Auto merge of #98758 - nnethercote:more-derive-output-improvements, r=Mark-Simulacrum

More derive output improvements

This PR includes:
- Some test improvements.
- Some cosmetic changes to derive output that make the code look more like what a human would write.
- Some more fundamental improvements to `cmp` and `partial_cmp` generation.

r? `@Mark-Simulacrum`

compiler/rustc_ast/src/ast.rs

@@ -2036,6 +2036,14 @@ pub fn is_implicit_self(&self) -> bool {
     pub fn is_unit(&self) -> bool {
         matches!(self, TyKind::Tup(tys) if tys.is_empty())
     }
+
+    pub fn is_simple_path(&self) -> Option<Symbol> {
+        if let TyKind::Path(None, Path { segments, .. }) = &self && segments.len() == 1 {
+            Some(segments[0].ident.name)
+        } else {
+            None
+        }
+    }
 }
 
 /// Syntax used to declare a trait object.

compiler/rustc_builtin_macros/src/deriving/clone.rs

@@ -2,8 +2,8 @@
 use crate::deriving::generic::*;
 use crate::deriving::path_std;
 
-use rustc_ast::ptr::P;
-use rustc_ast::{self as ast, Expr, Generics, ItemKind, MetaItem, VariantData};
+use rustc_ast::{self as ast, Generics, ItemKind, MetaItem, VariantData};
+use rustc_data_structures::fx::FxHashSet;
 use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_span::symbol::{kw, sym, Ident};
 use rustc_span::Span;
@@ -98,22 +98,31 @@ fn cs_clone_simple(
     trait_span: Span,
     substr: &Substructure<'_>,
     is_union: bool,
-) -> P<Expr> {
+) -> BlockOrExpr {
     let mut stmts = Vec::new();
+    let mut seen_type_names = FxHashSet::default();
     let mut process_variant = |variant: &VariantData| {
         for field in variant.fields() {
-            // let _: AssertParamIsClone<FieldTy>;
-            super::assert_ty_bounds(
-                cx,
-                &mut stmts,
-                field.ty.clone(),
-                field.span,
-                &[sym::clone, sym::AssertParamIsClone],
-            );
+            // This basic redundancy checking only prevents duplication of
+            // assertions like `AssertParamIsClone<Foo>` where the type is a
+            // simple name. That's enough to get a lot of cases, though.
+            if let Some(name) = field.ty.kind.is_simple_path() && !seen_type_names.insert(name) {
+                // Already produced an assertion for this type.
+            } else {
+                // let _: AssertParamIsClone<FieldTy>;
+                super::assert_ty_bounds(
+                    cx,
+                    &mut stmts,
+                    field.ty.clone(),
+                    field.span,
+                    &[sym::clone, sym::AssertParamIsClone],
+                );
+            }
         }
     };
 
     if is_union {
+        // Just a single assertion for unions, that the union impls `Copy`.
         // let _: AssertParamIsCopy<Self>;
         let self_ty = cx.ty_path(cx.path_ident(trait_span, Ident::with_dummy_span(kw::SelfUpper)));
         super::assert_ty_bounds(
@@ -139,8 +148,7 @@ fn cs_clone_simple(
             ),
         }
     }
-    stmts.push(cx.stmt_expr(cx.expr_deref(trait_span, cx.expr_self(trait_span))));
-    cx.expr_block(cx.block(trait_span, stmts))
+    BlockOrExpr::new_mixed(stmts, cx.expr_deref(trait_span, cx.expr_self(trait_span)))
 }
 
 fn cs_clone(
@@ -148,7 +156,7 @@ fn cs_clone(
     cx: &mut ExtCtxt<'_>,
     trait_span: Span,
     substr: &Substructure<'_>,
-) -> P<Expr> {
+) -> BlockOrExpr {
     let ctor_path;
     let all_fields;
     let fn_path = cx.std_path(&[sym::clone, sym::Clone, sym::clone]);
@@ -177,7 +185,7 @@ fn cs_clone(
         }
     }
 
-    match *vdata {
+    let expr = match *vdata {
         VariantData::Struct(..) => {
             let fields = all_fields
                 .iter()
@@ -201,5 +209,6 @@ fn cs_clone(
             cx.expr_call(trait_span, path, subcalls)
         }
         VariantData::Unit(..) => cx.expr_path(ctor_path),
-    }
+    };
+    BlockOrExpr::new_expr(expr)
 }

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

@@ -2,8 +2,8 @@
 use crate::deriving::generic::*;
 use crate::deriving::path_std;
 
-use rustc_ast::ptr::P;
-use rustc_ast::{self as ast, Expr, MetaItem};
+use rustc_ast::{self as ast, MetaItem};
+use rustc_data_structures::fx::FxHashSet;
 use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_span::symbol::{sym, Ident};
 use rustc_span::Span;
@@ -52,18 +52,26 @@ fn cs_total_eq_assert(
     cx: &mut ExtCtxt<'_>,
     trait_span: Span,
     substr: &Substructure<'_>,
-) -> P<Expr> {
+) -> BlockOrExpr {
     let mut stmts = Vec::new();
+    let mut seen_type_names = FxHashSet::default();
     let mut process_variant = |variant: &ast::VariantData| {
         for field in variant.fields() {
-            // let _: AssertParamIsEq<FieldTy>;
-            super::assert_ty_bounds(
-                cx,
-                &mut stmts,
-                field.ty.clone(),
-                field.span,
-                &[sym::cmp, sym::AssertParamIsEq],
-            );
+            // This basic redundancy checking only prevents duplication of
+            // assertions like `AssertParamIsEq<Foo>` where the type is a
+            // simple name. That's enough to get a lot of cases, though.
+            if let Some(name) = field.ty.kind.is_simple_path() && !seen_type_names.insert(name) {
+                // Already produced an assertion for this type.
+            } else {
+                // let _: AssertParamIsEq<FieldTy>;
+                super::assert_ty_bounds(
+                    cx,
+                    &mut stmts,
+                    field.ty.clone(),
+                    field.span,
+                    &[sym::cmp, sym::AssertParamIsEq],
+                );
+            }
         }
     };
 
@@ -78,5 +86,5 @@ fn cs_total_eq_assert(
         }
         _ => cx.span_bug(trait_span, "unexpected substructure in `derive(Eq)`"),
     }
-    cx.expr_block(cx.block(trait_span, stmts))
+    BlockOrExpr::new_stmts(stmts)
 }

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

@@ -3,7 +3,7 @@
 use crate::deriving::path_std;
 
 use rustc_ast::ptr::P;
-use rustc_ast::{self as ast, Expr, MetaItem};
+use rustc_ast::{self as ast, MetaItem};
 use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_span::symbol::{sym, Ident};
 use rustc_span::Span;
@@ -51,7 +51,7 @@ pub fn ordering_collapsed(
     cx.expr_call_global(span, fn_cmp_path, vec![lft, rgt])
 }
 
-pub fn cs_cmp(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> P<Expr> {
+pub fn cs_cmp(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> BlockOrExpr {
     let test_id = Ident::new(sym::cmp, span);
     let equals_path = cx.path_global(span, cx.std_path(&[sym::cmp, sym::Ordering, sym::Equal]));
 
@@ -70,7 +70,7 @@ pub fn cs_cmp(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> P<
     // cmp => cmp
     // }
     //
-    cs_fold(
+    let expr = cs_fold(
         // foldr nests the if-elses correctly, leaving the first field
         // as the outermost one, and the last as the innermost.
         false,
@@ -79,15 +79,12 @@ pub fn cs_cmp(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> P<
             //     ::std::cmp::Ordering::Equal => old,
             //     cmp => cmp
             // }
-
             let new = {
                 let [other_f] = other_fs else {
                     cx.span_bug(span, "not exactly 2 arguments in `derive(Ord)`");
                 };
-
                 let args =
                     vec![cx.expr_addr_of(span, self_f), cx.expr_addr_of(span, other_f.clone())];
-
                 cx.expr_call_global(span, cmp_path.clone(), args)
             };
 
@@ -96,7 +93,21 @@ pub fn cs_cmp(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> P<
 
             cx.expr_match(span, new, vec![eq_arm, neq_arm])
         },
-        cx.expr_path(equals_path.clone()),
+        |cx, args| match args {
+            Some((span, self_f, other_fs)) => {
+                let new = {
+                    let [other_f] = other_fs else {
+                            cx.span_bug(span, "not exactly 2 arguments in `derive(Ord)`");
+                        };
+                    let args =
+                        vec![cx.expr_addr_of(span, self_f), cx.expr_addr_of(span, other_f.clone())];
+                    cx.expr_call_global(span, cmp_path.clone(), args)
+                };
+
+                new
+            }
+            None => cx.expr_path(equals_path.clone()),
+        },
         Box::new(|cx, span, tag_tuple| {
             if tag_tuple.len() != 2 {
                 cx.span_bug(span, "not exactly 2 arguments in `derive(Ord)`")
@@ -107,5 +118,6 @@ pub fn cs_cmp(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> P<
         cx,
         span,
         substr,
-    )
+    );
+    BlockOrExpr::new_expr(expr)
 }

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

@@ -15,8 +15,6 @@ pub fn expand_deriving_partial_eq(
     item: &Annotatable,
     push: &mut dyn FnMut(Annotatable),
 ) {
-    // structures are equal if all fields are equal, and non equal, if
-    // any fields are not equal or if the enum variants are different
     fn cs_op(
         cx: &mut ExtCtxt<'_>,
         span: Span,
@@ -24,7 +22,7 @@ fn cs_op(
         op: BinOpKind,
         combiner: BinOpKind,
         base: bool,
-    ) -> P<Expr> {
+    ) -> BlockOrExpr {
         let op = |cx: &mut ExtCtxt<'_>, span: Span, self_f: P<Expr>, other_fs: &[P<Expr>]| {
             let [other_f] = other_fs else {
                 cx.span_bug(span, "not exactly 2 arguments in `derive(PartialEq)`");
@@ -33,7 +31,7 @@ fn cs_op(
             cx.expr_binary(span, op, self_f, other_f.clone())
         };
 
-        cs_fold1(
+        let expr = cs_fold(
             true, // use foldl
             |cx, span, subexpr, self_f, other_fs| {
                 let eq = op(cx, span, self_f, other_fs);
@@ -52,13 +50,14 @@ fn cs_op(
             cx,
             span,
             substr,
-        )
+        );
+        BlockOrExpr::new_expr(expr)
     }
 
-    fn cs_eq(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> P<Expr> {
+    fn cs_eq(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> BlockOrExpr {
         cs_op(cx, span, substr, BinOpKind::Eq, BinOpKind::And, true)
     }
-    fn cs_ne(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> P<Expr> {
+    fn cs_ne(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> BlockOrExpr {
         cs_op(cx, span, substr, BinOpKind::Ne, BinOpKind::Or, false)
     }
 

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

@@ -48,11 +48,10 @@ pub fn expand_deriving_partial_ord(
     trait_def.expand(cx, mitem, item, push)
 }
 
-pub fn cs_partial_cmp(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> P<Expr> {
+pub fn cs_partial_cmp(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> BlockOrExpr {
     let test_id = Ident::new(sym::cmp, span);
     let ordering = cx.path_global(span, cx.std_path(&[sym::cmp, sym::Ordering, sym::Equal]));
     let ordering_expr = cx.expr_path(ordering.clone());
-    let equals_expr = cx.expr_some(span, ordering_expr);
 
     let partial_cmp_path = cx.std_path(&[sym::cmp, sym::PartialOrd, sym::partial_cmp]);
 
@@ -69,7 +68,7 @@ pub fn cs_partial_cmp(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_
     // cmp => cmp
     // }
     //
-    cs_fold(
+    let expr = cs_fold(
         // foldr nests the if-elses correctly, leaving the first field
         // as the outermost one, and the last as the innermost.
         false,
@@ -95,7 +94,21 @@ pub fn cs_partial_cmp(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_
 
             cx.expr_match(span, new, vec![eq_arm, neq_arm])
         },
-        equals_expr,
+        |cx: &mut ExtCtxt<'_>, args: Option<(Span, P<Expr>, &[P<Expr>])>| match args {
+            Some((span, self_f, other_fs)) => {
+                let new = {
+                    let [other_f] = other_fs else {
+                            cx.span_bug(span, "not exactly 2 arguments in `derive(Ord)`");
+                        };
+                    let args =
+                        vec![cx.expr_addr_of(span, self_f), cx.expr_addr_of(span, other_f.clone())];
+                    cx.expr_call_global(span, partial_cmp_path.clone(), args)
+                };
+
+                new
+            }
+            None => cx.expr_some(span, ordering_expr.clone()),
+        },
         Box::new(|cx, span, tag_tuple| {
             if tag_tuple.len() != 2 {
                 cx.span_bug(span, "not exactly 2 arguments in `derive(PartialOrd)`")
@@ -110,5 +123,6 @@ pub fn cs_partial_cmp(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_
         cx,
         span,
         substr,
-    )
+    );
+    BlockOrExpr::new_expr(expr)
 }

compiler/rustc_builtin_macros/src/deriving/debug.rs

@@ -2,8 +2,7 @@
 use crate::deriving::generic::*;
 use crate::deriving::path_std;
 
-use rustc_ast::ptr::P;
-use rustc_ast::{self as ast, Expr, MetaItem};
+use rustc_ast::{self as ast, MetaItem};
 use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_span::symbol::{sym, Ident, Symbol};
 use rustc_span::Span;
@@ -42,7 +41,7 @@ pub fn expand_deriving_debug(
     trait_def.expand(cx, mitem, item, push)
 }
 
-fn show_substructure(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> P<Expr> {
+fn show_substructure(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> BlockOrExpr {
     let (ident, vdata, fields) = match substr.fields {
         Struct(vdata, fields) => (substr.type_ident, *vdata, fields),
         EnumMatching(_, _, v, fields) => (v.ident, &v.data, fields),
@@ -74,7 +73,8 @@ fn show_substructure(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>
     if fields.is_empty() {
         // Special case for no fields.
         let fn_path_write_str = cx.std_path(&[sym::fmt, sym::Formatter, sym::write_str]);
-        cx.expr_call_global(span, fn_path_write_str, vec![fmt, name])
+        let expr = cx.expr_call_global(span, fn_path_write_str, vec![fmt, name]);
+        BlockOrExpr::new_expr(expr)
     } else if fields.len() <= CUTOFF {
         // Few enough fields that we can use a specific-length method.
         let debug = if is_struct {
@@ -100,7 +100,8 @@ fn show_substructure(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>
             let field = cx.expr_addr_of(field.span, field);
             args.push(field);
         }
-        cx.expr_call_global(span, fn_path_debug, args)
+        let expr = cx.expr_call_global(span, fn_path_debug, args);
+        BlockOrExpr::new_expr(expr)
     } else {
         // Enough fields that we must use the any-length method.
         let mut name_exprs = Vec::with_capacity(fields.len());
@@ -176,8 +177,6 @@ fn show_substructure(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>
             stmts.push(names_let.unwrap());
         }
         stmts.push(values_let);
-        stmts.push(cx.stmt_expr(expr));
-
-        cx.expr_block(cx.block(span, stmts))
+        BlockOrExpr::new_mixed(stmts, expr)
     }
 }

compiler/rustc_builtin_macros/src/deriving/decodable.rs

@@ -62,7 +62,7 @@ fn decodable_substructure(
     trait_span: Span,
     substr: &Substructure<'_>,
     krate: Symbol,
-) -> P<Expr> {
+) -> BlockOrExpr {
     let decoder = substr.nonself_args[0].clone();
     let recurse = vec![
         Ident::new(krate, trait_span),
@@ -74,7 +74,7 @@ fn decodable_substructure(
     let blkarg = Ident::new(sym::_d, trait_span);
     let blkdecoder = cx.expr_ident(trait_span, blkarg);
 
-    match *substr.fields {
+    let expr = match *substr.fields {
         StaticStruct(_, ref summary) => {
             let nfields = match *summary {
                 Unnamed(ref fields, _) => fields.len(),
@@ -173,7 +173,8 @@ fn decodable_substructure(
             )
         }
         _ => cx.bug("expected StaticEnum or StaticStruct in derive(Decodable)"),
-    }
+    };
+    BlockOrExpr::new_expr(expr)
 }
 
 /// Creates a decoder for a single enum variant/struct:

compiler/rustc_builtin_macros/src/deriving/default.rs

 use crate::deriving::generic::ty::*;
 use crate::deriving::generic::*;
 
-use rustc_ast::ptr::P;
+use rustc_ast as ast;
 use rustc_ast::walk_list;
 use rustc_ast::EnumDef;
 use rustc_ast::VariantData;
-use rustc_ast::{Expr, MetaItem};
 use rustc_errors::Applicability;
 use rustc_expand::base::{Annotatable, DummyResult, ExtCtxt};
 use rustc_span::symbol::Ident;
@@ -16,7 +15,7 @@
 pub fn expand_deriving_default(
     cx: &mut ExtCtxt<'_>,
     span: Span,
-    mitem: &MetaItem,
+    mitem: &ast::MetaItem,
     item: &Annotatable,
     push: &mut dyn FnMut(Annotatable),
 ) {
@@ -59,12 +58,12 @@ fn default_struct_substructure(
     trait_span: Span,
     substr: &Substructure<'_>,
     summary: &StaticFields,
-) -> P<Expr> {
+) -> BlockOrExpr {
     // Note that `kw::Default` is "default" and `sym::Default` is "Default"!
     let default_ident = cx.std_path(&[kw::Default, sym::Default, kw::Default]);
     let default_call = |span| cx.expr_call_global(span, default_ident.clone(), Vec::new());
 
-    match summary {
+    let expr = match summary {
         Unnamed(ref fields, is_tuple) => {
             if !is_tuple {
                 cx.expr_ident(trait_span, substr.type_ident)
@@ -80,31 +79,27 @@ fn default_struct_substructure(
                 .collect();
             cx.expr_struct_ident(trait_span, substr.type_ident, default_fields)
         }
-    }
+    };
+    BlockOrExpr::new_expr(expr)
 }
 
 fn default_enum_substructure(
     cx: &mut ExtCtxt<'_>,
     trait_span: Span,
     enum_def: &EnumDef,
-) -> P<Expr> {
-    let Ok(default_variant) = extract_default_variant(cx, enum_def, trait_span) else {
-        return DummyResult::raw_expr(trait_span, true);
+) -> BlockOrExpr {
+    let expr = if let Ok(default_variant) = extract_default_variant(cx, enum_def, trait_span)
+        && let Ok(_) = validate_default_attribute(cx, default_variant)
+    {
+        // We now know there is exactly one unit variant with exactly one `#[default]` attribute.
+        cx.expr_path(cx.path(
+            default_variant.span,
+            vec![Ident::new(kw::SelfUpper, default_variant.span), default_variant.ident],
+        ))
+    } else {
+        DummyResult::raw_expr(trait_span, true)
     };
-
-    // At this point, we know that there is exactly one variant with a `#[default]` attribute. The
-    // attribute hasn't yet been validated.
-
-    if let Err(()) = validate_default_attribute(cx, default_variant) {
-        return DummyResult::raw_expr(trait_span, true);
-    }
-
-    // We now know there is exactly one unit variant with exactly one `#[default]` attribute.
-
-    cx.expr_path(cx.path(
-        default_variant.span,
-        vec![Ident::new(kw::SelfUpper, default_variant.span), default_variant.ident],
-    ))
+    BlockOrExpr::new_expr(expr)
 }
 
 fn extract_default_variant<'a>(

compiler/rustc_builtin_macros/src/deriving/encodable.rs

@@ -89,8 +89,7 @@
 use crate::deriving::generic::*;
 use crate::deriving::pathvec_std;
 
-use rustc_ast::ptr::P;
-use rustc_ast::{Expr, ExprKind, MetaItem, Mutability};
+use rustc_ast::{ExprKind, MetaItem, Mutability};
 use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_span::symbol::{sym, Ident, Symbol};
 use rustc_span::Span;
@@ -147,7 +146,7 @@ fn encodable_substructure(
     trait_span: Span,
     substr: &Substructure<'_>,
     krate: Symbol,
-) -> P<Expr> {
+) -> BlockOrExpr {
     let encoder = substr.nonself_args[0].clone();
     // throw an underscore in front to suppress unused variable warnings
     let blkarg = Ident::new(sym::_e, trait_span);
@@ -208,7 +207,7 @@ fn encodable_substructure(
             let fn_emit_struct_path =
                 cx.def_site_path(&[sym::rustc_serialize, sym::Encoder, sym::emit_struct]);
 
-            cx.expr_call_global(
+            let expr = cx.expr_call_global(
                 trait_span,
                 fn_emit_struct_path,
                 vec![
@@ -217,7 +216,8 @@ fn encodable_substructure(
                     cx.expr_usize(trait_span, fields.len()),
                     blk,
                 ],
-            )
+            );
+            BlockOrExpr::new_expr(expr)
         }
 
         EnumMatching(idx, _, variant, ref fields) => {
@@ -279,12 +279,12 @@ fn encodable_substructure(
             let blk = cx.lambda1(trait_span, call, blkarg);
             let fn_emit_enum_path: Vec<_> =
                 cx.def_site_path(&[sym::rustc_serialize, sym::Encoder, sym::emit_enum]);
-            let ret = cx.expr_call_global(
+            let expr = cx.expr_call_global(
                 trait_span,
                 fn_emit_enum_path,
                 vec![encoder, cx.expr_str(trait_span, substr.type_ident.name), blk],
             );
-            cx.expr_block(cx.block(trait_span, vec![me, cx.stmt_expr(ret)]))
+            BlockOrExpr::new_mixed(vec![me], expr)
         }
 
         _ => cx.bug("expected Struct or EnumMatching in derive(Encodable)"),

compiler/rustc_builtin_macros/src/deriving/generic/mod.rs

@@ -296,7 +296,7 @@ pub enum SubstructureFields<'a> {
 /// Combine the values of all the fields together. The last argument is
 /// all the fields of all the structures.
 pub type CombineSubstructureFunc<'a> =
-    Box<dyn FnMut(&mut ExtCtxt<'_>, Span, &Substructure<'_>) -> P<Expr> + 'a>;
+    Box<dyn FnMut(&mut ExtCtxt<'_>, Span, &Substructure<'_>) -> BlockOrExpr + 'a>;
 
 /// Deal with non-matching enum variants. The slice is the identifiers holding
 /// the variant index value for each of the `Self` arguments.
@@ -314,6 +314,48 @@ struct TypeParameter {
     ty: P<ast::Ty>,
 }
 
+// The code snippets built up for derived code are sometimes used as blocks
+// (e.g. in a function body) and sometimes used as expressions (e.g. in a match
+// arm). This structure avoids committing to either form until necessary,
+// avoiding the insertion of any unnecessary blocks.
+//
+// The statements come before the expression.
+pub struct BlockOrExpr(Vec<ast::Stmt>, Option<P<Expr>>);
+
+impl BlockOrExpr {
+    pub fn new_stmts(stmts: Vec<ast::Stmt>) -> BlockOrExpr {
+        BlockOrExpr(stmts, None)
+    }
+
+    pub fn new_expr(expr: P<Expr>) -> BlockOrExpr {
+        BlockOrExpr(vec![], Some(expr))
+    }
+
+    pub fn new_mixed(stmts: Vec<ast::Stmt>, expr: P<Expr>) -> BlockOrExpr {
+        BlockOrExpr(stmts, Some(expr))
+    }
+
+    // Converts it into a block.
+    fn into_block(mut self, cx: &ExtCtxt<'_>, span: Span) -> P<ast::Block> {
+        if let Some(expr) = self.1 {
+            self.0.push(cx.stmt_expr(expr));
+        }
+        cx.block(span, self.0)
+    }
+
+    // Converts it into an expression.
+    fn into_expr(self, cx: &ExtCtxt<'_>, span: Span) -> P<Expr> {
+        if self.0.is_empty() {
+            match self.1 {
+                None => cx.expr_block(cx.block(span, vec![])),
+                Some(expr) => expr,
+            }
+        } else {
+            cx.expr_block(self.into_block(cx, span))
+        }
+    }
+}
+
 /// This method helps to extract all the type parameters referenced from a
 /// type. For a type parameter `<T>`, it looks for either a `TyPath` that
 /// is not global and starts with `T`, or a `TyQPath`.
@@ -827,7 +869,7 @@ fn call_substructure_method(
         type_ident: Ident,
         nonself_args: &[P<Expr>],
         fields: &SubstructureFields<'_>,
-    ) -> P<Expr> {
+    ) -> BlockOrExpr {
         let span = trait_.span;
         let substructure = Substructure { type_ident, nonself_args, fields };
         let mut f = self.combine_substructure.borrow_mut();
@@ -902,7 +944,7 @@ fn create_method(
         generics: &Generics,
         explicit_self: Option<ast::ExplicitSelf>,
         arg_types: Vec<(Ident, P<ast::Ty>)>,
-        body: P<Expr>,
+        body: BlockOrExpr,
     ) -> P<ast::AssocItem> {
         let span = trait_.span;
         // Create the generics that aren't for `Self`.
@@ -921,7 +963,7 @@ fn create_method(
 
         let method_ident = Ident::new(self.name, span);
         let fn_decl = cx.fn_decl(args, ast::FnRetTy::Ty(ret_type));
-        let body_block = cx.block_expr(body);
+        let body_block = body.into_block(cx, span);
 
         let trait_lo_sp = span.shrink_to_lo();
 
@@ -986,7 +1028,7 @@ fn expand_struct_method_body<'b>(
         nonself_args: &[P<Expr>],
         use_temporaries: bool,
         is_packed: bool,
-    ) -> P<Expr> {
+    ) -> BlockOrExpr {
         let mut raw_fields = Vec::new(); // Vec<[fields of self], [fields of next Self arg], [etc]>
         let span = trait_.span;
         let mut patterns = Vec::new();
@@ -1047,16 +1089,14 @@ fn expand_struct_method_body<'b>(
         );
 
         if !is_packed {
-            body.span = span;
             body
         } else {
             // Do the let-destructuring.
             let mut stmts: Vec<_> = iter::zip(self_args, patterns)
                 .map(|(arg_expr, pat)| cx.stmt_let_pat(span, pat, arg_expr.clone()))
                 .collect();
-            stmts.push(cx.stmt_expr(body));
-
-            cx.expr_block(cx.block(span, stmts))
+            stmts.extend(std::mem::take(&mut body.0));
+            BlockOrExpr(stmts, body.1)
         }
     }
 
@@ -1067,7 +1107,7 @@ fn expand_static_struct_method_body(
         struct_def: &VariantData,
         type_ident: Ident,
         nonself_args: &[P<Expr>],
-    ) -> P<Expr> {
+    ) -> BlockOrExpr {
         let summary = trait_.summarise_struct(cx, struct_def);
 
         self.call_substructure_method(
@@ -1130,7 +1170,7 @@ fn expand_enum_method_body<'b>(
         type_ident: Ident,
         mut self_args: Vec<P<Expr>>,
         nonself_args: &[P<Expr>],
-    ) -> P<Expr> {
+    ) -> BlockOrExpr {
         let span = trait_.span;
         let variants = &enum_def.variants;
 
@@ -1199,7 +1239,11 @@ fn expand_enum_method_body<'b>(
                 }
 
                 // Here is the pat = `(&VariantK, &VariantK, ...)`
-                let single_pat = cx.pat_tuple(span, subpats);
+                let single_pat = if subpats.len() == 1 {
+                    subpats.pop().unwrap()
+                } else {
+                    cx.pat_tuple(span, subpats)
+                };
 
                 // For the BodyK, we need to delegate to our caller,
                 // passing it an EnumMatching to indicate which case
@@ -1253,13 +1297,9 @@ fn expand_enum_method_body<'b>(
                 // Self arg, assuming all are instances of VariantK.
                 // Build up code associated with such a case.
                 let substructure = EnumMatching(index, variants.len(), variant, field_tuples);
-                let arm_expr = self.call_substructure_method(
-                    cx,
-                    trait_,
-                    type_ident,
-                    nonself_args,
-                    &substructure,
-                );
+                let arm_expr = self
+                    .call_substructure_method(cx, trait_, type_ident, nonself_args, &substructure)
+                    .into_expr(cx, span);
 
                 cx.arm(span, single_pat, arm_expr)
             })
@@ -1271,13 +1311,16 @@ fn expand_enum_method_body<'b>(
                 // The index and actual variant aren't meaningful in this case,
                 // so just use whatever
                 let substructure = EnumMatching(0, variants.len(), v, Vec::new());
-                Some(self.call_substructure_method(
-                    cx,
-                    trait_,
-                    type_ident,
-                    nonself_args,
-                    &substructure,
-                ))
+                Some(
+                    self.call_substructure_method(
+                        cx,
+                        trait_,
+                        type_ident,
+                        nonself_args,
+                        &substructure,
+                    )
+                    .into_expr(cx, span),
+                )
             }
             _ if variants.len() > 1 && self_args.len() > 1 => {
                 // Since we know that all the arguments will match if we reach
@@ -1341,13 +1384,15 @@ fn expand_enum_method_body<'b>(
                 }
             }
 
-            let arm_expr = self.call_substructure_method(
-                cx,
-                trait_,
-                type_ident,
-                nonself_args,
-                &catch_all_substructure,
-            );
+            let arm_expr = self
+                .call_substructure_method(
+                    cx,
+                    trait_,
+                    type_ident,
+                    nonself_args,
+                    &catch_all_substructure,
+                )
+                .into_expr(cx, span);
 
             // Final wrinkle: the self_args are expressions that deref
             // down to desired places, but we cannot actually deref
@@ -1371,8 +1416,7 @@ fn expand_enum_method_body<'b>(
             //  }
             let all_match = cx.expr_match(span, match_arg, match_arms);
             let arm_expr = cx.expr_if(span, discriminant_test, all_match, Some(arm_expr));
-            index_let_stmts.push(cx.stmt_expr(arm_expr));
-            cx.expr_block(cx.block(span, index_let_stmts))
+            BlockOrExpr(index_let_stmts, Some(arm_expr))
         } else if variants.is_empty() {
             // As an additional wrinkle, For a zero-variant enum A,
             // currently the compiler
@@ -1423,7 +1467,7 @@ fn expand_enum_method_body<'b>(
             // derive Debug on such a type could here generate code
             // that needs the feature gate enabled.)
 
-            deriving::call_unreachable(cx, span)
+            BlockOrExpr(vec![], Some(deriving::call_unreachable(cx, span)))
         } else {
             // Final wrinkle: the self_args are expressions that deref
             // down to desired places, but we cannot actually deref
@@ -1431,8 +1475,12 @@ fn expand_enum_method_body<'b>(
             // expression; here add a layer of borrowing, turning
             // `(*self, *__arg_0, ...)` into `(&*self, &*__arg_0, ...)`.
             self_args.map_in_place(|self_arg| cx.expr_addr_of(span, self_arg));
-            let match_arg = cx.expr(span, ast::ExprKind::Tup(self_args));
-            cx.expr_match(span, match_arg, match_arms)
+            let match_arg = if self_args.len() == 1 {
+                self_args.pop().unwrap()
+            } else {
+                cx.expr(span, ast::ExprKind::Tup(self_args))
+            };
+            BlockOrExpr(vec![], Some(cx.expr_match(span, match_arg, match_arms)))
         }
     }
 
@@ -1443,7 +1491,7 @@ fn expand_static_enum_method_body(
         enum_def: &EnumDef,
         type_ident: Ident,
         nonself_args: &[P<Expr>],
-    ) -> P<Expr> {
+    ) -> BlockOrExpr {
         let summary = enum_def
             .variants
             .iter()
@@ -1606,71 +1654,6 @@ fn create_enum_variant_pattern(
     }
 }
 
-// helpful premade recipes
-
-fn cs_fold_fields<'a, F>(
-    use_foldl: bool,
-    mut f: F,
-    base: P<Expr>,
-    cx: &mut ExtCtxt<'_>,
-    all_fields: &[FieldInfo<'a>],
-) -> P<Expr>
-where
-    F: FnMut(&mut ExtCtxt<'_>, Span, P<Expr>, P<Expr>, &[P<Expr>]) -> P<Expr>,
-{
-    if use_foldl {
-        all_fields
-            .iter()
-            .fold(base, |old, field| f(cx, field.span, old, field.self_.clone(), &field.other))
-    } else {
-        all_fields
-            .iter()
-            .rev()
-            .fold(base, |old, field| f(cx, field.span, old, field.self_.clone(), &field.other))
-    }
-}
-
-fn cs_fold_enumnonmatch(
-    mut enum_nonmatch_f: EnumNonMatchCollapsedFunc<'_>,
-    cx: &mut ExtCtxt<'_>,
-    trait_span: Span,
-    substructure: &Substructure<'_>,
-) -> P<Expr> {
-    match *substructure.fields {
-        EnumNonMatchingCollapsed(tuple) => enum_nonmatch_f(cx, trait_span, tuple),
-        _ => cx.span_bug(trait_span, "cs_fold_enumnonmatch expected an EnumNonMatchingCollapsed"),
-    }
-}
-
-fn cs_fold_static(cx: &mut ExtCtxt<'_>, trait_span: Span) -> P<Expr> {
-    cx.span_bug(trait_span, "static function in `derive`")
-}
-
-/// Fold the fields. `use_foldl` controls whether this is done
-/// left-to-right (`true`) or right-to-left (`false`).
-pub fn cs_fold<F>(
-    use_foldl: bool,
-    f: F,
-    base: P<Expr>,
-    enum_nonmatch_f: EnumNonMatchCollapsedFunc<'_>,
-    cx: &mut ExtCtxt<'_>,
-    trait_span: Span,
-    substructure: &Substructure<'_>,
-) -> P<Expr>
-where
-    F: FnMut(&mut ExtCtxt<'_>, Span, P<Expr>, P<Expr>, &[P<Expr>]) -> P<Expr>,
-{
-    match *substructure.fields {
-        EnumMatching(.., ref all_fields) | Struct(_, ref all_fields) => {
-            cs_fold_fields(use_foldl, f, base, cx, all_fields)
-        }
-        EnumNonMatchingCollapsed(..) => {
-            cs_fold_enumnonmatch(enum_nonmatch_f, cx, trait_span, substructure)
-        }
-        StaticEnum(..) | StaticStruct(..) => cs_fold_static(cx, trait_span),
-    }
-}
-
 /// Function to fold over fields, with three cases, to generate more efficient and concise code.
 /// When the `substructure` has grouped fields, there are two cases:
 /// Zero fields: call the base case function with `None` (like the usual base case of `cs_fold`).
@@ -1679,11 +1662,11 @@ pub fn cs_fold<F>(
 /// fields.
 /// When the `substructure` is an `EnumNonMatchingCollapsed`, the result of `enum_nonmatch_f`
 /// is returned. Statics may not be folded over.
-pub fn cs_fold1<F, B>(
+pub fn cs_fold<F, B>(
     use_foldl: bool,
-    f: F,
+    mut f: F,
     mut b: B,
-    enum_nonmatch_f: EnumNonMatchCollapsedFunc<'_>,
+    mut enum_nonmatch_f: EnumNonMatchCollapsedFunc<'_>,
     cx: &mut ExtCtxt<'_>,
     trait_span: Span,
     substructure: &Substructure<'_>,
@@ -1708,12 +1691,18 @@ pub fn cs_fold1<F, B>(
                 (true, _) => (b(cx, None), &all_fields[..]),
             };
 
-            cs_fold_fields(use_foldl, f, base, cx, rest)
-        }
-        EnumNonMatchingCollapsed(..) => {
-            cs_fold_enumnonmatch(enum_nonmatch_f, cx, trait_span, substructure)
+            if use_foldl {
+                rest.iter().fold(base, |old, field| {
+                    f(cx, field.span, old, field.self_.clone(), &field.other)
+                })
+            } else {
+                rest.iter().rev().fold(base, |old, field| {
+                    f(cx, field.span, old, field.self_.clone(), &field.other)
+                })
+            }
         }
-        StaticEnum(..) | StaticStruct(..) => cs_fold_static(cx, trait_span),
+        EnumNonMatchingCollapsed(tuple) => enum_nonmatch_f(cx, trait_span, tuple),
+        StaticEnum(..) | StaticStruct(..) => cx.span_bug(trait_span, "static function in `derive`"),
     }
 }
 

compiler/rustc_builtin_macros/src/deriving/hash.rs

@@ -2,8 +2,7 @@
 use crate::deriving::generic::*;
 use crate::deriving::{self, path_std, pathvec_std};
 
-use rustc_ast::ptr::P;
-use rustc_ast::{Expr, MetaItem, Mutability};
+use rustc_ast::{MetaItem, Mutability};
 use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_span::symbol::sym;
 use rustc_span::Span;
@@ -45,7 +44,11 @@ pub fn expand_deriving_hash(
     hash_trait_def.expand(cx, mitem, item, push);
 }
 
-fn hash_substructure(cx: &mut ExtCtxt<'_>, trait_span: Span, substr: &Substructure<'_>) -> P<Expr> {
+fn hash_substructure(
+    cx: &mut ExtCtxt<'_>,
+    trait_span: Span,
+    substr: &Substructure<'_>,
+) -> BlockOrExpr {
     let [state_expr] = substr.nonself_args else {
         cx.span_bug(trait_span, "incorrect number of arguments in `derive(Hash)`");
     };
@@ -81,6 +84,5 @@ fn hash_substructure(cx: &mut ExtCtxt<'_>, trait_span: Span, substr: &Substructu
     stmts.extend(
         fields.iter().map(|FieldInfo { ref self_, span, .. }| call_hash(*span, self_.clone())),
     );
-
-    cx.expr_block(cx.block(trait_span, stmts))
+    BlockOrExpr::new_stmts(stmts)
 }

src/test/codegen/consts.rs

@@ -10,7 +10,7 @@
 // CHECK: @STATIC = {{.*}}, align 4
 
 // This checks the constants from inline_enum_const
-// CHECK: @alloc14 = {{.*}}, align 2
+// CHECK: @alloc12 = {{.*}}, align 2
 
 // This checks the constants from {low,high}_align_const, they share the same
 // constant, but the alignment differs, so the higher one should be used

src/test/ui/deriving/deriving-all-codegen.rs

@@ -39,6 +39,16 @@ struct Big {
 #[repr(packed)]
 struct Packed(u32);
 
+// An empty enum.
+#[derive(Clone, Copy, Debug, Hash, PartialEq, Eq, PartialOrd, Ord)]
+enum Enum0 {}
+
+// A single-variant enum.
+#[derive(Clone, Debug, Hash, PartialEq, Eq, PartialOrd, Ord)]
+enum Enum1 {
+    Single { x: u32 }
+}
+
 // A C-like, fieldless enum.
 #[derive(Clone, Copy, Debug, Default, Hash, PartialEq, Eq, PartialOrd, Ord)]
 enum Fieldless {
@@ -66,3 +76,11 @@ enum Fielded {
     Y(bool),
     Z(Option<i32>),
 }
+
+// A union. Most builtin traits are not derivable for unions.
+#[derive(Clone, Copy)]
+pub union Union {
+    pub b: bool,
+    pub u: u32,
+    pub i: i32,
+}