Revert "Add --rust-target to replace --unstable-rust"

This reverts commit 0bb7b9f.

It turns out our CI stopped running test expectations in an earlier
regression (from d73507e; fix incoming) and so this pull request actually
introduced a bunch of failures when compiling the test expectations and running
their unit tests :(

Author: fitzgen

src/codegen/mod.rs

@@ -350,7 +350,7 @@ impl CodeGenerator for Module {
             }
 
             if item.id() == ctx.root_module() {
-                if result.saw_union && !ctx.options().rust_features().untagged_union() {
+                if result.saw_union && !ctx.options().unstable_rust {
                     utils::prepend_union_types(ctx, &mut *result);
                 }
                 if result.saw_incomplete_array {
@@ -911,8 +911,8 @@ impl<'a> FieldCodegen<'a> for FieldData {
         let field_ty = ctx.resolve_type(self.ty());
         let ty = self.ty().to_rust_ty_or_opaque(ctx, &());
 
-        // NB: If supported, we use proper `union` types.
-        let ty = if parent.is_union() && !ctx.options().rust_features().untagged_union() {
+        // NB: In unstable rust we use proper `union` types.
+        let ty = if parent.is_union() && !ctx.options().unstable_rust {
             if ctx.options().enable_cxx_namespaces {
                 quote_ty!(ctx.ext_cx(), root::__BindgenUnionField<$ty>)
             } else {
@@ -1052,8 +1052,8 @@ impl BitfieldUnit {
                          -> P<ast::Item> {
         let ctor_name = self.ctor_name(ctx);
 
-        // If supported, add the const.
-        let fn_prefix = if ctx.options().rust_features().const_fn() {
+        // If we're generating unstable Rust, add the const.
+        let fn_prefix = if ctx.options().unstable_rust {
             quote_tokens!(ctx.ext_cx(), pub const fn)
         } else {
             quote_tokens!(ctx.ext_cx(), pub fn)
@@ -1115,8 +1115,8 @@ impl Bitfield {
         let offset = self.offset_into_unit();
         let mask = self.mask();
 
-        // If supported, add the const.
-        let fn_prefix = if ctx.options().rust_features().const_fn() {
+        // If we're generating unstable Rust, add the const.
+        let fn_prefix = if ctx.options().unstable_rust {
             quote_tokens!(ctx.ext_cx(), pub const fn)
         } else {
             quote_tokens!(ctx.ext_cx(), pub fn)
@@ -1445,7 +1445,7 @@ impl CodeGenerator for CompInfo {
         }
 
         let canonical_name = item.canonical_name(ctx);
-        let builder = if is_union && ctx.options().rust_features().untagged_union() {
+        let builder = if is_union && ctx.options().unstable_rust {
             aster::AstBuilder::new()
                 .item()
                 .pub_()
@@ -1552,7 +1552,7 @@ impl CodeGenerator for CompInfo {
                           ());
         }
 
-        if is_union && !ctx.options().rust_features().untagged_union() {
+        if is_union && !ctx.options().unstable_rust {
             let layout = layout.expect("Unable to get layout information?");
             let ty = BlobTyBuilder::new(layout).build();
             let field = StructFieldBuilder::named("bindgen_union_field")

src/features.rs

@@ -207,7 +207,7 @@ impl<'ctx, 'gen> MonotoneFramework for CannotDeriveCopy<'ctx, 'gen> {
                 }
 
                 if info.kind() == CompKind::Union {
-                    if !self.ctx.options().rust_features().untagged_union() {
+                    if !self.ctx.options().unstable_rust {
                         // NOTE: If there's no template parameters we can derive copy
                         // unconditionally, since arrays are magical for rustc, and
                         // __BindgenUnionField always implements copy.

src/ir/analysis/derive_copy.rs

@@ -208,7 +208,7 @@ impl<'ctx, 'gen> MonotoneFramework for CannotDeriveDebug<'ctx, 'gen> {
                 );
 
                 if info.kind() == CompKind::Union {
-                    if self.ctx.options().rust_features().untagged_union() {
+                    if self.ctx.options().unstable_rust {
                         trace!("    cannot derive Debug for Rust unions");
                         return self.insert(id);
                     }

src/ir/analysis/derive_debug.rs

@@ -242,7 +242,7 @@ impl<'ctx, 'gen> MonotoneFramework for CannotDeriveDefault<'ctx, 'gen> {
                 );
 
                 if info.kind() == CompKind::Union {
-                    if self.ctx.options().rust_features().untagged_union() {
+                    if self.ctx.options().unstable_rust {
                         trace!("    cannot derive Default for Rust unions");
                         return self.insert(id);
                     }