Put const data of format_args in const block.

Author: m-ou-se

compiler/rustc_ast_lowering/src/expr.rs

@@ -2270,6 +2270,15 @@ impl<'hir> LoweringContext<'_, 'hir> {
         )
     }
 
+    pub(super) fn expr_const(&mut self, span: Span, expr: hir::Expr<'hir>) -> hir::Expr<'hir> {
+        let node_id = self.next_node_id();
+        let def_id =
+            self.create_def(node_id, None, DefKind::InlineConst, DefPathData::LateAnonConst, span);
+        let hir_id = self.lower_node_id(node_id);
+        let body = self.record_body(&[], expr);
+        self.expr(span, hir::ExprKind::ConstBlock(hir::ConstBlock { hir_id, def_id, body }))
+    }
+
     fn expr_block_empty(&mut self, span: Span) -> &'hir hir::Expr<'hir> {
         let blk = self.block_all(span, &[], None);
         let expr = self.expr_block(blk);

compiler/rustc_ast_lowering/src/format.rs

@@ -525,11 +525,12 @@ fn expand_format_args<'hir>(
     pieces.push(make_piece(ctx, sym::num, zero, macsp));
 
     // ```
-    //   unsafe { <core::fmt::rt::Template>::new(&[pieces…]) }
+    //   unsafe { <core::fmt::rt::Template>::new(const { &[pieces…] }) }
     // ```
     let template_new =
         ctx.expr_lang_item_type_relative(macsp, hir::LangItem::FormatTemplate, sym::new);
     let pieces = ctx.expr_array_ref(macsp, ctx.arena.alloc_from_iter(pieces));
+    let pieces = ctx.expr_const(macsp, pieces);
     let template = ctx.expr(
         macsp,
         hir::ExprKind::Call(ctx.arena.alloc(template_new), ctx.arena.alloc_from_iter([pieces])),

compiler/rustc_ast_lowering/src/item.rs

@@ -1174,7 +1174,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         }
     }
 
-    fn record_body(
+    pub(super) fn record_body(
         &mut self,
         params: &'hir [hir::Param<'hir>],
         value: hir::Expr<'hir>,

library/core/src/fmt/rt.rs

@@ -67,22 +67,11 @@ pub union Piece {
 unsafe impl Send for Piece {}
 unsafe impl Sync for Piece {}
 
-// These are marked as #[stable] because of #[rustc_promotable] and #[rustc_const_stable].
-// With #[rustc_const_unstable], many format_args!() invocations would result in errors.
-//
-// There is still no way to use these on stable, because Piece itself is #[unstable] and not
-// reachable through any public path. (format_args!()'s expansion uses it as a lang item.)
 impl Piece {
-    #[rustc_promotable]
-    #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_stable(feature = "rust1", since = "1.0.0")]
     pub const fn str(s: &'static str) -> Self {
         Self { p: s as *const str as *const u8 }
     }
 
-    #[rustc_promotable]
-    #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_stable(feature = "rust1", since = "1.0.0")]
     pub const fn num(i: usize) -> Self {
         Self { i }
     }