nightly-diagnostics; warn on trait autoimpl with Sized bounded fns

.github/workflows/test.yml

@@ -15,6 +15,7 @@ jobs:
           - os: ubuntu-latest
             toolchain: nightly
             variant: docs
+            features: "--features nightly-diagnostics"
           - os: macos-latest
             toolchain: beta
           - os: windows-latest
@@ -41,13 +42,13 @@ jobs:
 
       - name: Build docs
         if: ${{ matrix.variant == 'docs' }}
-        run: cargo doc --all-features --all --no-deps
+        run: cargo doc ${{ matrix.features }} --all --no-deps
 
       - name: Test impl-tools-lib
-        run: cargo test --manifest-path lib/Cargo.toml --all-features ${{ matrix.targets }}
+        run: cargo test --manifest-path lib/Cargo.toml ${{ matrix.targets }}
 
       - name: Test impl-tools
-        run: cargo test --all-features ${{ matrix.targets }}
+        run: cargo test ${{ matrix.features }} ${{ matrix.targets }}
 
       - name: Test test-cfg
         working-directory: tests/test-cfg

Cargo.toml

@@ -25,6 +25,12 @@ edition.workspace = true
 [lib]
 proc-macro = true
 
+[features]
+# Enables better proc-macro diagnostics (including warnings); nightly only.
+nightly-diagnostics = ["proc-macro-error2/nightly"]
+# Squelch lint: "autoimpl on trait that has a method with Self: Sized bound"
+allow-trait-autoimpl-with-sized-fn-bound = ["impl-tools-lib/allow-trait-autoimpl-with-sized-fn-bound"]
+
 [dependencies.proc-macro-error2]
 version = "2.0"
 default-features = false

compile-errors/expect-self.stderr → compile-errors/common/expect-self.stderr

@@ -1,5 +1,5 @@
 error: expected `Self` or `A` or `A<...>` or `Trait for Self`, etc
- --> compile-errors/expect-self.rs:5:10
+ --> compile-errors/common/expect-self.rs:5:10
   |
 5 |     impl () {
   |          ^^

compile-errors/wrong-item-type.stderr → compile-errors/common/wrong-item-type.stderr

@@ -1,5 +1,5 @@
 error: #[autoimpl(Trait)] can only be used on enum or struct items
- --> compile-errors/wrong-item-type.rs:3:1
+ --> compile-errors/common/wrong-item-type.rs:3:1
   |
 3 | #[impl_tools::autoimpl(Copy)]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

compile-errors/nightly/expect-self-anon.stderr

@@ -0,0 +1,16 @@
+error: expected `Self` or `Trait for Self`
+ --> compile-errors/nightly/expect-self-anon.rs:6:14
+  |
+6 |         impl () {
+  |              ^^
+
+error: expected expression, found end of macro arguments
+  --> compile-errors/nightly/expect-self-anon.rs:2:13
+   |
+ 2 |       let _ = impl_tools::impl_anon! {
+   |  _____________^
+ 3 | |         #[derive(Clone, Debug)]
+ 4 | |         struct (i32 = 123);
+...  |
+11 | |     };
+   | |_____^

compile-errors/nightly/trait-autoimpl-with-sized-fn-bound.rs

@@ -0,0 +1,21 @@
+use impl_tools::autoimpl;
+use core::fmt::Debug;
+
+#[autoimpl(for<'a, T> &'a T, &'a mut T, Box<T> where T: trait + ?Sized)]
+trait G<V>
+where
+    V: Debug,
+{
+    fn g(&self) -> V;
+
+    fn s<X>(&self, f: impl Fn(V) -> X) -> X
+    where
+        Self: Sized,
+    {
+        f(self.g())
+    }
+}
+
+fn main() {
+    compile_error!("Warnings verification");
+}

compile-errors/nightly/trait-autoimpl-with-sized-fn-bound.stderr

@@ -0,0 +1,22 @@
+warning: autoimpl on trait that has a method with Self: Sized bound
+  --> compile-errors/nightly/trait-autoimpl-with-sized-fn-bound.rs:4:1
+   |
+ 4 | #[autoimpl(for<'a, T> &'a T, &'a mut T, Box<T> where T: trait + ?Sized)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: method impl uses default implementation, not deref
+  --> compile-errors/nightly/trait-autoimpl-with-sized-fn-bound.rs:11:5
+   |
+11 | /     fn s<X>(&self, f: impl Fn(V) -> X) -> X
+12 | |     where
+13 | |         Self: Sized,
+...  |
+16 | |     }
+   | |_____^
+   = note: this warning originates in the attribute macro `autoimpl` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: Warnings verification
+  --> compile-errors/nightly/trait-autoimpl-with-sized-fn-bound.rs:20:5
+   |
+20 |     compile_error!("Warnings verification");
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

compile-errors/stable/expect-self-anon.rs

@@ -0,0 +1,12 @@
+fn main() {
+    let _ = impl_tools::impl_anon! {
+        #[derive(Clone, Debug)]
+        struct (i32 = 123);
+
+        impl () {
+            fn get(&self) -> i32 {
+                self.0
+            }
+        }
+    };
+}

compile-errors/expect-self-anon.stderr → compile-errors/stable/expect-self-anon.stderr

@@ -1,5 +1,5 @@
 error: expected `Self` or `Trait for Self`
- --> compile-errors/expect-self-anon.rs:6:14
+ --> compile-errors/stable/expect-self-anon.rs:6:14
   |
 6 |         impl () {
   |              ^^

lib/Cargo.toml

@@ -11,6 +11,10 @@ repository.workspace = true
 rust-version.workspace = true
 edition.workspace = true
 
+[features]
+# Squelch lint: "autoimpl on trait that has a method with Self: Sized bound"
+allow-trait-autoimpl-with-sized-fn-bound = []
+
 [dependencies]
 quote = "1.0"
 proc-macro2 = "1.0"

lib/src/autoimpl/for_deref.rs

@@ -7,7 +7,7 @@
 
 use crate::generics::{GenericParam, Generics, TypeParamBound, WherePredicate};
 use proc_macro2::{Span, TokenStream};
-use proc_macro_error2::{emit_call_site_error, emit_error};
+use proc_macro_error2::{emit_call_site_error, emit_call_site_warning, emit_error};
 use quote::{quote, ToTokens, TokenStreamExt};
 use syn::punctuated::Punctuated;
 use syn::spanned::Spanned;
@@ -198,6 +198,12 @@ impl ForDeref {
                                 "cannot autoimpl trait with Deref";
                                 note = item.span() => "method has a bound on Self and no default implementation";
                             );
+                        } else if !cfg!(feature = "allow-trait-autoimpl-with-sized-fn-bound") {
+                            // TODO(rust proc_macro_lint): this should be a configurable lint
+                            emit_call_site_warning!(
+                                "autoimpl on trait that has a method with Self: Sized bound";
+                                note = item.span() => "method impl uses default implementation, not deref";
+                            );
                         }
 
                         continue;

src/lib.rs

@@ -240,10 +240,12 @@ pub fn impl_default(args: TokenStream, item: TokenStream) -> TokenStream {
 /// Traits using generics and trait items using generics are, for the most part,
 /// supported.
 ///
-/// Items with a where clause with a type bound on `Self` are not supported
-/// since the item is not guaranteed to exist on the definitive type.
-/// Exception: methods with a default implementation (in this case the item is
-/// skipped).
+/// Items with a where clause with a type bound on `Self` cannot be implemented
+/// via [`Deref`] since the item is not guaranteed to exist on the definitive
+/// type. Such items with a default implementation may be implemented using this
+/// that default implementation, though this results in a warning by default
+/// (requires feature "nightly-diagnostics").
+/// In other cases an error is reported.
 ///
 /// An example:
 /// ```

tests/compile-errors.rs

@@ -1,5 +1,11 @@
 #[test]
 fn compile_errors() {
     let t = trybuild::TestCases::new();
-    t.compile_fail("compile-errors/*.rs");
+    t.compile_fail("compile-errors/common/*.rs");
+
+    #[cfg(feature = "nightly-diagnostics")]
+    t.compile_fail("compile-errors/nightly/*.rs");
+
+    #[cfg(not(feature = "nightly-diagnostics"))]
+    t.compile_fail("compile-errors/stable/*.rs");
 }

tests/for_deref.rs

@@ -67,13 +67,6 @@ where
     V: Debug,
 {
     fn g(&self) -> V;
-
-    fn s<X>(&self, f: impl Fn(V) -> X) -> X
-    where
-        Self: Sized,
-    {
-        f(self.g())
-    }
 }
 
 #[test]
@@ -87,7 +80,6 @@ fn g() {
 
     fn impls_g(g: impl G<i32>) {
         assert_eq!(g.g(), 123);
-        assert!(g.s(|x| x == 123));
     }
 
     impls_g(S);