feat(derive): Add support for lower/upper in rename_all

Some programs do not use anything to separate word boundaries.

For example a struct may contain the field `build_dir` while the flag is
`--builddir`.

This is a port of TeXitoi/structopt#412

This is part of #2809

clap_derive/src/attrs.rs

@@ -80,6 +80,10 @@ pub enum CasingStyle {
     ScreamingSnake,
     /// Keep all letters lowercase and indicate word boundaries with underscores.
     Snake,
+    /// Keep all letters lowercase and remove word boundaries.
+    Lower,
+    /// Keep all letters uppercase and remove word boundaries.
+    Upper,
     /// Use the original attribute name defined in the code.
     Verbatim,
 }
@@ -206,6 +210,8 @@ impl CasingStyle {
             "pascal" | "pascalcase" => cs(Pascal),
             "screamingsnake" | "screamingsnakecase" => cs(ScreamingSnake),
             "snake" | "snakecase" => cs(Snake),
+            "lower" | "lowercase" => cs(Lower),
+            "upper" | "uppercase" => cs(Upper),
             "verbatim" | "verbatimcase" => cs(Verbatim),
             s => abort!(name, "unsupported casing: `{}`", s),
         }
@@ -226,6 +232,8 @@ impl Name {
                     Camel => s.to_mixed_case(),
                     ScreamingSnake => s.to_shouty_snake_case(),
                     Snake => s.to_snake_case(),
+                    Lower => s.to_snake_case().replace("_", ""),
+                    Upper => s.to_shouty_snake_case().replace("_", ""),
                     Verbatim => s,
                 };
                 quote_spanned!(ident.span()=> #s)
@@ -246,6 +254,8 @@ impl Name {
                     Camel => s.to_mixed_case(),
                     ScreamingSnake => s.to_shouty_snake_case(),
                     Snake => s.to_snake_case(),
+                    Lower => s.to_snake_case(),
+                    Upper => s.to_shouty_snake_case(),
                     Verbatim => s,
                 };
 

clap_derive/tests/argument_naming.rs

@@ -288,3 +288,31 @@ fn test_rename_all_is_propagation_can_be_overridden() {
         Opt::parse_from(&["test", "SECOND_VARIANT", "--foo-option"])
     );
 }
+
+#[test]
+fn test_lower_is_renamed() {
+    #[derive(Clap, Debug, PartialEq)]
+    struct Opt {
+        #[clap(rename_all = "lower", long)]
+        foo_option: bool,
+    }
+
+    assert_eq!(
+        Opt { foo_option: true },
+        Opt::parse_from(&["test", "--foooption"])
+    );
+}
+
+#[test]
+fn test_upper_is_renamed() {
+    #[derive(Clap, Debug, PartialEq)]
+    struct Opt {
+        #[clap(rename_all = "upper", long)]
+        foo_option: bool,
+    }
+
+    assert_eq!(
+        Opt { foo_option: true },
+        Opt::parse_from(&["test", "--FOOOPTION"])
+    );
+}