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Guard HIR lowered contracts with contract_checks #144438

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260 changes: 260 additions & 0 deletions compiler/rustc_ast_lowering/src/contract.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,260 @@
use crate::LoweringContext;

impl<'a, 'hir> LoweringContext<'a, 'hir> {
pub(super) fn lower_contract(
&mut self,
body: impl FnOnce(&mut Self) -> rustc_hir::Expr<'hir>,
contract: &rustc_ast::FnContract,
) -> rustc_hir::Expr<'hir> {
match (&contract.requires, &contract.ensures) {
(Some(req), Some(ens)) => {
// Lower the fn contract, which turns:
//
// { body }
//
// into:
//
// {
// let __postcond = if contracts_checks() {
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The suggestion is for this code to be either:

let __postcond = if false {
  // ...
}

or

let __postcond = if true {
  // ...
}

depending on the value of the -Z contract-checks

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@dawidl022 dawidl022 Aug 16, 2025 •
edited
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Thanks for the suggestion! I tried implementing this change in a branch, and the codegen test still fails (tests/codegen/cross-crate-inlining/leaf-inlining.rs) with the alignment contracts from #136578.

I have not looked into why the test is failing, but based on the name, I suspect the optimiser is not able to inline functions it was previously able to, because now they have the if false ... contract code, and maybe the inlining optimisation runs before the if false elimination run, (or there is no if false optimisation in the optimisation settings the test runs under, even though it is set to opt-level 3)?

I'm happy to investigate more deeply why this test fails, so that we understand how to fix this optimisation problem.

My branch: https://github.com/dawidl022/rust/tree/contracts/guarded-lowering-compiler-flag-alignment-tests

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If you want to debug this further you should first check if the branch has been eliminated in the final optimized mir. If it has, you should check if rustc runs the inline pass after dead code elimination or just before.

I believe that llvm inline pass will still run, so this may not be a problem and we may need to adjust the tests. But it's worth checking with the library team.

// contract_check_requires(PRECOND);
// Some(|ret_val| POSTCOND)
// } else {
// None
// };
// contract_check_ensures(__postcond, { body })
// }

let precond = self.lower_precond(req);
let postcond_checker = self.lower_postcond_checker(ens);

let contract_check =
self.lower_contract_check_with_postcond(Some(precond), postcond_checker);

let wrapped_body =
self.wrap_body_with_contract_check(body, contract_check, postcond_checker.span);
self.expr_block(wrapped_body)
}
(None, Some(ens)) => {
// Lower the fn contract, which turns:
//
// { body }
//
// into:
//
// {
// let __postcond = if contracts_check() {
// Some(|ret_val| POSTCOND)
// } else {
// None
// };
// __postcond({ body })
// }

let postcond_checker = self.lower_postcond_checker(ens);
let contract_check =
self.lower_contract_check_with_postcond(None, postcond_checker);

let wrapped_body =
self.wrap_body_with_contract_check(body, contract_check, postcond_checker.span);
self.expr_block(wrapped_body)
}
(Some(req), None) => {
// Lower the fn contract, which turns:
//
// { body }
//
// into:
//
// {
// if contracts_check() {
// contract_requires(PRECOND);
// }
// body
// }
let precond = self.lower_precond(req);
let precond_check = self.lower_contract_check_just_precond(precond);

let body = self.arena.alloc(body(self));

// Flatten the body into precond check, then body.
let wrapped_body = self.block_all(
body.span,
self.arena.alloc_from_iter([precond_check].into_iter()),
Some(body),
);
self.expr_block(wrapped_body)
}
(None, None) => body(self),
}
}

/// Lower the precondition check intrinsic.
fn lower_precond(&mut self, req: &Box<rustc_ast::Expr>) -> rustc_hir::Stmt<'hir> {
let lowered_req = self.lower_expr_mut(&req);
let req_span = self.mark_span_with_reason(
rustc_span::DesugaringKind::Contract,
lowered_req.span,
None,
);
let precond = self.expr_call_lang_item_fn_mut(
req_span,
rustc_hir::LangItem::ContractCheckRequires,
&*arena_vec![self; lowered_req],
);
self.stmt_expr(req.span, precond)
}

fn lower_postcond_checker(
&mut self,
ens: &Box<rustc_ast::Expr>,
) -> &'hir rustc_hir::Expr<'hir> {
let ens_span = self.lower_span(ens.span);
let ens_span =
self.mark_span_with_reason(rustc_span::DesugaringKind::Contract, ens_span, None);
let lowered_ens = self.lower_expr_mut(&ens);
self.expr_call_lang_item_fn(
ens_span,
rustc_hir::LangItem::ContractBuildCheckEnsures,
&*arena_vec![self; lowered_ens],
)
}

fn lower_contract_check_just_precond(
&mut self,
precond: rustc_hir::Stmt<'hir>,
) -> rustc_hir::Stmt<'hir> {
let stmts = self.arena.alloc_from_iter([precond].into_iter());

let then_block_stmts = self.block_all(precond.span, stmts, None);
let then_block = self.arena.alloc(self.expr_block(&then_block_stmts));

let precond_check = rustc_hir::ExprKind::If(
self.expr_call_lang_item_fn(
precond.span,
rustc_hir::LangItem::ContractChecks,
Default::default(),
),
then_block,
None,
);

let precond_check = self.expr(precond.span, precond_check);
self.stmt_expr(precond.span, precond_check)
}

fn lower_contract_check_with_postcond(
&mut self,
precond: Option<rustc_hir::Stmt<'hir>>,
postcond_checker: &'hir rustc_hir::Expr<'hir>,
) -> &'hir rustc_hir::Expr<'hir> {
let stmts = self.arena.alloc_from_iter(precond.into_iter());
let span = match precond {
Some(precond) => precond.span,
None => postcond_checker.span,
};

let postcond_checker = self.arena.alloc(self.expr_enum_variant_lang_item(
postcond_checker.span,
rustc_hir::lang_items::LangItem::OptionSome,
&*arena_vec![self; *postcond_checker],
));
let then_block_stmts = self.block_all(span, stmts, Some(postcond_checker));
let then_block = self.arena.alloc(self.expr_block(&then_block_stmts));

let none_expr = self.arena.alloc(self.expr_enum_variant_lang_item(
postcond_checker.span,
rustc_hir::lang_items::LangItem::OptionNone,
Default::default(),
));
let else_block = self.block_expr(none_expr);
let else_block = self.arena.alloc(self.expr_block(else_block));

let contract_check = rustc_hir::ExprKind::If(
self.expr_call_lang_item_fn(
span,
rustc_hir::LangItem::ContractChecks,
Default::default(),
),
then_block,
Some(else_block),
);
self.arena.alloc(self.expr(span, contract_check))
}

fn wrap_body_with_contract_check(
&mut self,
body: impl FnOnce(&mut Self) -> rustc_hir::Expr<'hir>,
contract_check: &'hir rustc_hir::Expr<'hir>,
postcond_span: rustc_span::Span,
) -> &'hir rustc_hir::Block<'hir> {
let check_ident: rustc_span::Ident =
rustc_span::Ident::from_str_and_span("__ensures_checker", postcond_span);
let (check_hir_id, postcond_decl) = {
// Set up the postcondition `let` statement.
let (checker_pat, check_hir_id) = self.pat_ident_binding_mode_mut(
postcond_span,
check_ident,
rustc_hir::BindingMode::NONE,
);
(
check_hir_id,
self.stmt_let_pat(
None,
postcond_span,
Some(contract_check),
self.arena.alloc(checker_pat),
rustc_hir::LocalSource::Contract,
),
)
};

// Install contract_ensures so we will intercept `return` statements,
// then lower the body.
self.contract_ensures = Some((postcond_span, check_ident, check_hir_id));
let body = self.arena.alloc(body(self));

// Finally, inject an ensures check on the implicit return of the body.
let body = self.inject_ensures_check(body, postcond_span, check_ident, check_hir_id);

// Flatten the body into precond, then postcond, then wrapped body.
let wrapped_body = self.block_all(
body.span,
self.arena.alloc_from_iter([postcond_decl].into_iter()),
Some(body),
);
wrapped_body
}

/// Create an `ExprKind::Ret` that is optionally wrapped by a call to check
/// a contract ensures clause, if it exists.
pub(super) fn checked_return(
&mut self,
opt_expr: Option<&'hir rustc_hir::Expr<'hir>>,
) -> rustc_hir::ExprKind<'hir> {
let checked_ret =
if let Some((check_span, check_ident, check_hir_id)) = self.contract_ensures {
let expr = opt_expr.unwrap_or_else(|| self.expr_unit(check_span));
Some(self.inject_ensures_check(expr, check_span, check_ident, check_hir_id))
} else {
opt_expr
};
rustc_hir::ExprKind::Ret(checked_ret)
}

/// Wraps an expression with a call to the ensures check before it gets returned.
pub(super) fn inject_ensures_check(
&mut self,
expr: &'hir rustc_hir::Expr<'hir>,
span: rustc_span::Span,
cond_ident: rustc_span::Ident,
cond_hir_id: rustc_hir::HirId,
) -> &'hir rustc_hir::Expr<'hir> {
let cond_fn = self.expr_ident(span, cond_ident, cond_hir_id);
let call_expr = self.expr_call_lang_item_fn_mut(
span,
rustc_hir::LangItem::ContractCheckEnsures,
arena_vec![self; *cond_fn, *expr],
);
self.arena.alloc(call_expr)
}
}
86 changes: 53 additions & 33 deletions compiler/rustc_ast_lowering/src/expr.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -380,36 +380,6 @@ impl<'hir> LoweringContext<'_, 'hir> {
})
}

/// Create an `ExprKind::Ret` that is optionally wrapped by a call to check
/// a contract ensures clause, if it exists.
fn checked_return(&mut self, opt_expr: Option<&'hir hir::Expr<'hir>>) -> hir::ExprKind<'hir> {
let checked_ret =
if let Some((check_span, check_ident, check_hir_id)) = self.contract_ensures {
let expr = opt_expr.unwrap_or_else(|| self.expr_unit(check_span));
Some(self.inject_ensures_check(expr, check_span, check_ident, check_hir_id))
} else {
opt_expr
};
hir::ExprKind::Ret(checked_ret)
}

/// Wraps an expression with a call to the ensures check before it gets returned.
pub(crate) fn inject_ensures_check(
&mut self,
expr: &'hir hir::Expr<'hir>,
span: Span,
cond_ident: Ident,
cond_hir_id: HirId,
) -> &'hir hir::Expr<'hir> {
let cond_fn = self.expr_ident(span, cond_ident, cond_hir_id);
let call_expr = self.expr_call_lang_item_fn_mut(
span,
hir::LangItem::ContractCheckEnsures,
arena_vec![self; *cond_fn, *expr],
);
self.arena.alloc(call_expr)
}

pub(crate) fn lower_const_block(&mut self, c: &AnonConst) -> hir::ConstBlock {
self.with_new_scopes(c.value.span, |this| {
let def_id = this.local_def_id(c.id);
Expand Down Expand Up @@ -2095,7 +2065,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
self.expr(span, hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Mut, e))
}

fn expr_unit(&mut self, sp: Span) -> &'hir hir::Expr<'hir> {
pub(super) fn expr_unit(&mut self, sp: Span) -> &'hir hir::Expr<'hir> {
self.arena.alloc(self.expr(sp, hir::ExprKind::Tup(&[])))
}

Expand Down Expand Up @@ -2133,6 +2103,43 @@ impl<'hir> LoweringContext<'_, 'hir> {
self.expr(span, hir::ExprKind::Call(e, args))
}

pub(super) fn expr_struct(
&mut self,
span: Span,
path: &'hir hir::QPath<'hir>,
fields: &'hir [hir::ExprField<'hir>],
) -> hir::Expr<'hir> {
self.expr(span, hir::ExprKind::Struct(path, fields, rustc_hir::StructTailExpr::None))
}

pub(super) fn expr_enum_variant(
&mut self,
span: Span,
path: &'hir hir::QPath<'hir>,
fields: &'hir [hir::Expr<'hir>],
) -> hir::Expr<'hir> {
let fields = self.arena.alloc_from_iter(fields.into_iter().enumerate().map(|(i, f)| {
hir::ExprField {
hir_id: self.next_id(),
ident: Ident::from_str(&i.to_string()),
expr: f,
span: f.span,
is_shorthand: false,
}
}));
self.expr_struct(span, path, fields)
}

pub(super) fn expr_enum_variant_lang_item(
&mut self,
span: Span,
lang_item: hir::LangItem,
fields: &'hir [hir::Expr<'hir>],
) -> hir::Expr<'hir> {
let path = self.arena.alloc(self.lang_item_path(span, lang_item));
self.expr_enum_variant(span, path, fields)
}

pub(super) fn expr_call(
&mut self,
span: Span,
Expand Down Expand Up @@ -2161,8 +2168,21 @@ impl<'hir> LoweringContext<'_, 'hir> {
self.arena.alloc(self.expr_call_lang_item_fn_mut(span, lang_item, args))
}

fn expr_lang_item_path(&mut self, span: Span, lang_item: hir::LangItem) -> hir::Expr<'hir> {
self.expr(span, hir::ExprKind::Path(hir::QPath::LangItem(lang_item, self.lower_span(span))))
pub(super) fn expr_lang_item_path(
&mut self,
span: Span,
lang_item: hir::LangItem,
) -> hir::Expr<'hir> {
let path = self.lang_item_path(span, lang_item);
self.expr(span, hir::ExprKind::Path(path))
}

pub(super) fn lang_item_path(
&mut self,
span: Span,
lang_item: hir::LangItem,
) -> hir::QPath<'hir> {
hir::QPath::LangItem(lang_item, self.lower_span(span))
}

/// `<LangItem>::name`
Expand Down
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