EXPERIMENT: derive Debug as a single function call

compiler/rustc_builtin_macros/src/deriving/debug.rs

@@ -6,7 +6,7 @@ use rustc_ast::ptr::P;
 use rustc_ast::{self as ast, Expr, LocalKind, MetaItem};
 use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_span::symbol::{sym, Ident};
-use rustc_span::{Span, DUMMY_SP};
+use rustc_span::Span;
 
 fn make_mut_borrow(cx: &mut ExtCtxt<'_>, sp: Span, expr: P<Expr>) -> P<Expr> {
     cx.expr(sp, ast::ExprKind::AddrOf(ast::BorrowKind::Ref, ast::Mutability::Mut, expr))
@@ -78,6 +78,65 @@ fn show_substructure(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>
         return cx.expr_block(block);
     }
 
+    if let ast::VariantData::Struct(..) = vdata {
+        let mut name_exprs = Vec::with_capacity(fields.len() + 1);
+        let mut value_exprs = Vec::with_capacity(fields.len());
+
+        name_exprs.push(name);
+        for field in fields {
+            name_exprs.push(cx.expr_lit(
+                field.span,
+                ast::LitKind::Str(field.name.unwrap().name, ast::StrStyle::Cooked),
+            ));
+
+            // Use double indirection to make sure this works for unsized types
+            let value_ref = cx.expr_addr_of(field.span, field.self_.clone());
+            value_exprs.push(cx.expr_addr_of(field.span, value_ref));
+        }
+
+        // `let names: &'static _ = &["TheType", "field1", "field2"];`
+        let lt_static = Some(cx.lifetime_static(span));
+        let ty_static_ref = cx.ty_rptr(span, cx.ty_infer(span), lt_static, ast::Mutability::Not);
+        let names_let = cx.stmt_let_ty(
+            span,
+            false,
+            Ident::new(sym::names, span),
+            Some(ty_static_ref),
+            cx.expr_array_ref(span, name_exprs),
+        );
+
+        // `let values: &[&dyn Debug] = &[&&self.field1, &&self.field2];`
+        let path_debug = cx.path_global(span, cx.std_path(&[sym::fmt, sym::Debug]));
+        let ty_dyn_debug = cx.ty(
+            span,
+            ast::TyKind::TraitObject(vec![cx.trait_bound(path_debug)], ast::TraitObjectSyntax::Dyn),
+        );
+        let ty_slice = cx.ty(
+            span,
+            ast::TyKind::Slice(cx.ty_rptr(span, ty_dyn_debug, None, ast::Mutability::Not)),
+        );
+        let values_let = cx.stmt_let_ty(
+            span,
+            false,
+            Ident::new(sym::values, span),
+            Some(cx.ty_rptr(span, ty_slice, None, ast::Mutability::Not)),
+            cx.expr_array_ref(span, value_exprs),
+        );
+
+        // `fmt::DebugStruct::internal_debug_from_slices(fmt, names, values)`
+        let fn_path_debug_internal =
+            cx.std_path(&[sym::fmt, sym::DebugStruct, sym::internal_debug_from_slices]);
+        let args = vec![
+            fmt,
+            cx.expr_ident(span, Ident::new(sym::names, span)),
+            cx.expr_ident(span, Ident::new(sym::values, span)),
+        ];
+        let expr = cx.expr_call_global(span, fn_path_debug_internal, args);
+
+        let block = cx.block(span, vec![names_let, values_let, cx.stmt_expr(expr)]);
+        return cx.expr_block(block);
+    }
+
     let builder = Ident::new(sym::debug_trait_builder, span);
     let builder_expr = cx.expr_ident(span, builder);
 
@@ -111,6 +170,9 @@ fn show_substructure(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>
             fn_path_finish = cx.std_path(&[sym::fmt, sym::DebugTuple, sym::finish]);
         }
         ast::VariantData::Struct(..) => {
+            cx.span_bug(span, "structs should have been handled above");
+
+            /*
             // normal struct/struct variant
             let fn_path_debug_struct = cx.std_path(&[sym::fmt, sym::Formatter, sym::debug_struct]);
             let expr = cx.expr_call_global(span, fn_path_debug_struct, vec![fmt, name]);
@@ -135,6 +197,7 @@ fn show_substructure(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>
                 stmts.push(stmt_let_underscore(cx, span, expr));
             }
             fn_path_finish = cx.std_path(&[sym::fmt, sym::DebugStruct, sym::finish]);
+            */
         }
     }
 

compiler/rustc_builtin_macros/src/deriving/decodable.rs

@@ -162,7 +162,7 @@ fn decodable_substructure(
                 cx.expr_match(trait_span, cx.expr_ident(trait_span, variant), arms),
             );
             let lambda = cx.lambda(trait_span, vec![blkarg, variant], result);
-            let variant_vec = cx.expr_vec(trait_span, variants);
+            let variant_vec = cx.expr_array(trait_span, variants);
             let variant_vec = cx.expr_addr_of(trait_span, variant_vec);
             let fn_read_enum_variant_path: Vec<_> =
                 cx.def_site_path(&[sym::rustc_serialize, sym::Decoder, sym::read_enum_variant]);

compiler/rustc_builtin_macros/src/format.rs

@@ -765,7 +765,7 @@ impl<'a, 'b> Context<'a, 'b> {
 
         // First, build up the static array which will become our precompiled
         // format "string"
-        let pieces = self.ecx.expr_vec_slice(self.fmtsp, self.str_pieces);
+        let pieces = self.ecx.expr_array_ref(self.fmtsp, self.str_pieces);
 
         // We need to construct a &[ArgumentV1] to pass into the fmt::Arguments
         // constructor. In general the expressions in this slice might be
@@ -838,7 +838,7 @@ impl<'a, 'b> Context<'a, 'b> {
             fmt_args.push(Context::format_arg(self.ecx, self.macsp, span, arg_ty, arg));
         }
 
-        let args_array = self.ecx.expr_vec(self.macsp, fmt_args);
+        let args_array = self.ecx.expr_array(self.macsp, fmt_args);
         let args_slice = self.ecx.expr_addr_of(
             self.macsp,
             if no_need_for_match {
@@ -868,7 +868,7 @@ impl<'a, 'b> Context<'a, 'b> {
         } else {
             // Build up the static array which will store our precompiled
             // nonstandard placeholders, if there are any.
-            let fmt = self.ecx.expr_vec_slice(self.macsp, self.pieces);
+            let fmt = self.ecx.expr_array_ref(self.macsp, self.pieces);
 
             let path = self.ecx.std_path(&[sym::fmt, sym::UnsafeArg, sym::new]);
             let unsafe_arg = self.ecx.expr_call_global(self.macsp, path, Vec::new());

compiler/rustc_builtin_macros/src/proc_macro_harness.rs

@@ -318,7 +318,7 @@ fn mk_decls(cx: &mut ExtCtxt<'_>, macros: &[ProcMacro]) -> P<ast::Item> {
                         proc_macro_ty_method_path(cx, custom_derive),
                         vec![
                             cx.expr_str(cd.span, cd.trait_name),
-                            cx.expr_vec_slice(
+                            cx.expr_array_ref(
                                 span,
                                 cd.attrs
                                     .iter()
@@ -365,7 +365,7 @@ fn mk_decls(cx: &mut ExtCtxt<'_>, macros: &[ProcMacro]) -> P<ast::Item> {
                 ast::Mutability::Not,
             ),
             ast::Mutability::Not,
-            cx.expr_vec_slice(span, decls),
+            cx.expr_array_ref(span, decls),
         )
         .map(|mut i| {
             let attr = cx.meta_word(span, sym::rustc_proc_macro_decls);

compiler/rustc_builtin_macros/src/test_harness.rs

@@ -351,7 +351,7 @@ fn mk_tests_slice(cx: &TestCtxt<'_>, sp: Span) -> P<ast::Expr> {
     debug!("building test vector from {} tests", cx.test_cases.len());
     let ecx = &cx.ext_cx;
 
-    ecx.expr_vec_slice(
+    ecx.expr_array_ref(
         sp,
         cx.test_cases
             .iter()

compiler/rustc_expand/src/build.rs

@@ -57,6 +57,10 @@ impl<'a> ExtCtxt<'a> {
         P(ast::Ty { id: ast::DUMMY_NODE_ID, span, kind, tokens: None })
     }
 
+    pub fn ty_infer(&self, span: Span) -> P<ast::Ty> {
+        self.ty(span, ast::TyKind::Infer)
+    }
+
     pub fn ty_path(&self, path: ast::Path) -> P<ast::Ty> {
         self.ty(path.span, ast::TyKind::Path(None, path))
     }
@@ -139,11 +143,26 @@ impl<'a> ExtCtxt<'a> {
         ast::Lifetime { id: ast::DUMMY_NODE_ID, ident: ident.with_span_pos(span) }
     }
 
+    pub fn lifetime_static(&self, span: Span) -> ast::Lifetime {
+        self.lifetime(span, Ident::new(kw::StaticLifetime, span))
+    }
+
     pub fn stmt_expr(&self, expr: P<ast::Expr>) -> ast::Stmt {
         ast::Stmt { id: ast::DUMMY_NODE_ID, span: expr.span, kind: ast::StmtKind::Expr(expr) }
     }
 
     pub fn stmt_let(&self, sp: Span, mutbl: bool, ident: Ident, ex: P<ast::Expr>) -> ast::Stmt {
+        self.stmt_let_ty(sp, mutbl, ident, None, ex)
+    }
+
+    pub fn stmt_let_ty(
+        &self,
+        sp: Span,
+        mutbl: bool,
+        ident: Ident,
+        ty: Option<P<ast::Ty>>,
+        ex: P<ast::Expr>,
+    ) -> ast::Stmt {
         let pat = if mutbl {
             let binding_mode = ast::BindingMode::ByValue(ast::Mutability::Mut);
             self.pat_ident_binding_mode(sp, ident, binding_mode)
@@ -152,7 +171,7 @@ impl<'a> ExtCtxt<'a> {
         };
         let local = P(ast::Local {
             pat,
-            ty: None,
+            ty,
             id: ast::DUMMY_NODE_ID,
             kind: LocalKind::Init(ex),
             span: sp,
@@ -310,11 +329,13 @@ impl<'a> ExtCtxt<'a> {
         self.expr_lit(sp, ast::LitKind::Bool(value))
     }
 
-    pub fn expr_vec(&self, sp: Span, exprs: Vec<P<ast::Expr>>) -> P<ast::Expr> {
+    /// `[expr1, expr2, ...]`
+    pub fn expr_array(&self, sp: Span, exprs: Vec<P<ast::Expr>>) -> P<ast::Expr> {
         self.expr(sp, ast::ExprKind::Array(exprs))
     }
-    pub fn expr_vec_slice(&self, sp: Span, exprs: Vec<P<ast::Expr>>) -> P<ast::Expr> {
-        self.expr_addr_of(sp, self.expr_vec(sp, exprs))
+    /// `&[expr1, expr2, ...]`
+    pub fn expr_array_ref(&self, sp: Span, exprs: Vec<P<ast::Expr>>) -> P<ast::Expr> {
+        self.expr_addr_of(sp, self.expr_array(sp, exprs))
     }
     pub fn expr_str(&self, sp: Span, s: Symbol) -> P<ast::Expr> {
         self.expr_lit(sp, ast::LitKind::Str(s, ast::StrStyle::Cooked))

compiler/rustc_span/src/symbol.rs

@@ -782,6 +782,7 @@ symbols! {
         integer_: "integer",
         integral,
         intel,
+        internal_debug_from_slices,
         into_future,
         into_iter,
         intra_doc_pointers,

library/core/src/fmt/builders.rs

@@ -1,6 +1,7 @@
 #![allow(unused_imports)]
 
 use crate::fmt::{self, Debug, Formatter};
+use crate::iter;
 
 struct PadAdapter<'buf, 'state> {
     buf: &'buf mut (dyn fmt::Write + 'buf),
@@ -101,6 +102,86 @@ pub(super) fn debug_struct_new<'a, 'b>(
 }
 
 impl<'a, 'b: 'a> DebugStruct<'a, 'b> {
+    /// Makes the full debug output using the data from the provided slices.
+    ///
+    /// This is a horrible API that encodes far more in the `names` slice than
+    /// any good API ever would.  It will never be stabilized.
+    ///
+    /// But this a great way for the derive in `rustc_builtin_macros` to be able
+    /// to just have a `'static` slice for `names`, and a runtime array for
+    /// `values`, rather than needing to codegen a call for every field.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(fmt_helpers_for_derive)]
+    /// use std::fmt;
+    ///
+    /// struct Bar {
+    ///     bar: i32,
+    ///     another: String,
+    /// }
+    ///
+    /// impl fmt::Debug for Bar {
+    ///     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+    ///         fmt::DebugStruct::internal_debug_from_slices(
+    ///             fmt,
+    ///             &["Bartender", "bar", "another"],
+    ///             &[&self.bar, &self.another],
+    ///         )
+    ///     }
+    /// }
+    ///
+    /// assert_eq!(
+    ///     format!("{:?}", Bar { bar: 10, another: "Hello World".to_string() }),
+    ///     "Bartender { bar: 10, another: \"Hello World\" }",
+    /// );
+    ///
+    /// struct Bat {
+    ///     bat: i32,
+    /// }
+    ///
+    /// impl fmt::Debug for Bat {
+    ///     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+    ///         fmt::DebugStruct::internal_debug_from_slices(
+    ///             fmt,
+    ///             &["Baton", "battle", ".."],
+    ///             &[&self.bat],
+    ///         )
+    ///     }
+    /// }
+    ///
+    /// assert_eq!(
+    ///     format!("{:?}", Bat { bat: 123 }),
+    ///     "Baton { battle: 123, .. }",
+    /// );
+    /// ```
+    //#[doc(hidden)]
+    #[unstable(feature = "fmt_helpers_for_derive", issue = "none")]
+    pub fn internal_debug_from_slices(
+        fmt: &mut fmt::Formatter<'_>,
+        names: &[&str],
+        values: &[&dyn Debug],
+    ) -> fmt::Result {
+        // The derive will never call this without a name.
+        debug_assert_ne!(names.len(), 0);
+
+        let Some((struct_name, field_names)) = names.split_first() else {
+            return Err(fmt::Error);
+        };
+
+        let mut builder = fmt.debug_struct(struct_name);
+        for (n, v) in iter::zip(field_names, values) {
+            builder.field(n, v);
+        }
+
+        if field_names.len() > values.len() {
+            builder.finish_non_exhaustive()
+        } else {
+            builder.finish()
+        }
+    }
+
     /// Adds a new field to the generated struct output.
     ///
     /// # Examples

library/core/src/fmt/mod.rs

@@ -690,7 +690,7 @@ pub(crate) mod macros {
     /// Derive macro generating an impl of the trait `Debug`.
     #[rustc_builtin_macro]
     #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
-    #[allow_internal_unstable(core_intrinsics)]
+    #[allow_internal_unstable(core_intrinsics, fmt_helpers_for_derive)]
     pub macro Debug($item:item) {
         /* compiler built-in */
     }

library/core/src/lib.rs

@@ -183,6 +183,7 @@
 #![feature(intra_doc_pointers)]
 #![feature(intrinsics)]
 #![feature(lang_items)]
+#![feature(let_else)]
 #![feature(link_llvm_intrinsics)]
 #![feature(min_specialization)]
 #![feature(mixed_integer_ops)]