Overhaul how contracts are lowered on fn-like bodies

compiler/rustc_ast_lowering/src/expr.rs

@@ -379,16 +379,16 @@ impl<'hir> LoweringContext<'_, 'hir> {
         })
     }
 
-    /// Create an `ExprKind::Ret` that is preceded by a call to check contract ensures clause.
+    /// 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(Some((span, fresh_ident))) =
-            self.contract.as_ref().map(|c| c.ensures.as_ref().map(|e| (e.expr.span, e.fresh_ident)))
-        {
-            let expr = opt_expr.unwrap_or_else(|| self.expr_unit(span));
-            Some(self.inject_ensures_check(expr, span, fresh_ident.0, fresh_ident.2))
-        } else {
-            opt_expr
-        };
+        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)
     }
 
@@ -1090,7 +1090,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
             } else {
                 None
             };
-            let body_id = this.lower_fn_body(decl, |this| {
+            // FIXME(contracts): Support contracts on closures?
+            let body_id = this.lower_fn_body(decl, None, |this| {
                 this.coroutine_kind = coroutine_kind;
                 let e = this.lower_expr_mut(body);
                 coroutine_kind = this.coroutine_kind;

compiler/rustc_ast_lowering/src/item.rs

@@ -211,36 +211,6 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 ..
             }) => {
                 self.with_new_scopes(*fn_sig_span, |this| {
-                    assert!(this.contract.is_none());
-                    if let Some(contract) = contract {
-                        let requires = contract.requires.clone();
-                        let ensures = contract.ensures.clone();
-                        let ensures = ensures.map(|ens| {
-                            // FIXME: this needs to be a fresh (or illegal) identifier to prevent
-                            // accidental capture of a parameter or global variable.
-                            let checker_ident: Ident =
-                                Ident::from_str_and_span("__ensures_checker", ens.span);
-                            let (checker_pat, checker_hir_id) = this.pat_ident_binding_mode_mut(
-                                ens.span,
-                                checker_ident,
-                                hir::BindingMode::NONE,
-                            );
-
-                            crate::FnContractLoweringEnsures {
-                                expr: ens,
-                                fresh_ident: (checker_ident, checker_pat, checker_hir_id),
-                            }
-                        });
-
-                        // Note: `with_new_scopes` will reinstall the outer
-                        // item's contract (if any) after its callback finishes.
-                        this.contract.replace(crate::FnContractLoweringInfo {
-                            span,
-                            requires,
-                            ensures,
-                        });
-                    }
-
                     // Note: we don't need to change the return type from `T` to
                     // `impl Future<Output = T>` here because lower_body
                     // only cares about the input argument patterns in the function
@@ -254,6 +224,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         coroutine_kind,
                         body.as_deref(),
                         attrs,
+                        contract.as_deref(),
                     );
 
                     let itctx = ImplTraitContext::Universal;
@@ -803,6 +774,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 (generics, kind, expr.is_some())
             }
             AssocItemKind::Fn(box Fn { sig, generics, body: None, .. }) => {
+                // FIXME(contracts): Deny contract here since it won't apply to
+                // any impl method or callees.
                 let names = self.lower_fn_params_to_names(&sig.decl);
                 let (generics, sig) = self.lower_method_sig(
                     generics,
@@ -814,7 +787,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 );
                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Required(names)), false)
             }
-            AssocItemKind::Fn(box Fn { sig, generics, body: Some(body), .. }) => {
+            AssocItemKind::Fn(box Fn { sig, generics, body: Some(body), contract, .. }) => {
                 let body_id = self.lower_maybe_coroutine_body(
                     sig.span,
                     i.span,
@@ -823,6 +796,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     sig.header.coroutine_kind,
                     Some(body),
                     attrs,
+                    contract.as_deref(),
                 );
                 let (generics, sig) = self.lower_method_sig(
                     generics,
@@ -931,7 +905,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     hir::ImplItemKind::Const(ty, body)
                 },
             ),
-            AssocItemKind::Fn(box Fn { sig, generics, body, .. }) => {
+            AssocItemKind::Fn(box Fn { sig, generics, body, contract, .. }) => {
                 let body_id = self.lower_maybe_coroutine_body(
                     sig.span,
                     i.span,
@@ -940,6 +914,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     sig.header.coroutine_kind,
                     body.as_deref(),
                     attrs,
+                    contract.as_deref(),
                 );
                 let (generics, sig) = self.lower_method_sig(
                     generics,
@@ -1088,72 +1063,89 @@ impl<'hir> LoweringContext<'_, 'hir> {
     pub(super) fn lower_fn_body(
         &mut self,
         decl: &FnDecl,
+        contract: Option<&FnContract>,
         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
     ) -> hir::BodyId {
         self.lower_body(|this| {
             let params =
                 this.arena.alloc_from_iter(decl.inputs.iter().map(|x| this.lower_param(x)));
-            let result = body(this);
-
-            let opt_contract = this.contract.take();
 
+            // Optionally lower the fn contract, which turns:
+            //
             // { body }
-            // ==>
-            // { contract_requires(PRECOND); { body } }
-            let Some(contract) = opt_contract else { return (params, result) };
-            let result_ref = this.arena.alloc(result);
-            let lit_unit = |this: &mut LoweringContext<'_, 'hir>| {
-                this.expr(contract.span, hir::ExprKind::Tup(&[]))
-            };
-
-            let precond: hir::Stmt<'hir> = if let Some(req) = contract.requires {
-                let lowered_req = this.lower_expr_mut(&req);
-                let precond = this.expr_call_lang_item_fn_mut(
-                    req.span,
-                    hir::LangItem::ContractCheckRequires,
-                    &*arena_vec![this; lowered_req],
-                );
-                this.stmt_expr(req.span, precond)
-            } else {
-                let u = lit_unit(this);
-                this.stmt_expr(contract.span, u)
-            };
-
-            let (postcond_checker, result) = if let Some(ens) = contract.ensures {
-                let crate::FnContractLoweringEnsures { expr: ens, fresh_ident } = ens;
-                let lowered_ens: hir::Expr<'hir> = this.lower_expr_mut(&ens);
-                let postcond_checker = this.expr_call_lang_item_fn(
-                    ens.span,
-                    hir::LangItem::ContractBuildCheckEnsures,
-                    &*arena_vec![this; lowered_ens],
-                );
-                let checker_binding_pat = fresh_ident.1;
-                (
-                    this.stmt_let_pat(
+            //
+            // into:
+            //
+            // { contract_requires(PRECOND); let __postcond = |ret_val| POSTCOND; postcond({ body }) }
+            if let Some(contract) = contract {
+                let precond = if let Some(req) = &contract.requires {
+                    // Lower the precondition check intrinsic.
+                    let lowered_req = this.lower_expr_mut(&req);
+                    let precond = this.expr_call_lang_item_fn_mut(
+                        req.span,
+                        hir::LangItem::ContractCheckRequires,
+                        &*arena_vec![this; lowered_req],
+                    );
+                    Some(this.stmt_expr(req.span, precond))
+                } else {
+                    None
+                };
+                let (postcond, body) = if let Some(ens) = &contract.ensures {
+                    let ens_span = this.lower_span(ens.span);
+                    // Set up the postcondition `let` statement.
+                    let check_ident: Ident =
+                        Ident::from_str_and_span("__ensures_checker", ens_span);
+                    let (checker_pat, check_hir_id) = this.pat_ident_binding_mode_mut(
+                        ens_span,
+                        check_ident,
+                        hir::BindingMode::NONE,
+                    );
+                    let lowered_ens = this.lower_expr_mut(&ens);
+                    let postcond_checker = this.expr_call_lang_item_fn(
+                        ens_span,
+                        hir::LangItem::ContractBuildCheckEnsures,
+                        &*arena_vec![this; lowered_ens],
+                    );
+                    let postcond = this.stmt_let_pat(
                         None,
-                        ens.span,
+                        ens_span,
                         Some(postcond_checker),
-                        this.arena.alloc(checker_binding_pat),
+                        this.arena.alloc(checker_pat),
                         hir::LocalSource::Contract,
-                    ),
-                    this.inject_ensures_check(result_ref, ens.span, fresh_ident.0, fresh_ident.2),
-                )
-            } else {
-                let u = lit_unit(this);
-                (this.stmt_expr(contract.span, u), &*result_ref)
-            };
+                    );
 
-            let block = this.block_all(
-                contract.span,
-                arena_vec![this; precond, postcond_checker],
-                Some(result),
-            );
-            (params, this.expr_block(block))
+                    // Install contract_ensures so we will intercept `return` statements,
+                    // then lower the body.
+                    this.contract_ensures = Some((ens_span, check_ident, check_hir_id));
+                    let body = this.arena.alloc(body(this));
+
+                    // Finally, inject an ensures check on the implicit return of the body.
+                    let body = this.inject_ensures_check(body, ens_span, check_ident, check_hir_id);
+                    (Some(postcond), body)
+                } else {
+                    let body = &*this.arena.alloc(body(this));
+                    (None, body)
+                };
+                // Flatten the body into precond, then postcond, then wrapped body.
+                let wrapped_body = this.block_all(
+                    body.span,
+                    this.arena.alloc_from_iter([precond, postcond].into_iter().flatten()),
+                    Some(body),
+                );
+                (params, this.expr_block(wrapped_body))
+            } else {
+                (params, body(this))
+            }
         })
     }
 
-    fn lower_fn_body_block(&mut self, decl: &FnDecl, body: &Block) -> hir::BodyId {
-        self.lower_fn_body(decl, |this| this.lower_block_expr(body))
+    fn lower_fn_body_block(
+        &mut self,
+        decl: &FnDecl,
+        body: &Block,
+        contract: Option<&FnContract>,
+    ) -> hir::BodyId {
+        self.lower_fn_body(decl, contract, |this| this.lower_block_expr(body))
     }
 
     pub(super) fn lower_const_body(&mut self, span: Span, expr: Option<&Expr>) -> hir::BodyId {
@@ -1179,12 +1171,13 @@ impl<'hir> LoweringContext<'_, 'hir> {
         coroutine_kind: Option<CoroutineKind>,
         body: Option<&Block>,
         attrs: &'hir [hir::Attribute],
+        contract: Option<&FnContract>,
     ) -> hir::BodyId {
         let Some(body) = body else {
             // Functions without a body are an error, except if this is an intrinsic. For those we
             // create a fake body so that the entire rest of the compiler doesn't have to deal with
             // this as a special case.
-            return self.lower_fn_body(decl, |this| {
+            return self.lower_fn_body(decl, contract, |this| {
                 if attrs.iter().any(|a| a.name_or_empty() == sym::rustc_intrinsic) {
                     let span = this.lower_span(span);
                     let empty_block = hir::Block {
@@ -1209,8 +1202,9 @@ impl<'hir> LoweringContext<'_, 'hir> {
         };
         let Some(coroutine_kind) = coroutine_kind else {
             // Typical case: not a coroutine.
-            return self.lower_fn_body_block(decl, body);
+            return self.lower_fn_body_block(decl, body, contract);
         };
+        // FIXME(contracts): Support contracts on async fn.
         self.lower_body(|this| {
             let (parameters, expr) = this.lower_coroutine_body_with_moved_arguments(
                 decl,

compiler/rustc_ast_lowering/src/lib.rs

@@ -87,19 +87,6 @@ mod path;
 
 rustc_fluent_macro::fluent_messages! { "../messages.ftl" }
 
-#[derive(Debug, Clone)]
-struct FnContractLoweringInfo<'hir> {
-    pub span: Span,
-    pub requires: Option<ast::ptr::P<ast::Expr>>,
-    pub ensures: Option<FnContractLoweringEnsures<'hir>>,
-}
-
-#[derive(Debug, Clone)]
-struct FnContractLoweringEnsures<'hir> {
-    expr: ast::ptr::P<ast::Expr>,
-    fresh_ident: (Ident, hir::Pat<'hir>, HirId),
-}
-
 struct LoweringContext<'a, 'hir> {
     tcx: TyCtxt<'hir>,
     resolver: &'a mut ResolverAstLowering,
@@ -114,7 +101,7 @@ struct LoweringContext<'a, 'hir> {
     /// Collect items that were created by lowering the current owner.
     children: Vec<(LocalDefId, hir::MaybeOwner<'hir>)>,
 
-    contract: Option<FnContractLoweringInfo<'hir>>,
+    contract_ensures: Option<(Span, Ident, HirId)>,
 
     coroutine_kind: Option<hir::CoroutineKind>,
 
@@ -164,7 +151,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             bodies: Vec::new(),
             attrs: SortedMap::default(),
             children: Vec::default(),
-            contract: None,
+            contract_ensures: None,
             current_hir_id_owner: hir::CRATE_OWNER_ID,
             item_local_id_counter: hir::ItemLocalId::ZERO,
             ident_and_label_to_local_id: Default::default(),
@@ -851,15 +838,15 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         let was_in_loop_condition = self.is_in_loop_condition;
         self.is_in_loop_condition = false;
 
-        let old_contract = self.contract.take();
+        let old_contract = self.contract_ensures.take();
 
         let catch_scope = self.catch_scope.take();
         let loop_scope = self.loop_scope.take();
         let ret = f(self);
         self.catch_scope = catch_scope;
         self.loop_scope = loop_scope;
 
-        self.contract = old_contract;
+        self.contract_ensures = old_contract;
 
         self.is_in_loop_condition = was_in_loop_condition;
 

tests/ui/contracts/associated-item.rs

@@ -0,0 +1,18 @@
+// Ensure we don't ICE when lowering contracts on an associated item.
+
+//@ compile-flags: --crate-type=lib
+//@ check-pass
+
+#![feature(contracts)]
+//~^ WARN the feature `contracts` is incomplete and may not be safe to use
+
+extern crate core;
+
+use core::contracts::requires;
+
+struct Foo;
+
+impl Foo {
+    #[requires(align > 0 && (align & (align - 1)) == 0)]
+    pub fn foo(align: i32) {}
+}

tests/ui/contracts/associated-item.stderr

@@ -0,0 +1,11 @@
+warning: the feature `contracts` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/associated-item.rs:6:12
+   |
+LL | #![feature(contracts)]
+   |            ^^^^^^^^^
+   |
+   = note: see issue #128044 <https://github.com/rust-lang/rust/issues/128044> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+warning: 1 warning emitted
+