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Use ZST for fmt unsafety
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This allows the format_args! macro to keep the pre-expansion code out of
the unsafe block without doing gymnastics with nested `match`
expressions. This reduces codegen.
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@camsteffen
camsteffen committed Sep 21, 2021
1 parent db1fb85 commit 09b37d7
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Showing 8 changed files with 108 additions and 100 deletions.
32 changes: 14 additions & 18 deletions compiler/rustc_builtin_macros/src/format.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -845,8 +845,7 @@ impl<'a, 'b> Context<'a, 'b> {
self.ecx.expr_match(self.macsp, head, vec![arm])
};

let ident = Ident::from_str_and_span("args", self.macsp);
let args_slice = self.ecx.expr_ident(self.macsp, ident);
let args_slice = self.ecx.expr_addr_of(self.macsp, args_match);

// Now create the fmt::Arguments struct with all our locals we created.
let (fn_name, fn_args) = if self.all_pieces_simple {
Expand All @@ -856,25 +855,22 @@ impl<'a, 'b> Context<'a, 'b> {
// nonstandard placeholders, if there are any.
let fmt = self.ecx.expr_vec_slice(self.macsp, self.pieces);

("new_v1_formatted", vec![pieces, args_slice, fmt])
let path = self.ecx.std_path(&[sym::fmt, sym::UnsafeArg, sym::new]);
let unsafe_arg = self.ecx.expr_call_global(self.macsp, path, Vec::new());
let unsafe_expr = self.ecx.expr_block(P(ast::Block {
stmts: vec![self.ecx.stmt_expr(unsafe_arg)],
id: ast::DUMMY_NODE_ID,
rules: BlockCheckMode::Unsafe(UnsafeSource::CompilerGenerated),
span: self.macsp,
tokens: None,
could_be_bare_literal: false,
}));

("new_v1_formatted", vec![pieces, args_slice, fmt, unsafe_expr])
};

let path = self.ecx.std_path(&[sym::fmt, sym::Arguments, Symbol::intern(fn_name)]);
let arguments = self.ecx.expr_call_global(self.macsp, path, fn_args);
let body = self.ecx.expr_block(P(ast::Block {
stmts: vec![self.ecx.stmt_expr(arguments)],
id: ast::DUMMY_NODE_ID,
rules: BlockCheckMode::Unsafe(UnsafeSource::CompilerGenerated),
span: self.macsp,
tokens: None,
could_be_bare_literal: false,
}));

let ident = Ident::from_str_and_span("args", self.macsp);
let binding_mode = ast::BindingMode::ByRef(ast::Mutability::Not);
let pat = self.ecx.pat_ident_binding_mode(self.macsp, ident, binding_mode);
let arm = self.ecx.arm(self.macsp, pat, body);
self.ecx.expr_match(self.macsp, args_match, vec![arm])
self.ecx.expr_call_global(self.macsp, path, fn_args)
}

fn format_arg(
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1 change: 1 addition & 0 deletions compiler/rustc_span/src/symbol.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -253,6 +253,7 @@ symbols! {
TyCtxt,
TyKind,
Unknown,
UnsafeArg,
Vec,
Yield,
_DECLS,
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53 changes: 44 additions & 9 deletions library/core/src/fmt/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -265,6 +265,26 @@ pub struct ArgumentV1<'a> {
formatter: fn(&Opaque, &mut Formatter<'_>) -> Result,
}

/// This struct represents the unsafety of constructing an `Arguments`.
/// It exists, rather than an unsafe function, in order to simplify the expansion
/// of `format_args!(..)` and reduce the scope of the `unsafe` block.
#[allow(missing_debug_implementations)]
#[doc(hidden)]
#[non_exhaustive]
#[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
pub struct UnsafeArg;

impl UnsafeArg {
/// See documentation where `UnsafeArg` is required to know when it is safe to
/// create and use `UnsafeArg`.
#[doc(hidden)]
#[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
#[inline(always)]
pub unsafe fn new() -> Self {
Self
}
}

// This guarantees a single stable value for the function pointer associated with
// indices/counts in the formatting infrastructure.
//
Expand Down Expand Up @@ -337,22 +357,37 @@ impl<'a> Arguments<'a> {
#[inline]
#[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
#[rustc_const_unstable(feature = "const_fmt_arguments_new", issue = "none")]
pub const unsafe fn new_v1(
pieces: &'a [&'static str],
args: &'a [ArgumentV1<'a>],
) -> Arguments<'a> {
pub const 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 }
}

/// This function is used to specify nonstandard formatting parameters.
/// The `pieces` array must be at least as long as `fmt` to construct
/// a valid Arguments structure. Also, any `Count` within `fmt` that is
/// `CountIsParam` or `CountIsNextParam` has to point to an argument
/// created with `argumentusize`. However, failing to do so doesn't cause
/// unsafety, but will ignore invalid .
///
/// An `UnsafeArg` is required because the following invariants must be held
/// in order for this function to be safe:
/// 1. The `pieces` slice must be at least as long as `fmt`.
/// 2. Every [`rt::v1::Argument::position`] value within `fmt` must be a
/// valid index of `args`.
/// 3. Every [`Count::Param`] within `fmt` must contain a valid index of
/// `args`.
#[cfg(not(bootstrap))]
#[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 const fn new_v1_formatted(
pieces: &'a [&'static str],
args: &'a [ArgumentV1<'a>],
fmt: &'a [rt::v1::Argument],
_unsafe_arg: UnsafeArg,
) -> Arguments<'a> {
Arguments { pieces, fmt: Some(fmt), args }
}

#[cfg(bootstrap)]
#[doc(hidden)]
#[inline]
#[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
Expand Down
5 changes: 1 addition & 4 deletions library/core/src/panicking.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -47,10 +47,7 @@ pub 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(
// SAFETY: Arguments::new_v1 is safe with exactly one str and zero args
unsafe { fmt::Arguments::new_v1(&[expr], &[]) },
);
panic_fmt(fmt::Arguments::new_v1(&[expr], &[]));
}

#[inline]
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10 changes: 4 additions & 6 deletions src/test/pretty/dollar-crate.pp
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Original file line number Diff line number Diff line change
Expand Up @@ -10,11 +10,9 @@

fn main() {
{
::std::io::_print(match match () { () => [], } {
ref args => unsafe {
::core::fmt::Arguments::new_v1(&["rust\n"],
args)
}
});
::std::io::_print(::core::fmt::Arguments::new_v1(&["rust\n"],
&match () {
() => [],
}));
};
}
56 changes: 23 additions & 33 deletions src/test/pretty/issue-4264.pp
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Original file line number Diff line number Diff line change
Expand Up @@ -32,39 +32,29 @@
({
let res =
((::alloc::fmt::format as
for<'r> fn(Arguments<'r>) -> String {format})((match (match (()
as
())
{
()
=>
([]
as
[ArgumentV1; 0]),
}
as
[ArgumentV1; 0])
{
ref args
=>
unsafe
{
((::core::fmt::Arguments::new_v1
as
unsafe fn(&[&'static str], &[ArgumentV1]) -> Arguments {Arguments::new_v1})((&([("test"
as
&str)]
as
[&str; 1])
as
&[&str; 1]),
(args
as
&[ArgumentV1; 0]))
as
Arguments)
}
}
for<'r> fn(Arguments<'r>) -> String {format})(((::core::fmt::Arguments::new_v1
as
fn(&[&'static str], &[ArgumentV1]) -> Arguments {Arguments::new_v1})((&([("test"
as
&str)]
as
[&str; 1])
as
&[&str; 1]),
(&(match (()
as
())
{
()
=>
([]
as
[ArgumentV1; 0]),
}
as
[ArgumentV1; 0])
as
&[ArgumentV1; 0]))
as
Arguments))
as String);
Expand Down
16 changes: 6 additions & 10 deletions src/test/ui/attributes/key-value-expansion.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -17,16 +17,12 @@ LL | bug!();

error: unexpected token: `{
let res =
::alloc::fmt::format(match match (&"u8",) {
(arg0,) =>
[::core::fmt::ArgumentV1::new(arg0,
::core::fmt::Display::fmt)],
} {
ref args => unsafe {
::core::fmt::Arguments::new_v1(&[""],
args)
}
});
::alloc::fmt::format(::core::fmt::Arguments::new_v1(&[""],
&match (&"u8",) {
(arg0,) =>
[::core::fmt::ArgumentV1::new(arg0,
::core::fmt::Display::fmt)],
}));
res
}.as_str()`
--> $DIR/key-value-expansion.rs:48:23
Expand Down
35 changes: 15 additions & 20 deletions src/tools/clippy/clippy_utils/src/higher.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -524,28 +524,12 @@ impl FormatArgsExpn<'tcx> {
if let ExpnKind::Macro(_, name) = expr.span.ctxt().outer_expn_data().kind;
let name = name.as_str();
if name.ends_with("format_args") || name.ends_with("format_args_nl");

if let ExprKind::Match(inner_match, [arm], _) = expr.kind;

// `match match`, if you will
if let ExprKind::Match(args, [inner_arm], _) = inner_match.kind;
if let ExprKind::Tup(value_args) = args.kind;
if let Some(value_args) = value_args
.iter()
.map(|e| match e.kind {
ExprKind::AddrOf(_, _, e) => Some(e),
_ => None,
})
.collect();
if let ExprKind::Array(args) = inner_arm.body.kind;

if let ExprKind::Block(Block { stmts: [], expr: Some(expr), .. }, _) = arm.body.kind;
if let ExprKind::Call(_, call_args) = expr.kind;
if let Some((strs_ref, fmt_expr)) = match call_args {
if let ExprKind::Call(_, args) = expr.kind;
if let Some((strs_ref, args, fmt_expr)) = match args {
// Arguments::new_v1
[strs_ref, _] => Some((strs_ref, None)),
[strs_ref, args] => Some((strs_ref, args, None)),
// Arguments::new_v1_formatted
[strs_ref, _, fmt_expr] => Some((strs_ref, Some(fmt_expr))),
[strs_ref, args, fmt_expr, _unsafe_arg] => Some((strs_ref, args, Some(fmt_expr))),
_ => None,
};
if let ExprKind::AddrOf(BorrowKind::Ref, _, strs_arr) = strs_ref.kind;
Expand All @@ -561,6 +545,17 @@ impl FormatArgsExpn<'tcx> {
None
})
.collect();
if let ExprKind::AddrOf(BorrowKind::Ref, _, args) = args.kind;
if let ExprKind::Match(args, [arm], _) = args.kind;
if let ExprKind::Tup(value_args) = args.kind;
if let Some(value_args) = value_args
.iter()
.map(|e| match e.kind {
ExprKind::AddrOf(_, _, e) => Some(e),
_ => None,
})
.collect();
if let ExprKind::Array(args) = arm.body.kind;
then {
Some(FormatArgsExpn {
format_string_span: strs_ref.span,
Expand Down

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