Implement unsafe trait support (continued)

crates/formality-check/src/impls.rs

@@ -2,7 +2,7 @@ use anyhow::bail;
 
 use fn_error_context::context;
 use formality_core::Downcasted;
-use formality_prove::Env;
+use formality_prove::{Env, Safety};
 use formality_rust::{
     grammar::{
         AssociatedTy, AssociatedTyBoundData, AssociatedTyValue, AssociatedTyValueBoundData, Fn,
@@ -19,7 +19,7 @@ use formality_types::{
 impl super::Check<'_> {
     #[context("check_trait_impl({v:?})")]
     pub(super) fn check_trait_impl(&self, v: &TraitImpl) -> Fallible<()> {
-        let TraitImpl { binder } = v;
+        let TraitImpl { binder, safety } = v;
 
         let mut env = Env::default();
 
@@ -45,6 +45,8 @@ impl super::Check<'_> {
             trait_items,
         } = trait_decl.binder.instantiate_with(&trait_ref.parameters)?;
 
+        self.check_safety_matches(&trait_decl.safety, safety)?;
+
         for impl_item in &impl_items {
             self.check_trait_impl_item(&env, &where_clauses, &trait_items, impl_item)?;
         }
@@ -71,6 +73,21 @@ impl super::Check<'_> {
         Ok(())
     }
 
+    /// Validate `unsafe trait` and `unsafe impl` line up
+    fn check_safety_matches(&self, trait_decl: &Safety, trait_impl: &Safety) -> Fallible<()> {
+        match trait_decl {
+            Safety::Safe => anyhow::ensure!(
+                matches!(trait_impl, Safety::Safe),
+                "implementing the trait is not `unsafe`"
+            ),
+            Safety::Unsafe => anyhow::ensure!(
+                matches!(trait_impl, Safety::Unsafe),
+                "the trait requires an `unsafe impl` declaration"
+            ),
+        }
+        Ok(())
+    }
+
     fn check_trait_impl_item(
         &self,
         env: &Env,

crates/formality-check/src/traits.rs

@@ -8,7 +8,11 @@ use formality_types::grammar::Fallible;
 impl super::Check<'_> {
     #[context("check_trait({:?})", t.id)]
     pub(super) fn check_trait(&self, t: &Trait) -> Fallible<()> {
-        let Trait { id: _, binder } = t;
+        let Trait {
+            safety: _,
+            id: _,
+            binder,
+        } = t;
         let mut env = Env::default();
 
         let TraitBoundData {

crates/formality-prove/src/decls.rs

@@ -104,8 +104,10 @@ impl Decls {
 
 /// An "impl decl" indicates that a trait is implemented for a given set of types.
 /// One "impl decl" is created for each impl in the Rust source.
-#[term(impl $binder)]
+#[term($?safety impl $binder)]
 pub struct ImplDecl {
+    /// The safety this impl declares, which needs to match the implemented trait's safety.
+    pub safety: Safety,
     /// The binder covers the generic variables from the impl
     pub binder: Binder<ImplDeclBoundData>,
 }
@@ -122,8 +124,11 @@ pub struct ImplDeclBoundData {
 
 /// A declaration that some trait will *not* be implemented for a type; derived from negative impls
 /// like `impl !Foo for Bar`.
-#[term(impl $binder)]
+#[term($?safety impl $binder)]
 pub struct NegImplDecl {
+    /// The safety this negative impl declares
+    pub safety: Safety,
+
     /// Binder comes the generics on the impl
     pub binder: Binder<NegImplDeclBoundData>,
 }
@@ -135,15 +140,26 @@ pub struct NegImplDeclBoundData {
     pub where_clause: Wcs,
 }
 
+#[term]
+#[derive(Default)]
+pub enum Safety {
+    #[default]
+    Safe,
+    Unsafe,
+}
+
 /// A "trait declaration" declares a trait that exists, its generics, and its where-clauses.
 /// It doesn't capture the trait items, which will be transformed into other sorts of rules.
 ///
 /// In Rust syntax, it covers the `trait Foo: Bar` part of the declaration, but not what appears in the `{...}`.
-#[term(trait $id $binder)]
+#[term($?safety trait $id $binder)]
 pub struct TraitDecl {
     /// The name of the trait
     pub id: TraitId,
 
+    /// Whether the trait is `unsafe` or not
+    pub safety: Safety,
+
     /// The binder here captures the generics of the trait; it always begins with a `Self` type.
     pub binder: Binder<TraitDeclBoundData>,
 }

crates/formality-rust/src/grammar.rs

@@ -1,6 +1,7 @@
 use std::sync::Arc;
 
 use formality_core::{term, Upcast};
+use formality_prove::Safety;
 use formality_types::{
     grammar::{
         AdtId, AliasTy, AssociatedItemId, Binder, Const, CrateId, Fallible, FieldId, FnId, Lt,
@@ -160,8 +161,9 @@ pub struct Variant {
     pub fields: Vec<Field>,
 }
 
-#[term(trait $id $binder)]
+#[term($?safety trait $id $binder)]
 pub struct Trait {
+    pub safety: Safety,
     pub id: TraitId,
     pub binder: TraitBinder<TraitBoundData>,
 }
@@ -241,8 +243,9 @@ pub struct AssociatedTyBoundData {
     pub where_clauses: Vec<WhereClause>,
 }
 
-#[term(impl $binder)]
+#[term($?safety impl $binder)]
 pub struct TraitImpl {
+    pub safety: Safety,
     pub binder: Binder<TraitImplBoundData>,
 }
 
@@ -267,8 +270,9 @@ impl TraitImplBoundData {
     }
 }
 
-#[term(impl $binder)]
+#[term($?safety impl $binder)]
 pub struct NegTraitImpl {
+    pub safety: Safety,
     pub binder: Binder<NegTraitImplBoundData>,
 }
 

crates/formality-rust/src/prove.rs

@@ -80,7 +80,7 @@ impl Crate {
         self.items
             .iter()
             .flat_map(|item| match item {
-                CrateItem::Trait(Trait { id, binder }) => {
+                CrateItem::Trait(Trait { id, binder, safety }) => {
                     let (
                         vars,
                         TraitBoundData {
@@ -89,6 +89,7 @@ impl Crate {
                         },
                     ) = binder.open();
                     Some(prove::TraitDecl {
+                        safety: safety.clone(),
                         id: id.clone(),
                         binder: Binder::new(
                             vars,
@@ -110,7 +111,7 @@ impl Crate {
         self.items
             .iter()
             .flat_map(|item| match item {
-                CrateItem::TraitImpl(TraitImpl { binder }) => {
+                CrateItem::TraitImpl(TraitImpl { binder, safety }) => {
                     let (
                         vars,
                         TraitImplBoundData {
@@ -122,6 +123,7 @@ impl Crate {
                         },
                     ) = binder.open();
                     Some(prove::ImplDecl {
+                        safety: safety.clone(),
                         binder: Binder::new(
                             vars,
                             prove::ImplDeclBoundData {
@@ -140,7 +142,7 @@ impl Crate {
         self.items
             .iter()
             .flat_map(|item| match item {
-                CrateItem::NegTraitImpl(NegTraitImpl { binder }) => {
+                CrateItem::NegTraitImpl(NegTraitImpl { binder, safety }) => {
                     let (
                         vars,
                         NegTraitImplBoundData {
@@ -151,6 +153,7 @@ impl Crate {
                         },
                     ) = binder.open();
                     Some(prove::NegImplDecl {
+                        safety: safety.clone(),
                         binder: Binder::new(
                             vars,
                             prove::NegImplDeclBoundData {
@@ -169,7 +172,7 @@ impl Crate {
         self.items
             .iter()
             .flat_map(|item| match item {
-                CrateItem::TraitImpl(TraitImpl { binder }) => {
+                CrateItem::TraitImpl(TraitImpl { binder, safety: _ }) => {
                     let (
                         impl_vars,
                         TraitImplBoundData {
@@ -226,6 +229,7 @@ impl Crate {
             .iter()
             .flat_map(|item| match item {
                 CrateItem::Trait(Trait {
+                    safety: _,
                     id: trait_id,
                     binder,
                 }) => {