Add FromStr impl for Edition and BoxError alias

This commit moves the parsing code we did for Edition in the Manifest
type to a FromStr impl so that we can instead use this in other places
as well if need be and have the parsing code for Manifest be more about
parsing out fields and not doing adhoc type conversions in that parsing
if we can move it into the type itself somehow. We also alias `Box<dyn Error>`
to `BoxError` so that we can easily use that for our Error type for now.

src/config.rs

@@ -1,8 +1,9 @@
+use super::BoxError;
 use super::Edition;
 use super::Result;
-use std::error::Error;
 use std::fs;
 use std::path::Path;
+use std::str::FromStr;
 
 pub struct Manifest {
     pub crate_name: String,
@@ -22,21 +23,14 @@ impl Manifest {
 
             match field {
                 "name" => crate_name = Some(value.replace('"', "")),
-                "edition" => {
-                    edition = Some(match value.replace('"', "").parse()? {
-                        2015 => Edition::E2015,
-                        2018 => Edition::E2018,
-                        2021 => Edition::E2021,
-                        edition => return Err(format!("Edition {edition} is unsupported").into()),
-                    })
-                }
+                "edition" => edition = Some(Edition::from_str(&value.replace('"', ""))?),
                 field => return Err(format!("Field {field} is unsupported").into()),
             }
         }
 
         Ok(Self {
-            crate_name: crate_name.ok_or::<Box<dyn Error>>("name is a required field".into())?,
-            edition: edition.ok_or::<Box<dyn Error>>("edition is a required field".into())?,
+            crate_name: crate_name.ok_or::<BoxError>("name is a required field".into())?,
+            edition: edition.ok_or::<BoxError>("edition is a required field".into())?,
         })
     }
 }

src/lib.rs

@@ -8,8 +8,10 @@ use std::fmt::Display;
 use std::fs;
 use std::path::PathBuf;
 use std::process::Command;
+use std::str::FromStr;
 
-pub type Result<T> = std::result::Result<T, Box<dyn Error>>;
+pub type Result<T> = std::result::Result<T, BoxError>;
+pub type BoxError = Box<dyn Error>;
 
 pub fn build() -> Result<()> {
     let root_dir = root_dir()?;
@@ -201,6 +203,18 @@ impl Display for Edition {
     }
 }
 
+impl FromStr for Edition {
+    type Err = BoxError;
+    fn from_str(input: &str) -> Result<Self> {
+        match input {
+            "2015" => Ok(Self::E2015),
+            "2018" => Ok(Self::E2018),
+            "2021" => Ok(Self::E2021),
+            edition => Err(format!("Edition {edition} is not supported").into()),
+        }
+    }
+}
+
 pub enum CrateType {
     Bin,
     Lib,