Add a value name hint to Flag

A flag created with `Flag::StringFlag("instance-name")` produces a
synopsis like "--instance-name=NAME" by default. Callers can customize
it further with the ValueNameHintMethod.

Assisted-by: Gemini

Author: jemoreira

base/cvd/cuttlefish/flag_parser/flag.cpp

@@ -30,9 +30,11 @@
 
 #include "absl/cleanup/cleanup.h"
 #include "absl/log/check.h"
+#include "absl/strings/ascii.h"
 #include "absl/strings/str_cat.h"
 #include "absl/strings/str_join.h"
 #include "absl/strings/str_replace.h"
+#include "absl/strings/str_split.h"
 #include "absl/strings/strip.h"
 
 #include "cuttlefish/common/libs/utils/contains.h"
@@ -57,6 +59,15 @@ bool MatchNormalized(std::string_view str1, std::string_view str2) {
   return n1 == n2;
 }
 
+std::string DefaultValueNameHint(std::string_view name) {
+  std::vector<std::string_view> parts =
+      absl::StrSplit(name, absl::ByAnyChar("-_"), absl::SkipEmpty());
+  if (parts.empty()) {
+    return "VAL";
+  }
+  return absl::AsciiStrToUpper(parts.back());
+}
+
 }  // namespace
 
 Flag Flag::StringFlag(std::string name) {
@@ -80,6 +91,14 @@ Flag& Flag::Help(std::string help) & {
 }
 Flag Flag::Help(std::string help) && { return std::move(Help(help)); }
 
+Flag& Flag::ValueNameHint(std::string value_name_hint) & {
+  value_name_hint_ = std::move(value_name_hint);
+  return *this;
+}
+Flag Flag::ValueNameHint(std::string value_name_hint) && {
+  return std::move(ValueNameHint(value_name_hint));
+}
+
 Flag& Flag::Getter(std::function<std::string()> getter) & {
   getter_ = std::move(getter);
   return *this;
@@ -111,7 +130,7 @@ std::string Flag::Synopsis() const {
   for (const std::string& alias : aliases_) {
     switch (style_) {
       case Flag::Style::String:
-        options.emplace_back(absl::StrCat("--", alias, "=VAL"));
+        options.emplace_back(absl::StrCat("--", alias, "=", value_name_hint_));
         break;
       case Flag::Style::Bool:
         options.emplace_back("--[no]" + alias);
@@ -127,6 +146,7 @@ std::string Flag::CurrentValue() const { return getter_(); }
 
 Flag::Flag(std::string name, Flag::Style style) : style_(style) {
   ValidateAlias(name);
+  value_name_hint_ = DefaultValueNameHint(name);
   aliases_.emplace_back(std::move(name));
 }
 

base/cvd/cuttlefish/flag_parser/flag.h

@@ -40,6 +40,9 @@ class Flag {
   /* Set help text, visible in the class ostream writer method. Optional. */
   Flag& Help(std::string) &;
   Flag Help(std::string) &&;
+  /* Set the value name hint used in the synopsis. Optional. */
+  Flag& ValueNameHint(std::string) &;
+  Flag ValueNameHint(std::string) &&;
   /* Set a loader that displays the current value in help text. Optional. */
   Flag& Getter(std::function<std::string()>) &;
   Flag Getter(std::function<std::string()>) &&;
@@ -86,6 +89,7 @@ class Flag {
   std::vector<std::string> aliases_;
   Style style_;
   std::string help_;
+  std::string value_name_hint_ = "VAL";
   std::function<std::string()> getter_ = []() { return ""; };
   std::function<Result<void>(std::string_view value)> setter_;
   std::vector<std::function<Result<void>()>> validators_;

base/cvd/cuttlefish/flag_parser/flag_test.cpp

@@ -84,4 +84,29 @@ TEST(FlagParser, EndOfOptionMark) {
   ASSERT_TRUE(flag);
 }
 
+TEST(FlagParser, ValueNameHint_Customized) {
+  auto flag = Flag::StringFlag("myflag").ValueNameHint("myvalue");
+  EXPECT_EQ(flag.Synopsis(), "--myflag=myvalue");
+}
+
+TEST(FlagParser, ValueNameHint_CustomizedWithAlias) {
+  auto flag = Flag::StringFlag("myflag").Alias("mf").ValueNameHint("myvalue");
+  EXPECT_EQ(flag.Synopsis(), "--myflag=myvalue, --mf=myvalue");
+}
+
+TEST(FlagParser, ValueNameHint_DefaultDerived) {
+  auto flag = Flag::StringFlag("myflag");
+  EXPECT_EQ(flag.Synopsis(), "--myflag=MYFLAG");
+}
+
+TEST(FlagParser, ValueNameHint_DefaultDerivedDashed) {
+  auto flag = Flag::StringFlag("verbosity-level");
+  EXPECT_EQ(flag.Synopsis(), "--verbosity-level=LEVEL");
+}
+
+TEST(FlagParser, ValueNameHint_DefaultDerivedSnaked) {
+  auto flag = Flag::StringFlag("verbosity_level");
+  EXPECT_EQ(flag.Synopsis(), "--verbosity_level=LEVEL");
+}
+
 }  // namespace cuttlefish