Don't allow new const panic through format flattening.

panic!("a {}", "b") is still not allowed in const,
even if the hir flattens to panic!("a b").

compiler/rustc_ast_lowering/src/format.rs

@@ -13,9 +13,12 @@ use std::borrow::Cow;
 
 impl<'hir> LoweringContext<'_, 'hir> {
     pub(crate) fn lower_format_args(&mut self, sp: Span, fmt: &FormatArgs) -> hir::ExprKind<'hir> {
+        // Never call the const constructor of `fmt::Arguments` if the
+        // format_args!() had any arguments _before_ flattening/inlining.
+        let allow_const = fmt.arguments.all_args().is_empty();
         let fmt = flatten_format_args(Cow::Borrowed(fmt));
         let fmt = inline_literals(fmt);
-        expand_format_args(self, sp, &fmt)
+        expand_format_args(self, sp, &fmt, allow_const)
     }
 }
 
@@ -342,6 +345,7 @@ fn expand_format_args<'hir>(
     ctx: &mut LoweringContext<'_, 'hir>,
     macsp: Span,
     fmt: &FormatArgs,
+    allow_const: bool,
 ) -> hir::ExprKind<'hir> {
     let mut incomplete_lit = String::new();
     let lit_pieces =
@@ -411,6 +415,18 @@ fn expand_format_args<'hir>(
 
     let arguments = fmt.arguments.all_args();
 
+    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(
+            macsp,
+            hir::LangItem::FormatArguments,
+            sym::new_const,
+        ));
+        let new_args = ctx.arena.alloc_from_iter([lit_pieces]);
+        return hir::ExprKind::Call(new, new_args);
+    }
+
     // If the args array contains exactly all the original arguments once,
     // in order, we can use a simple array instead of a `match` construction.
     // However, if there's a yield point in any argument except the first one,

compiler/rustc_span/src/symbol.rs

@@ -984,6 +984,7 @@ symbols! {
         never_type_fallback,
         new,
         new_binary,
+        new_const,
         new_debug,
         new_display,
         new_lower_exp,

library/core/src/fmt/mod.rs

@@ -392,8 +392,31 @@ enum FlagV1 {
 }
 
 impl<'a> Arguments<'a> {
+    #[doc(hidden)]
+    #[inline]
+    #[unstable(feature = "fmt_internals", issue = "none")]
+    #[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");
+        }
+        Arguments { pieces, fmt: None, args: &[] }
+    }
+
     /// When using the format_args!() macro, this function is used to generate the
     /// Arguments structure.
+    #[cfg(not(bootstrap))]
+    #[doc(hidden)]
+    #[inline]
+    #[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
+    pub fn new_v1(pieces: &'a [&'static str], args: &'a [ArgumentV1<'a>]) -> Arguments<'a> {
+        if pieces.len() < args.len() || pieces.len() > args.len() + 1 {
+            panic!("invalid args");
+        }
+        Arguments { pieces, fmt: None, args }
+    }
+
+    #[cfg(bootstrap)]
     #[doc(hidden)]
     #[inline]
     #[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
@@ -417,8 +440,7 @@ impl<'a> Arguments<'a> {
     #[doc(hidden)]
     #[inline]
     #[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
-    #[rustc_const_unstable(feature = "const_fmt_arguments_new", issue = "none")]
+    pub fn new_v1_formatted(
-    pub const fn new_v1_formatted(
         pieces: &'a [&'static str],
         args: &'a [ArgumentV1<'a>],
         fmt: &'a [rt::v1::Argument],

library/core/src/panicking.rs

@@ -111,7 +111,7 @@ pub const fn panic(expr: &'static str) -> ! {
     // truncation and padding (even though none is used here). Using
     // Arguments::new_v1 may allow the compiler to omit Formatter::pad from the
     // output binary, saving up to a few kilobytes.
-    panic_fmt(fmt::Arguments::new_v1(&[expr], &[]));
+    panic_fmt(fmt::Arguments::new_const(&[expr]));
 }
 
 /// Like `panic`, but without unwinding and track_caller to reduce the impact on codesize.
@@ -120,7 +120,7 @@ pub const fn panic(expr: &'static str) -> ! {
 #[lang = "panic_nounwind"] // needed by codegen for non-unwinding panics
 #[rustc_nounwind]
 pub fn panic_nounwind(expr: &'static str) -> ! {
-    panic_nounwind_fmt(fmt::Arguments::new_v1(&[expr], &[]));
+    panic_nounwind_fmt(fmt::Arguments::new_const(&[expr]));
 }
 
 #[inline]

tests/ui/borrowck/issue-64453.stderr

@@ -1,4 +1,4 @@
-error: `Arguments::<'a>::new_v1` is not yet stable as a const fn
+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!("");

tests/ui/consts/const-eval/format.rs

@@ -1,12 +1,13 @@
 const fn failure() {
     panic!("{:?}", 0);
     //~^ ERROR cannot call non-const formatting macro in constant functions
+    //~| ERROR cannot call non-const fn `Arguments::<'_>::new_v1` in constant functions
 }
 
 const fn print() {
     println!("{:?}", 0);
     //~^ ERROR cannot call non-const formatting macro in constant functions
-    //~| ERROR `Arguments::<'a>::new_v1` is not yet stable as a const fn
+    //~| ERROR cannot call non-const fn `Arguments::<'_>::new_v1` in constant functions
     //~| ERROR cannot call non-const fn `_print` in constant functions
 }
 

tests/ui/consts/const-eval/format.stderr

@@ -7,8 +7,17 @@ LL |     panic!("{:?}", 0);
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
    = note: this error originates in the macro `$crate::const_format_args` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
+error[E0015]: cannot call non-const fn `Arguments::<'_>::new_v1` in constant functions
+  --> $DIR/format.rs:2:5
+   |
+LL |     panic!("{:?}", 0);
+   |     ^^^^^^^^^^^^^^^^^
+   |
+   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+   = note: this error originates in the macro `$crate::const_format_args` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
+
 error[E0015]: cannot call non-const formatting macro in constant functions
-  --> $DIR/format.rs:7:22
+  --> $DIR/format.rs:8:22
    |
 LL |     println!("{:?}", 0);
    |                      ^
@@ -16,17 +25,17 @@ LL |     println!("{:?}", 0);
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
    = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: `Arguments::<'a>::new_v1` is not yet stable as a const fn
+error[E0015]: cannot call non-const fn `Arguments::<'_>::new_v1` in constant functions
-  --> $DIR/format.rs:7:5
+  --> $DIR/format.rs:8:5
    |
 LL |     println!("{:?}", 0);
    |     ^^^^^^^^^^^^^^^^^^^
    |
-   = help: add `#![feature(const_fmt_arguments_new)]` to the crate attributes to enable
+   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
    = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0015]: cannot call non-const fn `_print` in constant functions
-  --> $DIR/format.rs:7:5
+  --> $DIR/format.rs:8:5
    |
 LL |     println!("{:?}", 0);
    |     ^^^^^^^^^^^^^^^^^^^
@@ -63,19 +72,19 @@ LL |     panic!("{:?}", 0);
    = note: this note originates in the macro `$crate::const_format_args` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: erroneous constant used
-  --> $DIR/format.rs:7:14
+  --> $DIR/format.rs:8:14
    |
 LL |     println!("{:?}", 0);
    |              ^^^^^^
 
 note: erroneous constant used
-  --> $DIR/format.rs:7:14
+  --> $DIR/format.rs:8:14
    |
 LL |     println!("{:?}", 0);
    |              ^^^^^^
 
 note: erroneous constant used
-  --> $DIR/format.rs:7:22
+  --> $DIR/format.rs:8:22
    |
 LL |     println!("{:?}", 0);
    |                      ^
@@ -83,13 +92,13 @@ LL |     println!("{:?}", 0);
    = note: this note originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: erroneous constant used
-  --> $DIR/format.rs:7:22
+  --> $DIR/format.rs:8:22
    |
 LL |     println!("{:?}", 0);
    |                      ^
    |
    = note: this note originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to 4 previous errors
+error: aborting due to 5 previous errors
 
 For more information about this error, try `rustc --explain E0015`.