Emit less MIR for format_args! with just a str literal

Author: saethlin

compiler/rustc_ast_lowering/src/format.rs

@@ -5,6 +5,7 @@ use rustc_ast::*;
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_hir as hir;
 use rustc_span::{
+    hygiene::DesugaringKind,
     sym,
     symbol::{kw, Ident},
     Span, Symbol,
@@ -425,14 +426,36 @@ fn expand_format_args<'hir>(
 
     if allow_const && arguments.is_empty() && argmap.is_empty() {
         // Generate:
-        //     <core::fmt::Arguments>::new_const(lit_pieces)
-        let new = ctx.arena.alloc(ctx.expr_lang_item_type_relative(
+        //     const { <core::fmt::Arguments>::new_const(lit_pieces) }
+        let span = ctx.mark_span_with_reason(
+            DesugaringKind::FormatArgs,
             macsp,
+            ctx.allow_format_args.clone(),
+        );
+        let new = ctx.arena.alloc(ctx.expr_lang_item_type_relative(
+            span,
             hir::LangItem::FormatArguments,
             sym::new_const,
         ));
         let new_args = ctx.arena.alloc_from_iter([lit_pieces]);
-        return hir::ExprKind::Call(new, new_args);
+        let call = hir::ExprKind::Call(new, new_args);
+
+        let node_id = ctx.next_node_id();
+        let call_expr = hir::Expr { hir_id: ctx.lower_node_id(node_id), kind: call, span };
+
+        let parent_def_id = ctx.current_hir_id_owner;
+        let node_id = ctx.next_node_id();
+        let def_id = ctx.create_def(
+            parent_def_id.def_id,
+            node_id,
+            hir::definitions::DefPathData::AnonConst,
+            span,
+        );
+        return hir::ExprKind::ConstBlock(hir::AnonConst {
+            def_id,
+            hir_id: ctx.lower_node_id(node_id),
+            body: ctx.lower_body(|_this| (&[], call_expr)),
+        });
     }
 
     // If the args array contains exactly all the original arguments once,

compiler/rustc_ast_lowering/src/item.rs

@@ -84,6 +84,7 @@ impl<'a, 'hir> ItemLowerer<'a, 'hir> {
             allow_try_trait: Some([sym::try_trait_v2, sym::yeet_desugar_details][..].into()),
             allow_gen_future: Some([sym::gen_future, sym::closure_track_caller][..].into()),
             allow_into_future: Some([sym::into_future][..].into()),
+            allow_format_args: Some([sym::const_fmt_arguments_new, sym::fmt_internals][..].into()),
             generics_def_id_map: Default::default(),
         };
         lctx.with_hir_id_owner(owner, |lctx| f(lctx));

compiler/rustc_ast_lowering/src/lib.rs

@@ -137,6 +137,7 @@ struct LoweringContext<'a, 'hir> {
     allow_try_trait: Option<Lrc<[Symbol]>>,
     allow_gen_future: Option<Lrc<[Symbol]>>,
     allow_into_future: Option<Lrc<[Symbol]>>,
+    allow_format_args: Option<Lrc<[Symbol]>>,
 
     /// Mapping from generics `def_id`s to TAIT generics `def_id`s.
     /// For each captured lifetime (e.g., 'a), we create a new lifetime parameter that is a generic

compiler/rustc_span/src/hygiene.rs

@@ -1152,6 +1152,7 @@ pub enum DesugaringKind {
     ForLoop,
     WhileLoop,
     Replace,
+    FormatArgs,
 }
 
 impl DesugaringKind {
@@ -1168,6 +1169,7 @@ impl DesugaringKind {
             DesugaringKind::ForLoop => "`for` loop",
             DesugaringKind::WhileLoop => "`while` loop",
             DesugaringKind::Replace => "drop and replace",
+            DesugaringKind::FormatArgs => "`format_args!` expansion",
         }
     }
 }

compiler/rustc_span/src/symbol.rs

@@ -511,6 +511,7 @@ symbols! {
         const_eval_select,
         const_evaluatable_checked,
         const_extern_fn,
+        const_fmt_arguments_new,
         const_fn,
         const_fn_floating_point_arithmetic,
         const_fn_fn_ptr_basics,
@@ -718,6 +719,7 @@ symbols! {
         fmaf32,
         fmaf64,
         fmt,
+        fmt_internals,
         fmul_fast,
         fn_align,
         fn_must_use,

library/core/src/fmt/mod.rs

@@ -393,7 +393,8 @@ impl<'a> Arguments<'a> {
     #[rustc_const_unstable(feature = "const_fmt_arguments_new", issue = "none")]
     pub const fn new_const(pieces: &'a [&'static str]) -> Self {
         if pieces.len() > 1 {
-            panic!("invalid args");
+            // panic! expands to a call to this function, so we do this to avoid recursion.
+            crate::panicking::panic("invalid args");
         }
         Arguments { pieces, fmt: None, args: &[] }
     }

tests/ui/borrowck/issue-64453.rs

@@ -3,7 +3,6 @@ struct Value;
 
 static settings_dir: String = format!("");
 //~^ ERROR cannot call non-const fn
-//~| ERROR is not yet stable as a const
 
 fn from_string(_: String) -> Value {
     Value

tests/ui/borrowck/issue-64453.stderr

@@ -1,12 +1,3 @@
-error: `Arguments::<'a>::new_const` is not yet stable as a const fn
-  --> $DIR/issue-64453.rs:4:31
-   |
-LL | static settings_dir: String = format!("");
-   |                               ^^^^^^^^^^^
-   |
-   = help: add `#![feature(const_fmt_arguments_new)]` to the crate attributes to enable
-   = note: this error originates in the macro `$crate::__export::format_args` which comes from the expansion of the macro `format` (in Nightly builds, run with -Z macro-backtrace for more info)
-
 error[E0015]: cannot call non-const fn `format` in statics
   --> $DIR/issue-64453.rs:4:31
    |
@@ -18,12 +9,12 @@ LL | static settings_dir: String = format!("");
    = note: this error originates in the macro `format` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0507]: cannot move out of static item `settings_dir`
-  --> $DIR/issue-64453.rs:14:37
+  --> $DIR/issue-64453.rs:13:37
    |
 LL |     let settings_data = from_string(settings_dir);
    |                                     ^^^^^^^^^^^^ move occurs because `settings_dir` has type `String`, which does not implement the `Copy` trait
 
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 
 Some errors have detailed explanations: E0015, E0507.
 For more information about an error, try `rustc --explain E0015`.