Auto merge of rust-lang#131663 - veera-sivarajan:fix-128709-final, r=<try>

Evaluate `std::fmt::Arguments::new_const()` during Compile Time

Fixes rust-lang#128709

This PR aims to optimize calls to string formating macros without any arguments by evaluating `std::fmt::Arguments::new_const()` in a const context.

Currently,
`println!("hola")` compiles to `std::io::_print(std::fmt::Arguments::new_const(&["hola\n"]))`.

With this PR,
`println!("hola")` compiles to `std::io::_print(const { std::fmt::Arguments::new_const(&["hola\n"]) })`.

This is accomplished in two steps:

1.  Const stabilize `std::fmt::Arguments::new_const()`.
2.  Wrap calls to `std::fmt::Arguments::new_const()` in an inline const block when lowering the AST to HIR.

This reduces the generated code to a `memcpy` instead of multiple `getelementptr` and `store` instructions even with `-C no-prepopulate-passes -C opt-level=0`. Godbolt for code comparison: https://rust.godbolt.org/z/P7Px7de6c

This is a safe and sound transformation because `std::fmt::Arguments::new_const()` is a trivial constructor function taking a slice containing a `'static` string literal as input.

CC rust-lang#99012

Author: bors

compiler/rustc_ast_lowering/src/format.rs

@@ -5,6 +5,7 @@ use rustc_ast::visit::Visitor;
 use rustc_ast::*;
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_hir as hir;
+use rustc_hir::def::DefKind;
 use rustc_session::config::FmtDebug;
 use rustc_span::symbol::{Ident, kw};
 use rustc_span::{Span, Symbol, sym};
@@ -24,6 +25,28 @@ impl<'hir> LoweringContext<'_, 'hir> {
         expand_format_args(self, sp, &fmt, allow_const)
     }
 
+    /// Wraps given `ExprKind` in an inline const block.
+    ///
+    /// Caller must ensure it's safe and sound to do so.
+    fn wrap_in_const_context(
+        &mut self,
+        sp: Span,
+        kind: hir::ExprKind<'hir>,
+    ) -> hir::ExprKind<'hir> {
+        let expr = hir::Expr { hir_id: self.next_id(), kind, span: self.lower_span(sp) };
+        let const_node_id = self.next_node_id();
+        let parent_def_id = self.current_def_id_parent;
+        let def_id =
+            self.create_def(parent_def_id, const_node_id, kw::Empty, DefKind::InlineConst, sp);
+        let hir_id = self.lower_node_id(const_node_id);
+        let const_block = self.with_new_scopes(sp, |this| hir::ConstBlock {
+            def_id,
+            hir_id,
+            body: this.with_def_id_parent(def_id, |this| this.lower_body(|_| (&[], expr))),
+        });
+        hir::ExprKind::ConstBlock(const_block)
+    }
+
     /// Try to convert a literal into an interned string
     fn try_inline_lit(&self, lit: token::Lit) -> Option<Symbol> {
         match LitKind::from_token_lit(lit) {
@@ -464,14 +487,14 @@ fn expand_format_args<'hir>(
 
     if allow_const && arguments.is_empty() && argmap.is_empty() {
         // Generate:
-        //     <core::fmt::Arguments>::new_const(lit_pieces)
+        //    const { <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);
+        return ctx.wrap_in_const_context(macsp, hir::ExprKind::Call(new, new_args));
     }
 
     // If the args array contains exactly all the original arguments once,

library/core/src/fmt/mod.rs

@@ -333,7 +333,7 @@ pub struct Arguments<'a> {
 #[unstable(feature = "fmt_internals", issue = "none")]
 impl<'a> Arguments<'a> {
     #[inline]
-    #[rustc_const_unstable(feature = "const_fmt_arguments_new", issue = "none")]
+    #[rustc_const_stable(feature = "const_fmt_arguments_new", since = "CURRENT_RUSTC_VERSION")]
     pub const fn new_const<const N: usize>(pieces: &'a [&'static str; N]) -> Self {
         const { assert!(N <= 1) };
         Arguments { pieces, fmt: None, args: &[] }

library/core/src/lib.rs

@@ -120,7 +120,6 @@
 #![feature(const_char_encode_utf16)]
 #![feature(const_eval_select)]
 #![feature(const_exact_div)]
-#![feature(const_fmt_arguments_new)]
 #![feature(const_hash)]
 #![feature(const_heap)]
 #![feature(const_index_range_slice_index)]

library/core/src/macros/mod.rs

@@ -1038,7 +1038,7 @@ pub(crate) mod builtin {
     ///
     /// This macro will be removed once `format_args` is allowed in const contexts.
     #[unstable(feature = "const_format_args", issue = "none")]
-    #[allow_internal_unstable(fmt_internals, const_fmt_arguments_new)]
+    #[allow_internal_unstable(fmt_internals)]
     #[rustc_builtin_macro]
     #[macro_export]
     macro_rules! const_format_args {

tests/codegen/issues/issue-128709-format-without-args.rs

@@ -0,0 +1,17 @@
+//@ compile-flags: -C no-prepopulate-passes -C opt-level=0
+
+#![crate_type = "lib"]
+
+// String formating macros without any arguments should compile
+// to a `memcpy` followed by a call to a library function.
+
+#[no_mangle]
+pub fn code() {
+    // CHECK-LABEL: @code
+    // CHECK-NOT: getelementptr
+    // CHECK-NOT: store
+    // CHECK-NOT: ; call core::fmt::Arguments::new_const
+    // CHECK: call void @llvm.memcpy
+    // CHECK-NEXT: ; call std::io::stdio::_print
+    println!("hello world");
+}

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
    |
@@ -18,7 +9,7 @@ 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
@@ -28,7 +19,7 @@ help: consider cloning the value if the performance cost is acceptable
 LL |     let settings_data = from_string(settings_dir.clone());
    |                                                 ++++++++
 
-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`.

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

@@ -0,0 +1,5 @@
+pub fn main() {
+    const A: std::fmt::Arguments = std::fmt::Arguments::new_const(&[&"hola"]);
+    //~^ use of unstable library feature
+    //~| temporary value dropped while borrowed
+}

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

@@ -0,0 +1,23 @@
+error[E0658]: use of unstable library feature 'fmt_internals'
+  --> $DIR/const-format-arguments.rs:3:36
+   |
+LL |     const A: std::fmt::Arguments = std::fmt::Arguments::new_const(&[&"hola"]);
+   |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(fmt_internals)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/const-format-arguments.rs:3:68
+   |
+LL |     const A: std::fmt::Arguments = std::fmt::Arguments::new_const(&[&"hola"]);
+   |                                    --------------------------------^^^^^^^^^-
+   |                                    |                               |        |
+   |                                    |                               |        temporary value is freed at the end of this statement
+   |                                    |                               creates a temporary value which is freed while still in use
+   |                                    using this value as a constant requires that borrow lasts for `'static`
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0658, E0716.
+For more information about an error, try `rustc --explain E0658`.

tests/ui/rfcs/rfc-2632-const-trait-impl/issue-79450.rs

@@ -1,6 +1,5 @@
 //@ compile-flags: -Znext-solver
 #![allow(incomplete_features)]
-#![feature(const_fmt_arguments_new)]
 #![feature(const_trait_impl, effects)]
 
 #[const_trait]

tests/ui/rfcs/rfc-2632-const-trait-impl/issue-79450.stderr

@@ -1,5 +1,5 @@
 error[E0015]: cannot call non-const fn `_print` in constant functions
-  --> $DIR/issue-79450.rs:11:9
+  --> $DIR/issue-79450.rs:10:9
    |
 LL |         println!("lul");
    |         ^^^^^^^^^^^^^^^