intermediate step: express contracts as part of function header and lower it into the previously added contract lang items.

Author: pnkfelix

compiler/rustc_ast/src/ast.rs

@@ -3335,11 +3335,18 @@ pub struct Impl {
     pub items: ThinVec<P<AssocItem>>,
 }
 
+#[derive(Clone, Encodable, Decodable, Debug, Default)]
+pub struct FnContract {
+    pub requires: Option<P<Expr>>,
+    pub ensures: Option<P<Expr>>,
+}
+
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub struct Fn {
     pub defaultness: Defaultness,
     pub generics: Generics,
     pub sig: FnSig,
+    pub contract: Option<P<FnContract>>,
     pub body: Option<P<Block>>,
 }
 
@@ -3637,7 +3644,7 @@ mod size_asserts {
     static_assert_size!(Block, 32);
     static_assert_size!(Expr, 72);
     static_assert_size!(ExprKind, 40);
-    static_assert_size!(Fn, 160);
+    static_assert_size!(Fn, 168);
     static_assert_size!(ForeignItem, 88);
     static_assert_size!(ForeignItemKind, 16);
     static_assert_size!(GenericArg, 24);

compiler/rustc_ast/src/mut_visit.rs

@@ -144,6 +144,10 @@ pub trait MutVisitor: Sized {
         walk_flat_map_assoc_item(self, i, ctxt)
     }
 
+    fn visit_contract(&mut self, c: &mut P<FnContract>) {
+        walk_contract(self, c);
+    }
+
     fn visit_fn_decl(&mut self, d: &mut P<FnDecl>) {
         walk_fn_decl(self, d);
     }
@@ -969,6 +973,16 @@ fn walk_fn<T: MutVisitor>(vis: &mut T, kind: FnKind<'_>) {
     }
 }
 
+fn walk_contract<T: MutVisitor>(vis: &mut T, contract: &mut P<FnContract>) {
+    let FnContract { requires, ensures } = contract.deref_mut();
+    if let Some(pred) = requires {
+        vis.visit_expr(pred);
+    }
+    if let Some(pred) = ensures {
+        vis.visit_expr(pred);
+    }
+}
+
 fn walk_fn_decl<T: MutVisitor>(vis: &mut T, decl: &mut P<FnDecl>) {
     let FnDecl { inputs, output } = decl.deref_mut();
     inputs.flat_map_in_place(|param| vis.flat_map_param(param));
@@ -1200,8 +1214,11 @@ impl WalkItemKind for ItemKind {
             ItemKind::Const(item) => {
                 visit_const_item(item, vis);
             }
-            ItemKind::Fn(box Fn { defaultness, generics, sig, body }) => {
+            ItemKind::Fn(box Fn { defaultness, generics, sig, contract, body }) => {
                 visit_defaultness(vis, defaultness);
+                if let Some(contract) = contract {
+                    vis.visit_contract(contract)
+                };
                 vis.visit_fn(
                     FnKind::Fn(FnCtxt::Free, ident, sig, visibility, generics, body),
                     span,
@@ -1319,8 +1336,11 @@ impl WalkItemKind for AssocItemKind {
             AssocItemKind::Const(item) => {
                 visit_const_item(item, visitor);
             }
-            AssocItemKind::Fn(box Fn { defaultness, generics, sig, body }) => {
+            AssocItemKind::Fn(box Fn { defaultness, generics, sig, contract, body }) => {
                 visit_defaultness(visitor, defaultness);
+                if let Some(contract) = contract {
+                    visitor.visit_contract(contract);
+                }
                 visitor.visit_fn(
                     FnKind::Fn(FnCtxt::Assoc(ctxt), ident, sig, visibility, generics, body),
                     span,
@@ -1466,8 +1486,11 @@ impl WalkItemKind for ForeignItemKind {
                 visitor.visit_ty(ty);
                 visit_opt(expr, |expr| visitor.visit_expr(expr));
             }
-            ForeignItemKind::Fn(box Fn { defaultness, generics, sig, body }) => {
+            ForeignItemKind::Fn(box Fn { defaultness, generics, sig, contract, body }) => {
                 visit_defaultness(visitor, defaultness);
+                if let Some(contract) = contract {
+                    visitor.visit_contract(contract);
+                }
                 visitor.visit_fn(
                     FnKind::Fn(FnCtxt::Foreign, ident, sig, visibility, generics, body),
                     span,

compiler/rustc_ast/src/visit.rs

@@ -66,7 +66,15 @@ impl BoundKind {
 #[derive(Copy, Clone, Debug)]
 pub enum FnKind<'a> {
     /// E.g., `fn foo()`, `fn foo(&self)`, or `extern "Abi" fn foo()`.
-    Fn(FnCtxt, &'a Ident, &'a FnSig, &'a Visibility, &'a Generics, &'a Option<P<Block>>),
+    Fn(
+        FnCtxt,
+        &'a Ident,
+        &'a FnSig,
+        &'a Visibility,
+        &'a Generics,
+        &'a Option<P<FnContract>>,
+        &'a Option<P<Block>>,
+    ),
 
     /// E.g., `|x, y| body`.
     Closure(&'a ClosureBinder, &'a Option<CoroutineKind>, &'a FnDecl, &'a Expr),
@@ -75,7 +83,7 @@ pub enum FnKind<'a> {
 impl<'a> FnKind<'a> {
     pub fn header(&self) -> Option<&'a FnHeader> {
         match *self {
-            FnKind::Fn(_, _, sig, _, _, _) => Some(&sig.header),
+            FnKind::Fn(_, _, sig, _, _, _, _) => Some(&sig.header),
             FnKind::Closure(..) => None,
         }
     }
@@ -89,7 +97,7 @@ impl<'a> FnKind<'a> {
 
     pub fn decl(&self) -> &'a FnDecl {
         match self {
-            FnKind::Fn(_, _, sig, _, _, _) => &sig.decl,
+            FnKind::Fn(_, _, sig, _, _, _, _) => &sig.decl,
             FnKind::Closure(_, _, decl, _) => decl,
         }
     }
@@ -189,6 +197,9 @@ pub trait Visitor<'ast>: Sized {
     fn visit_closure_binder(&mut self, b: &'ast ClosureBinder) -> Self::Result {
         walk_closure_binder(self, b)
     }
+    fn visit_contract(&mut self, c: &'ast FnContract) -> Self::Result {
+        walk_contract(self, c)
+    }
     fn visit_where_predicate(&mut self, p: &'ast WherePredicate) -> Self::Result {
         walk_where_predicate(self, p)
     }
@@ -375,8 +386,8 @@ impl WalkItemKind for ItemKind {
                 try_visit!(visitor.visit_ty(ty));
                 visit_opt!(visitor, visit_expr, expr);
             }
-            ItemKind::Fn(box Fn { defaultness: _, generics, sig, body }) => {
-                let kind = FnKind::Fn(FnCtxt::Free, ident, sig, vis, generics, body);
+            ItemKind::Fn(box Fn { defaultness: _, generics, sig, contract, body }) => {
+                let kind = FnKind::Fn(FnCtxt::Free, ident, sig, vis, generics, contract, body);
                 try_visit!(visitor.visit_fn(kind, span, id));
             }
             ItemKind::Mod(_unsafety, mod_kind) => match mod_kind {
@@ -708,8 +719,8 @@ impl WalkItemKind for ForeignItemKind {
                 try_visit!(visitor.visit_ty(ty));
                 visit_opt!(visitor, visit_expr, expr);
             }
-            ForeignItemKind::Fn(box Fn { defaultness: _, generics, sig, body }) => {
-                let kind = FnKind::Fn(FnCtxt::Foreign, ident, sig, vis, generics, body);
+            ForeignItemKind::Fn(box Fn { defaultness: _, generics, sig, contract, body }) => {
+                let kind = FnKind::Fn(FnCtxt::Foreign, ident, sig, vis, generics, contract, body);
                 try_visit!(visitor.visit_fn(kind, span, id));
             }
             ForeignItemKind::TyAlias(box TyAlias {
@@ -793,6 +804,17 @@ pub fn walk_closure_binder<'a, V: Visitor<'a>>(
     V::Result::output()
 }
 
+pub fn walk_contract<'a, V: Visitor<'a>>(visitor: &mut V, c: &'a FnContract) -> V::Result {
+    let FnContract { requires, ensures } = c;
+    if let Some(pred) = requires {
+        visitor.visit_expr(pred);
+    }
+    if let Some(pred) = ensures {
+        visitor.visit_expr(pred);
+    }
+    V::Result::output()
+}
+
 pub fn walk_where_predicate<'a, V: Visitor<'a>>(
     visitor: &mut V,
     predicate: &'a WherePredicate,
@@ -851,11 +873,20 @@ pub fn walk_fn_decl<'a, V: Visitor<'a>>(
 
 pub fn walk_fn<'a, V: Visitor<'a>>(visitor: &mut V, kind: FnKind<'a>) -> V::Result {
     match kind {
-        FnKind::Fn(_ctxt, _ident, FnSig { header, decl, span: _ }, _vis, generics, body) => {
+        FnKind::Fn(
+            _ctxt,
+            _ident,
+            FnSig { header, decl, span: _ },
+            _vis,
+            generics,
+            contract,
+            body,
+        ) => {
             // Identifier and visibility are visited as a part of the item.
             try_visit!(visitor.visit_fn_header(header));
             try_visit!(visitor.visit_generics(generics));
             try_visit!(visitor.visit_fn_decl(decl));
+            visit_opt!(visitor, visit_contract, contract);
             visit_opt!(visitor, visit_block, body);
         }
         FnKind::Closure(binder, coroutine_kind, decl, body) => {
@@ -885,8 +916,9 @@ impl WalkItemKind for AssocItemKind {
                 try_visit!(visitor.visit_ty(ty));
                 visit_opt!(visitor, visit_expr, expr);
             }
-            AssocItemKind::Fn(box Fn { defaultness: _, generics, sig, body }) => {
-                let kind = FnKind::Fn(FnCtxt::Assoc(ctxt), ident, sig, vis, generics, body);
+            AssocItemKind::Fn(box Fn { defaultness: _, generics, sig, contract, body }) => {
+                let kind =
+                    FnKind::Fn(FnCtxt::Assoc(ctxt), ident, sig, vis, generics, contract, body);
                 try_visit!(visitor.visit_fn(kind, span, id));
             }
             AssocItemKind::Type(box TyAlias {

compiler/rustc_ast_lowering/src/expr.rs

@@ -336,7 +336,20 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     hir::ExprKind::Continue(self.lower_jump_destination(e.id, *opt_label))
                 }
                 ExprKind::Ret(e) => {
-                    let e = e.as_ref().map(|x| self.lower_expr(x));
+                    let mut e = e.as_ref().map(|x| self.lower_expr(x));
+                    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 checker_fn = self.expr_ident(span, fresh_ident.0, fresh_ident.2);
+                        let args = if let Some(e) = e {
+                            std::slice::from_ref(e)
+                        } else {
+                            std::slice::from_ref(self.expr_unit(span))
+                        };
+                        e = Some(self.expr_call(span, checker_fn, args));
+                    }
                     hir::ExprKind::Ret(e)
                 }
                 ExprKind::Yeet(sub_expr) => self.lower_expr_yeet(e.span, sub_expr.as_deref()),
@@ -2116,7 +2129,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         self.arena.alloc(self.expr_call_mut(span, e, args))
     }
 
-    fn expr_call_lang_item_fn_mut(
+    pub(super) fn expr_call_lang_item_fn_mut(
         &mut self,
         span: Span,
         lang_item: hir::LangItem,
@@ -2126,7 +2139,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         self.expr_call_mut(span, path, args)
     }
 
-    fn expr_call_lang_item_fn(
+    pub(super) fn expr_call_lang_item_fn(
         &mut self,
         span: Span,
         lang_item: hir::LangItem,

compiler/rustc_ast_lowering/src/item.rs

@@ -205,8 +205,38 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 sig: FnSig { decl, header, span: fn_sig_span },
                 generics,
                 body,
+                contract,
                 ..
             }) => {
+                if let Some(contract) = contract {
+                    assert!(self.contract.is_none());
+                    let requires = contract.requires.clone();
+                    let ensures = contract.ensures.clone();
+                    let ensures = if let Some(ens) = ensures {
+                        // 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) = self.pat_ident_binding_mode_mut(
+                            ens.span,
+                            checker_ident,
+                            hir::BindingMode::NONE,
+                        );
+
+                        Some(crate::FnContractLoweringEnsures {
+                            expr: ens,
+                            fresh_ident: (checker_ident, checker_pat, checker_hir_id),
+                        })
+                    } else {
+                        None
+                    };
+
+                    self.contract.replace(crate::FnContractLoweringInfo {
+                        span,
+                        requires,
+                        ensures,
+                    });
+                }
                 self.with_new_scopes(*fn_sig_span, |this| {
                     // Note: we don't need to change the return type from `T` to
                     // `impl Future<Output = T>` here because lower_body
@@ -1030,10 +1060,82 @@ impl<'hir> LoweringContext<'_, 'hir> {
         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
     ) -> hir::BodyId {
         self.lower_body(|this| {
-            (
-                this.arena.alloc_from_iter(decl.inputs.iter().map(|x| this.lower_param(x))),
-                body(this),
-            )
+            let params =
+                this.arena.alloc_from_iter(decl.inputs.iter().map(|x| this.lower_param(x)));
+            let result = body(this);
+
+            let contract = this.contract.take();
+
+            // { body }
+            // ==>
+            // { rustc_contract_requires(PRECOND); { body } }
+            let result: hir::Expr<'hir> = if let Some(_contract) = contract {
+                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, _opt_ident, 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(
+                            None,
+                            ens.span,
+                            Some(postcond_checker),
+                            this.arena.alloc(checker_binding_pat),
+                            hir::LocalSource::Contract,
+                        ),
+                        Some((fresh_ident.0, fresh_ident.2)),
+                        {
+                            let checker_fn =
+                                this.expr_ident(ens.span, fresh_ident.0, fresh_ident.2);
+                            let span = this.mark_span_with_reason(
+                                DesugaringKind::Contract,
+                                ens.span,
+                                None,
+                            );
+                            this.expr_call_mut(
+                                span,
+                                checker_fn,
+                                std::slice::from_ref(this.arena.alloc(result)),
+                            )
+                        },
+                    )
+                } else {
+                    let u = lit_unit(this);
+                    (this.stmt_expr(_contract.span, u), None, result)
+                };
+
+                let block = this.block_all(
+                    _contract.span,
+                    arena_vec![this; precond, postcond_checker],
+                    Some(this.arena.alloc(result)),
+                );
+                this.expr_block(block)
+            } else {
+                result
+            };
+
+            (params, result)
         })
     }