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Remove 'simple array' lowering of format_args!() for >1 args. #139205

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Remove 'simple array' lowering of format_args!() for >1 args. #139205

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47 changes: 4 additions & 43 deletions compiler/rustc_ast_lowering/src/format.rs
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Original file line number Diff line number Diff line change
@@ -1,7 +1,5 @@
use core::ops::ControlFlow;
use std::borrow::Cow;

use rustc_ast::visit::Visitor;
use rustc_ast::*;
use rustc_data_structures::fx::FxIndexMap;
use rustc_hir as hir;
Expand Down Expand Up @@ -476,16 +474,10 @@ fn expand_format_args<'hir>(
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,
// we don't do this, because an Argument cannot be kept across yield points.
//
// This is an optimization, speeding up compilation about 1-2% in some cases.
// See https://github.com/rust-lang/rust/pull/106770#issuecomment-1380790609
let use_simple_array = argmap.len() == arguments.len()
&& argmap.iter().enumerate().all(|(i, (&(j, _), _))| i == j)
&& arguments.iter().skip(1).all(|arg| !may_contain_yield_point(&arg.expr));
// If the args array contains just one argument,
// we can use a simple array instead of a `match` construction.
let use_simple_array =
argmap.len() == 1 && arguments.len() == 1 && argmap.first().unwrap().0.0 == 0;

let args = if arguments.is_empty() {
// Generate:
Expand Down Expand Up @@ -514,9 +506,6 @@ fn expand_format_args<'hir>(
// Generate:
// &[
// <core::fmt::Argument>::new_display(&arg0),
// <core::fmt::Argument>::new_lower_hex(&arg1),
// <core::fmt::Argument>::new_debug(&arg2),
// …
// ]
let elements = ctx.arena.alloc_from_iter(arguments.iter().zip(argmap).map(
|(arg, ((_, ty), placeholder_span))| {
Expand Down Expand Up @@ -635,34 +624,6 @@ fn expand_format_args<'hir>(
}
}

fn may_contain_yield_point(e: &ast::Expr) -> bool {
struct MayContainYieldPoint;

impl Visitor<'_> for MayContainYieldPoint {
type Result = ControlFlow<()>;

fn visit_expr(&mut self, e: &ast::Expr) -> ControlFlow<()> {
if let ast::ExprKind::Await(_, _) | ast::ExprKind::Yield(_) = e.kind {
ControlFlow::Break(())
} else {
visit::walk_expr(self, e)
}
}

fn visit_mac_call(&mut self, _: &ast::MacCall) -> ControlFlow<()> {
// Macros should be expanded at this point.
unreachable!("unexpanded macro in ast lowering");
}

fn visit_item(&mut self, _: &ast::Item) -> ControlFlow<()> {
// Do not recurse into nested items.
ControlFlow::Continue(())
}
}

MayContainYieldPoint.visit_expr(e).is_break()
}

fn for_all_argument_indexes(template: &mut [FormatArgsPiece], mut f: impl FnMut(&mut usize)) {
for piece in template {
let FormatArgsPiece::Placeholder(placeholder) = piece else { continue };
Expand Down
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