declaration",
@@ -129,20 +126,3 @@ cc_library(
         "@llvm-project//llvm:Support",
     ],
 )
-
-cc_library(
-    name = "value",
-    srcs = ["value.cpp"],
-    hdrs = ["value.h"],
-    deps = [
-        ":address",
-        ":field_path",
-        ":stack",
-        "//common:ostream",
-        "//executable_semantics/ast:function_definition",
-        "//executable_semantics/ast:statement",
-        "//executable_semantics/common:arena",
-        "//executable_semantics/common:error",
-        "@llvm-project//llvm:Support",
-    ],
-)

executable_semantics/interpreter/interpreter.cpp

@@ -362,7 +362,8 @@ auto Interpreter::StepLvalue() -> Transition {
   Ptr<Action> act = stack.Top()->todo.Top();
   Ptr<const Expression> exp = cast<LValAction>(*act).Exp();
   if (tracing_output) {
-    llvm::outs() << "--- step lvalue " << *exp << " --->\n";
+    llvm::outs() << "--- step lvalue " << *exp << " (" << exp->SourceLoc()
+                 << ") --->\n";
   }
   switch (exp->Tag()) {
     case Expression::Kind::IdentifierExpression: {
@@ -449,7 +450,8 @@ auto Interpreter::StepExp() -> Transition {
   Ptr<Action> act = stack.Top()->todo.Top();
   Ptr<const Expression> exp = cast<ExpressionAction>(*act).Exp();
   if (tracing_output) {
-    llvm::outs() << "--- step exp " << *exp << " --->\n";
+    llvm::outs() << "--- step exp " << *exp << " (" << exp->SourceLoc()
+                 << ") --->\n";
   }
   switch (exp->Tag()) {
     case Expression::Kind::IndexExpression: {
@@ -647,7 +649,8 @@ auto Interpreter::StepPattern() -> Transition {
   Ptr<Action> act = stack.Top()->todo.Top();
   Ptr<const Pattern> pattern = cast<PatternAction>(*act).Pat();
   if (tracing_output) {
-    llvm::outs() << "--- step pattern " << *pattern << " --->\n";
+    llvm::outs() << "--- step pattern " << *pattern << " ("
+                 << pattern->SourceLoc() << ") --->\n";
   }
   switch (pattern->Tag()) {
     case Pattern::Kind::AutoPattern: {
@@ -745,7 +748,7 @@ auto Interpreter::StepStmt() -> Transition {
   if (tracing_output) {
     llvm::outs() << "--- step stmt ";
     stmt->PrintDepth(1, llvm::outs());
-    llvm::outs() << " --->\n";
+    llvm::outs() << " (" << stmt->SourceLoc() << ") --->\n";
   }
   switch (stmt->Tag()) {
     case Statement::Kind::Match: {

executable_semantics/interpreter/value.cpp

@@ -9,6 +9,7 @@
 #include "common/check.h"
 #include "executable_semantics/common/arena.h"
 #include "executable_semantics/common/error.h"
+#include "executable_semantics/interpreter/frame.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/Support/Casting.h"
 
@@ -232,11 +233,15 @@ void Value::Print(llvm::raw_ostream& out) const {
     case Value::Kind::VariableType:
       out << cast<VariableType>(*this).Name();
       break;
-    case Value::Kind::ContinuationValue:
-      out << "continuation";
-      // TODO: Find a way to print useful information about the continuation
-      // without creating a dependency cycle.
+    case Value::Kind::ContinuationValue: {
+      out << "{";
+      llvm::ListSeparator sep(" :: ");
+      for (Ptr<Frame> frame : cast<ContinuationValue>(*this).Stack()) {
+        out << sep << *frame;
+      }
+      out << "}";
       break;
+    }
     case Value::Kind::StringType:
       out << "String";
       break;