fix(derive): Reduce compiler error noise

It looks like CI hasn't been running on this and we've introduced some
problems.  It looks like we had an off-by-one error in the check for
MSRV for deciding to run ui tests.

clap_derive/src/dummies.rs

@@ -52,6 +52,9 @@ pub fn subcommand(name: &Ident) {
             fn augment_subcommands_for_update(_app: clap::App<'_>) -> clap::App<'_> {
                 unimplemented!()
             }
+            fn has_subcommand(name: &str) -> bool {
+                unimplemented!()
+            }
         }
     });
 }
@@ -77,6 +80,9 @@ pub fn arg_enum(name: &Ident) {
             fn from_str(_input: &str, _case_insensitive: bool) -> Result<Self, String> {
                 unimplemented!()
             }
+            fn as_arg(&self) -> Option<&'static str> {
+                unimplemented!()
+            }
         }
     })
 }

clap_derive/tests/ui/arg_enum_on_struct.stderr

@@ -5,12 +5,3 @@ error: `#[derive(ArgEnum)]` only supports enums
   |          ^^^^^^^
   |
   = note: this error originates in the derive macro `ArgEnum` (in Nightly builds, run with -Z macro-backtrace for more info)
-
-error[E0046]: not all trait items implemented, missing: `as_arg`
- --> $DIR/arg_enum_on_struct.rs:3:10
-  |
-3 | #[derive(ArgEnum, Debug)]
-  |          ^^^^^^^ missing `as_arg` in implementation
-  |
-  = note: this error originates in the derive macro `ArgEnum` (in Nightly builds, run with -Z macro-backtrace for more info)
-  = help: implement the missing item: `fn as_arg(&self) -> Option<&'static str> { todo!() }`

clap_derive/tests/ui/enum_flatten.stderr

@@ -4,12 +4,3 @@ error: `flatten` is usable only with single-typed tuple variants
 14 | /     #[clap(flatten)]
 15 | |     Variant1,
    | |____________^
-
-error[E0046]: not all trait items implemented, missing: `has_subcommand`
-  --> $DIR/enum_flatten.rs:11:10
-   |
-11 | #[derive(Clap, Debug)]
-   |          ^^^^ missing `has_subcommand` in implementation
-   |
-   = note: this error originates in the derive macro `Clap` (in Nightly builds, run with -Z macro-backtrace for more info)
-   = help: implement the missing item: `fn has_subcommand(_: &str) -> bool { todo!() }`

clap_derive/tests/ui/external_subcommand_wrong_type.stderr

@@ -16,30 +16,3 @@ error: The enum variant marked with `external_attribute` must be a single-typed
 18 | /     #[clap(external_subcommand)]
 19 | |     Other { a: String },
    | |_______________________^
-
-error[E0046]: not all trait items implemented, missing: `has_subcommand`
- --> $DIR/external_subcommand_wrong_type.rs:4:10
-  |
-4 | #[derive(Clap, Debug)]
-  |          ^^^^ missing `has_subcommand` in implementation
-  |
-  = note: this error originates in the derive macro `Clap` (in Nightly builds, run with -Z macro-backtrace for more info)
-  = help: implement the missing item: `fn has_subcommand(_: &str) -> bool { todo!() }`
-
-error[E0046]: not all trait items implemented, missing: `has_subcommand`
-  --> $DIR/external_subcommand_wrong_type.rs:10:10
-   |
-10 | #[derive(Clap, Debug)]
-   |          ^^^^ missing `has_subcommand` in implementation
-   |
-   = note: this error originates in the derive macro `Clap` (in Nightly builds, run with -Z macro-backtrace for more info)
-   = help: implement the missing item: `fn has_subcommand(_: &str) -> bool { todo!() }`
-
-error[E0046]: not all trait items implemented, missing: `has_subcommand`
-  --> $DIR/external_subcommand_wrong_type.rs:16:10
-   |
-16 | #[derive(Clap, Debug)]
-   |          ^^^^ missing `has_subcommand` in implementation
-   |
-   = note: this error originates in the derive macro `Clap` (in Nightly builds, run with -Z macro-backtrace for more info)
-   = help: implement the missing item: `fn has_subcommand(_: &str) -> bool { todo!() }`

clap_derive/tests/ui/multiple_external_subcommand.stderr

@@ -3,12 +3,3 @@ error: Only one variant can be marked with `external_subcommand`, this is the se
    |
 14 |     #[clap(external_subcommand)]
    |            ^^^^^^^^^^^^^^^^^^^
-
-error[E0046]: not all trait items implemented, missing: `has_subcommand`
- --> $DIR/multiple_external_subcommand.rs:9:10
-  |
-9 | #[derive(Clap, Debug)]
-  |          ^^^^ missing `has_subcommand` in implementation
-  |
-  = note: this error originates in the derive macro `Clap` (in Nightly builds, run with -Z macro-backtrace for more info)
-  = help: implement the missing item: `fn has_subcommand(_: &str) -> bool { todo!() }`

clap_derive/tests/ui/subcommand_on_struct.stderr

@@ -5,12 +5,3 @@ error: `#[derive(Subcommand)]` only supports enums
   |          ^^^^^^^^^^
   |
   = note: this error originates in the derive macro `Subcommand` (in Nightly builds, run with -Z macro-backtrace for more info)
-
-error[E0046]: not all trait items implemented, missing: `has_subcommand`
- --> $DIR/subcommand_on_struct.rs:3:10
-  |
-3 | #[derive(Subcommand, Debug)]
-  |          ^^^^^^^^^^ missing `has_subcommand` in implementation
-  |
-  = note: this error originates in the derive macro `Subcommand` (in Nightly builds, run with -Z macro-backtrace for more info)
-  = help: implement the missing item: `fn has_subcommand(_: &str) -> bool { todo!() }`