Add throws_expr! and try_expr! for wrapping closures/async blocks

macros/src/args.rs

@@ -51,13 +51,19 @@ impl Args {
     }
 }
 
+impl Default for Args {
+    fn default() -> Self {
+        Self {
+            error: Some(default_error()),
+            wrapper: Some(result()),
+        }
+    }
+}
+
 impl Parse for Args {
     fn parse(input: ParseStream) -> Result<Args> {
         if input.is_empty() {
-            return Ok(Args {
-                error: Some(default_error()),
-                wrapper: Some(result()),
-            });
+            return Ok(Args::default());
         }
 
         let error = match input.peek(Token![as]) {

macros/src/lib.rs

@@ -19,3 +19,13 @@ pub fn try_fn(args: TokenStream, input: TokenStream) -> TokenStream {
     assert!(args.to_string() == "", "try_fn does not take arguments");
     Throws::new(None).fold(input)
 }
+
+#[proc_macro]
+pub fn throws_expr(input: TokenStream) -> TokenStream {
+    Throws::new(Some(Args::default())).fold(input)
+}
+
+#[proc_macro]
+pub fn try_expr(input: TokenStream) -> TokenStream {
+    Throws::new(None).fold(input)
+}

macros/src/throws.rs

@@ -32,11 +32,17 @@ impl Throws {
         } else if let Ok(impl_item_fn) = syn::parse(input.clone()) {
             let impl_item_fn = self.fold_impl_item_fn(impl_item_fn);
             quote::quote!(#impl_item_fn).into()
-        } else if let Ok(trait_item_fn) = syn::parse(input) {
+        } else if let Ok(trait_item_fn) = syn::parse(input.clone()) {
             let trait_item_fn = self.fold_trait_item_fn(trait_item_fn);
             quote::quote!(#trait_item_fn).into()
+        } else if let Ok(expr_closure) = syn::parse(input.clone()) {
+            let expr_closure = self.fold_expr_closure(expr_closure);
+            quote::quote!(#expr_closure).into()
+        } else if let Ok(expr_async) = syn::parse(input) {
+            let expr_async = self.fold_expr_async(expr_async);
+            quote::quote!(#expr_async).into()
         } else {
-            panic!("#[throws] attribute can only be applied to functions and methods")
+            panic!("#[throws] attribute can only be applied to functions, methods, closures or async blocks")
         }
     }
 }
@@ -99,11 +105,37 @@ impl Fold for Throws {
     }
 
     fn fold_expr_closure(&mut self, i: syn::ExprClosure) -> syn::ExprClosure {
-        i // TODO
+        if !self.outer_fn {
+            return i;
+        }
+
+        let output = match i.output {
+            syn::ReturnType::Default => syn::parse_quote!(-> _),
+            output => output,
+        };
+        let output = self.fold_return_type(output);
+
+        self.outer_fn = false;
+
+        let inner = self.fold_expr(*i.body);
+        let body = Box::new(make_fn_expr(&self.return_type, &inner));
+
+        syn::ExprClosure { output, body, ..i }
     }
 
     fn fold_expr_async(&mut self, i: syn::ExprAsync) -> syn::ExprAsync {
-        i // TODO
+        if !self.outer_fn {
+            return i;
+        }
+
+        // update self.return_type
+        let _ = self.fold_return_type(syn::parse_quote!(-> _));
+        self.outer_fn = false;
+
+        let inner = self.fold_block(i.block);
+        let block = make_fn_block(&self.return_type, &inner);
+
+        syn::ExprAsync { block, ..i }
     }
 
     fn fold_return_type(&mut self, i: syn::ReturnType) -> syn::ReturnType {
@@ -147,7 +179,7 @@ fn make_fn_block(ty: &syn::Type, inner: &syn::Block) -> syn::Block {
     let mut block: syn::Block = syn::parse2(quote::quote! {{
         #[allow(clippy::diverging_sub_expression)]
         {
-            let __ret = { #inner };
+            let __ret = #inner;
 
             #[allow(unreachable_code)]
             <#ty as ::culpa::__internal::_Succeed>::from_ok(__ret)
@@ -158,6 +190,19 @@ fn make_fn_block(ty: &syn::Type, inner: &syn::Block) -> syn::Block {
     block
 }
 
+fn make_fn_expr(ty: &syn::Type, inner: &syn::Expr) -> syn::Expr {
+    syn::parse2(quote::quote! {{
+        #[allow(clippy::diverging_sub_expression)]
+        {
+            let __ret = { #inner };
+
+            #[allow(unreachable_code)]
+            <#ty as ::culpa::__internal::_Succeed>::from_ok(__ret)
+        }
+    }})
+    .unwrap()
+}
+
 fn ok(ty: &syn::Type, expr: &syn::Expr) -> syn::Expr {
     syn::parse2(quote::quote!(<#ty as ::culpa::__internal::_Succeed>::from_ok(#expr))).unwrap()
 }

src/lib.rs

@@ -113,6 +113,48 @@
 //! support `throws` syntax on functions that return `Poll` (so you can't use this syntax when
 //! implementing a `Future` by hand, for example). I hope to come up with a way to support `Poll`
 //! in the future.
+//!
+//! # Annotating Expressions
+//!
+//! Attributes on expressions are still unstable, so there are separate non-attribute macro
+//! [`throws_expr!`] and [`try_expr!`] available to wrap closures or async blocks. `throws_expr!`
+//! does not have any way to pass an error type, so only supports usage with a contextual "default
+//! error type". If you must override it for a single closure, you can do so by putting it in a
+//! block with a separate `use` or type alias, or simply use `try_expr!` with a normal closure
+//! return type annotation.
+//!
+//! ## Example
+//!
+//! ```
+//! use std::io::{self, Read, Error};
+//!
+//! use culpa::{throw, throws_expr, try_expr};
+//!
+//! let closure = throws_expr!(|| {
+//!     let mut file = std::fs::File::open("The_House_of_the_Spirits.txt")?;
+//!     let mut text = String::new();
+//!     file.read_to_string(&mut text)?;
+//!
+//!     if !text.starts_with("Barrabas came to us by sea, the child Clara wrote") {
+//!         throw!(Error::from_raw_os_error(22));
+//!     }
+//!
+//!     println!("Okay!");
+//! });
+//!
+//! let string_throwing_closure = {
+//!     type Error = &'static str;
+//!     throws_expr!(|| throw!("This is not for you."))
+//! };
+//!
+//! let string_throwing_closure = try_expr!(|| -> Result<_, &'static str> {
+//!     throw!("The air trembles. A breath of change passes…")
+//! });
+//!
+//! let maybe_random = try_expr!(|| -> Option<_> {
+//!     4
+//! });
+//! ```
 
 #[doc(inline)]
 /// Annotates a function that "throws" a Result.
@@ -126,6 +168,22 @@ pub use culpa_macros::throws;
 /// See the main crate docs for more details.
 pub use culpa_macros::try_fn;
 
+#[doc(inline)]
+/// Annotates an expression (closure or async block) that "throws" a Result.
+///
+/// Workaround for attributes on expressions being unstable.
+///
+/// See the main crate docs for more details.
+pub use culpa_macros::throws_expr;
+
+#[doc(inline)]
+/// Annotates an expression (closure or async block) that implicitly wraps a try block.
+///
+/// Workaround for attributes on expressions being unstable.
+///
+/// See the main crate docs for more details.
+pub use culpa_macros::try_expr;
+
 /// Throw an error.
 ///
 /// This macro is equivalent to `Err($err)?`.

tests/throws_expr_async_block.rs

@@ -0,0 +1,87 @@
+use culpa::{throw, throws_expr};
+
+#[test]
+#[allow(clippy::unused_unit)]
+fn unit() {
+    type Error = ();
+    let ok = Result::<(), ()>::Ok(());
+    assert_eq!(ok, poll(throws_expr!(async {})));
+    assert_eq!(ok, poll(throws_expr!(async { () })));
+    assert_eq!(
+        ok,
+        poll(throws_expr!(async {
+            return;
+        }))
+    );
+    assert_eq!(
+        ok,
+        poll(throws_expr!(async {
+            return ();
+        }))
+    );
+}
+
+#[test]
+fn integer() {
+    type Error = ();
+    let ok = Result::<i32, ()>::Ok(1);
+    assert_eq!(ok, poll(throws_expr!(async { 1 })));
+    assert_eq!(
+        ok,
+        poll(throws_expr!(async {
+            return 1;
+        }))
+    );
+}
+
+#[test]
+fn throws_unit() {
+    type Error = ();
+    let err = Result::<(), ()>::Err(());
+    assert_eq!(err, poll(throws_expr!(async { throw!(()) })));
+}
+
+#[test]
+fn throws_integer() {
+    type Error = i32;
+    let err = Result::<(), i32>::Err(1);
+    assert_eq!(err, poll(throws_expr!(async { throw!(1) })));
+}
+
+#[test]
+fn has_inner_fn() {
+    type Error = ();
+    assert_eq!(
+        Result::<(), ()>::Ok(()),
+        poll(throws_expr!(async {
+            async fn foo() -> i32 {
+                5
+            }
+            assert_eq!(5, foo().await);
+        })),
+    );
+}
+
+#[test]
+fn has_inner_closure() {
+    type Error = ();
+    assert_eq!(
+        Result::<(), ()>::Ok(()),
+        poll(throws_expr!(async {
+            assert_eq!(5, async { 5 }.await);
+        })),
+    );
+}
+
+fn poll<F: std::future::Future>(f: F) -> F::Output {
+    struct NoopWake;
+    impl std::task::Wake for NoopWake {
+        fn wake(self: std::sync::Arc<Self>) {}
+    }
+    let std::task::Poll::Ready(output) = std::pin::pin!(f).poll(
+        &mut std::task::Context::from_waker(&std::sync::Arc::new(NoopWake).into()),
+    ) else {
+        panic!("future was not ready")
+    };
+    output
+}

tests/throws_expr_closure.rs

@@ -0,0 +1,76 @@
+#![allow(unused_braces)]
+#![allow(clippy::redundant_closure_call)]
+
+use culpa::{throw, throws_expr};
+
+#[test]
+#[rustfmt::skip]
+fn unit() {
+    type Error = ();
+    let ok = Result::<(), ()>::Ok(());
+    assert_eq!(ok, throws_expr!(|| {})());
+    assert_eq!(ok, throws_expr!(|| ())());
+    assert_eq!(ok, throws_expr!(|| -> () {})());
+    assert_eq!(ok, throws_expr!(|| { return; })());
+    assert_eq!(ok, throws_expr!(|| { return (); })());
+    assert_eq!(ok, throws_expr!(|| -> () { return; })());
+    assert_eq!(ok, throws_expr!(|| -> () { return (); })());
+}
+
+#[test]
+#[rustfmt::skip]
+fn integer() {
+    type Error = ();
+    let ok = Result::<i32, ()>::Ok(1);
+    assert_eq!(ok, throws_expr!(|| { 1 })());
+    assert_eq!(ok, throws_expr!(|| 1)());
+    assert_eq!(ok, throws_expr!(|| -> i32 { 1 })());
+    assert_eq!(ok, throws_expr!(|| { return 1; })());
+    assert_eq!(ok, throws_expr!(|| -> i32 { return 1; })());
+    assert_eq!(ok, throws_expr!(|| -> _ { 1 })());
+}
+
+#[test]
+#[rustfmt::skip]
+fn throws_unit() {
+    type Error = ();
+    let err = Result::<(), ()>::Err(());
+    assert_eq!(err, throws_expr!(|| { throw!(()) })());
+    assert_eq!(err, throws_expr!(|| throw!(()))());
+    assert_eq!(err, throws_expr!(|| -> () { throw!(()) })());
+}
+
+#[test]
+#[rustfmt::skip]
+fn throws_integer() {
+    type Error = i32;
+    let err = Result::<(), i32>::Err(1);
+    assert_eq!(err, throws_expr!(|| { throw!(1)} )());
+    assert_eq!(err, throws_expr!(|| throw!(1))());
+    assert_eq!(err, throws_expr!(|| -> () { throw!(1) })());
+}
+
+#[test]
+fn has_inner_fn() {
+    type Error = ();
+    assert_eq!(
+        Result::<(), ()>::Ok(()),
+        throws_expr!(|| {
+            fn foo() -> i32 {
+                5
+            }
+            assert_eq!(5, foo());
+        })(),
+    );
+}
+
+#[test]
+fn has_inner_closure() {
+    type Error = ();
+    assert_eq!(
+        Result::<(), ()>::Ok(()),
+        throws_expr!(|| {
+            assert_eq!(5, (|| 5)());
+        })(),
+    );
+}