crosvm: Change expected field to String in InvalidValue

Change the type of argument::Error::InvalidValue::expected from "&'static str" to String.
It allows the lower level parse error object to handle the output of the expected value so that
the rule of parsing will not be duplicated.

For example, instead of:
```
v.parse::<Settings>()
  .map_err(|e| argument::Error::InvalidValue {
             value: v.to_string(),
	     expected: "The value of setting should null or cras",
  })?;

```
we can have:
```
v.parse::<Settings>()
  .map_err(|e| argument::Error::InvalidValue {
	   value: v.to_string(),
           expected: e.to_string(),
	   })?;
```
and the expected value can be handled in the Display implementaion of
Settings::ParseError.

BUG=b:140866281
TEST=cargo build

Change-Id: Ieba12a21103945fe0e47c70a190a4e5d985af537
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2103605
Tested-by: Judy Hsiao <judyhsiao@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Chih-Yang Hsia <paulhsia@chromium.org>
Reviewed-by: Zach Reizner <zachr@chromium.org>
Auto-Submit: Judy Hsiao <judyhsiao@chromium.org>
Commit-Queue: Judy Hsiao <judyhsiao@chromium.org>

1 files changed, 4 insertions, 7 deletions

diff --git a/src/argument.rs b/src/argument.rs
index f59d503..9c00ae5 100644
--- a/src/argument.rs
+++ b/src/argument.rs
@@ -25,7 +25,7 @@
 //!             let v: u32 = value.unwrap().parse().map_err(|_| {
 //!                 Error::InvalidValue {
 //!                     value: value.unwrap().to_owned(),
-//!                     expected: "this value for `cpus` needs to be integer",
+//!                     expected: String::from("this value for `cpus` needs to be integer"),
 //!                 }
 //!             })?;
 //!         }
@@ -56,10 +56,7 @@ pub enum Error {
     /// The argument was required.
     ExpectedArgument(String),
     /// The argument's given value is invalid.
-    InvalidValue {
-        value: String,
-        expected: &'static str,
-    },
+    InvalidValue { value: String, expected: String },
     /// The argument was already given and none more are expected.
     TooManyArguments(String),
     /// The argument expects a value.
@@ -447,7 +444,7 @@ mod tests {
                     "cpus" => {
                         let c: u32 = value.unwrap().parse().map_err(|_| Error::InvalidValue {
                             value: value.unwrap().to_owned(),
-                            expected: "this value for `cpus` needs to be integer",
+                            expected: String::from("this value for `cpus` needs to be integer"),
                         })?;
                         assert_eq!(c, 3);
                     }
@@ -521,7 +518,7 @@ mod tests {
                                     _ => {
                                         return Err(Error::InvalidValue {
                                             value: v.to_string(),
-                                            expected: "2D or 3D",
+                                            expected: String::from("2D or 3D"),
                                         })
                                     }
                                 }