Clarify args terminology.

The deriving code has inconsistent terminology to describe args.

In some places it distinguishes between:
- the `&self` arg (if present), versus
- all other args.

In other places it distinguishes between:
- the `&self` arg (if present) and any other arguments with the same
  type (in practice there is at most one, e.g. in `PartialEq::eq`),
  versus
- all other args.

The terms "self_args" and "nonself_args" are sometimes used for the
former distinction, and sometimes for the latter. "args" is also
sometimes used for "all other args".

This commit makes the code consistently uses "self_args"/"nonself_args"
for the former and "selflike_args"/"nonselflike_args" for the latter.
This change makes the code easier to read.

The commit also adds a panic on an impossible path (the `Self_` case) in
`extract_arg_details`.

compiler/rustc_builtin_macros/src/deriving/clone.rs

@@ -80,7 +80,7 @@ pub fn expand_deriving_clone(
             name: sym::clone,
             generics: Bounds::empty(),
             explicit_self: true,
-            args: Vec::new(),
+            nonself_args: Vec::new(),
             ret_ty: Self_,
             attributes: attrs,
             unify_fieldless_variants: false,

compiler/rustc_builtin_macros/src/deriving/cmp/eq.rs

@@ -32,7 +32,7 @@ pub fn expand_deriving_eq(
             name: sym::assert_receiver_is_total_eq,
             generics: Bounds::empty(),
             explicit_self: true,
-            args: vec![],
+            nonself_args: vec![],
             ret_ty: Unit,
             attributes: attrs,
             unify_fieldless_variants: true,

compiler/rustc_builtin_macros/src/deriving/cmp/ord.rs

@@ -28,7 +28,7 @@ pub fn expand_deriving_ord(
             name: sym::cmp,
             generics: Bounds::empty(),
             explicit_self: true,
-            args: vec![(self_ref(), sym::other)],
+            nonself_args: vec![(self_ref(), sym::other)],
             ret_ty: Path(path_std!(cmp::Ordering)),
             attributes: attrs,
             unify_fieldless_variants: true,

compiler/rustc_builtin_macros/src/deriving/cmp/partial_eq.rs

@@ -69,7 +69,7 @@ fn cs_ne(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> BlockOr
                 name: $name,
                 generics: Bounds::empty(),
                 explicit_self: true,
-                args: vec![(self_ref(), sym::other)],
+                nonself_args: vec![(self_ref(), sym::other)],
                 ret_ty: Path(path_local!(bool)),
                 attributes: attrs,
                 unify_fieldless_variants: true,

compiler/rustc_builtin_macros/src/deriving/cmp/partial_ord.rs

@@ -26,7 +26,7 @@ pub fn expand_deriving_partial_ord(
         name: sym::partial_cmp,
         generics: Bounds::empty(),
         explicit_self: true,
-        args: vec![(self_ref(), sym::other)],
+        nonself_args: vec![(self_ref(), sym::other)],
         ret_ty,
         attributes: attrs,
         unify_fieldless_variants: true,

compiler/rustc_builtin_macros/src/deriving/debug.rs

@@ -28,7 +28,7 @@ pub fn expand_deriving_debug(
             name: sym::fmt,
             generics: Bounds::empty(),
             explicit_self: true,
-            args: vec![(fmtr, sym::f)],
+            nonself_args: vec![(fmtr, sym::f)],
             ret_ty: Path(path_std!(fmt::Result)),
             attributes: Vec::new(),
             unify_fieldless_variants: false,
@@ -53,7 +53,7 @@ fn show_substructure(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>
     // We want to make sure we have the ctxt set so that we can use unstable methods
     let span = cx.with_def_site_ctxt(span);
     let name = cx.expr_lit(span, ast::LitKind::Str(ident.name, ast::StrStyle::Cooked));
-    let fmt = substr.nonself_args[0].clone();
+    let fmt = substr.nonselflike_args[0].clone();
 
     // Struct and tuples are similar enough that we use the same code for both,
     // with some extra pieces for structs due to the field names.

compiler/rustc_builtin_macros/src/deriving/decodable.rs

@@ -36,7 +36,10 @@ pub fn expand_deriving_rustc_decodable(
                 )],
             },
             explicit_self: false,
-            args: vec![(Ref(Box::new(Path(Path::new_local(typaram))), Mutability::Mut), sym::d)],
+            nonself_args: vec![(
+                Ref(Box::new(Path(Path::new_local(typaram))), Mutability::Mut),
+                sym::d,
+            )],
             ret_ty: Path(Path::new_(
                 pathvec_std!(result::Result),
                 vec![
@@ -63,7 +66,7 @@ fn decodable_substructure(
     substr: &Substructure<'_>,
     krate: Symbol,
 ) -> BlockOrExpr {
-    let decoder = substr.nonself_args[0].clone();
+    let decoder = substr.nonselflike_args[0].clone();
     let recurse = vec![
         Ident::new(krate, trait_span),
         Ident::new(sym::Decodable, trait_span),

compiler/rustc_builtin_macros/src/deriving/default.rs

@@ -34,7 +34,7 @@ pub fn expand_deriving_default(
             name: kw::Default,
             generics: Bounds::empty(),
             explicit_self: false,
-            args: Vec::new(),
+            nonself_args: Vec::new(),
             ret_ty: Self_,
             attributes: attrs,
             unify_fieldless_variants: false,

compiler/rustc_builtin_macros/src/deriving/encodable.rs

@@ -120,7 +120,10 @@ pub fn expand_deriving_rustc_encodable(
                 )],
             },
             explicit_self: true,
-            args: vec![(Ref(Box::new(Path(Path::new_local(typaram))), Mutability::Mut), sym::s)],
+            nonself_args: vec![(
+                Ref(Box::new(Path(Path::new_local(typaram))), Mutability::Mut),
+                sym::s,
+            )],
             ret_ty: Path(Path::new_(
                 pathvec_std!(result::Result),
                 vec![
@@ -147,7 +150,7 @@ fn encodable_substructure(
     substr: &Substructure<'_>,
     krate: Symbol,
 ) -> BlockOrExpr {
-    let encoder = substr.nonself_args[0].clone();
+    let encoder = substr.nonselflike_args[0].clone();
     // throw an underscore in front to suppress unused variable warnings
     let blkarg = Ident::new(sym::_e, trait_span);
     let blkencoder = cx.expr_ident(trait_span, blkarg);

compiler/rustc_builtin_macros/src/deriving/hash.rs

@@ -30,7 +30,7 @@ pub fn expand_deriving_hash(
             name: sym::hash,
             generics: Bounds { bounds: vec![(typaram, vec![path_std!(hash::Hasher)])] },
             explicit_self: true,
-            args: vec![(Ref(Box::new(Path(arg)), Mutability::Mut), sym::state)],
+            nonself_args: vec![(Ref(Box::new(Path(arg)), Mutability::Mut), sym::state)],
             ret_ty: Unit,
             attributes: vec![],
             unify_fieldless_variants: true,
@@ -49,7 +49,7 @@ fn hash_substructure(
     trait_span: Span,
     substr: &Substructure<'_>,
 ) -> BlockOrExpr {
-    let [state_expr] = substr.nonself_args else {
+    let [state_expr] = substr.nonselflike_args else {
         cx.span_bug(trait_span, "incorrect number of arguments in `derive(Hash)`");
     };
     let call_hash = |span, thing_expr| {