Handle desugaring in impl trait bound suggestion

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -254,27 +254,21 @@ fn suggest_restriction(
         let pred = trait_ref.without_const().to_predicate(tcx).to_string();
         let pred = pred.replace(&impl_trait_str, &type_param_name);
         let mut sugg = vec![
+            // Find the last of the generic parameters contained within the span of
+            // the generics
             match generics
                 .params
                 .iter()
-                .filter(|p| match p.kind {
-                    hir::GenericParamKind::Type {
-                        synthetic: Some(hir::SyntheticTyParamKind::ImplTrait),
-                        ..
-                    } => false,
-                    _ => true,
-                })
-                .last()
+                .map(|p| p.bounds_span().unwrap_or(p.span))
+                .filter(|&span| generics.span.contains(span) && span.desugaring_kind().is_none())
+                .max_by_key(|span| span.hi())
             {
                 // `fn foo(t: impl Trait)`
                 //        ^ suggest `<T: Trait>` here
                 None => (generics.span, format!("<{}>", type_param)),
                 // `fn foo<A>(t: impl Trait)`
                 //        ^^^ suggest `<A, T: Trait>` here
-                Some(param) => (
-                    param.bounds_span().unwrap_or(param.span).shrink_to_hi(),
-                    format!(", {}", type_param),
-                ),
+                Some(span) => (span.shrink_to_hi(), format!(", {}", type_param)),
             },
             // `fn foo(t: impl Trait)`
             //                       ^ suggest `where <T as Trait>::A: Bound`

src/test/ui/suggestions/impl-trait-with-missing-bounds.rs

@@ -39,6 +39,14 @@ fn bak(constraints: impl  Iterator + std::fmt::Debug) {
     }
 }
 
+#[rustfmt::skip]
+fn baw<>(constraints: impl Iterator) {
+    for constraint in constraints {
+        qux(constraint);
+//~^ ERROR `<impl Iterator as Iterator>::Item` doesn't implement `Debug`
+    }
+}
+
 fn qux(_: impl std::fmt::Debug) {}
 
 fn main() {}

src/test/ui/suggestions/impl-trait-with-missing-bounds.stderr

@@ -73,6 +73,21 @@ help: introduce a type parameter with a trait bound instead of using `impl Trait
 LL | fn bak<I: Iterator + std::fmt::Debug>(constraints: I) where <I as Iterator>::Item: Debug {
    |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^              ^  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 5 previous errors
+error[E0277]: `<impl Iterator as Iterator>::Item` doesn't implement `Debug`
+  --> $DIR/impl-trait-with-missing-bounds.rs:45:13
+   |
+LL |         qux(constraint);
+   |             ^^^^^^^^^^ `<impl Iterator as Iterator>::Item` cannot be formatted using `{:?}` because it doesn't implement `Debug`
+...
+LL | fn qux(_: impl std::fmt::Debug) {}
+   |                --------------- required by this bound in `qux`
+   |
+   = help: the trait `Debug` is not implemented for `<impl Iterator as Iterator>::Item`
+help: introduce a type parameter with a trait bound instead of using `impl Trait`
+   |
+LL | fn baw<I: Iterator>(constraints: I) where <I as Iterator>::Item: Debug {
+   |       ^^^^^^^^^^^^^              ^  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 6 previous errors
 
 For more information about this error, try `rustc --explain E0277`.

src/test/ui/suggestions/issue-79843-impl-trait-with-missing-bounds-on-async-fn.rs

@@ -0,0 +1,32 @@
+// Regression test: if we suggest replacing an `impl Trait` argument to an async
+// fn with a named type parameter in order to add bounds, the suggested function
+// signature should be well-formed.
+//
+// edition:2018
+
+trait Foo {
+    type Bar;
+    fn bar(&self) -> Self::Bar;
+}
+
+async fn run(_: &(), foo: impl Foo) -> std::io::Result<()> {
+    let bar = foo.bar();
+    assert_is_send(&bar);
+//~^ ERROR: `<impl Foo as Foo>::Bar` cannot be sent between threads safely
+
+    Ok(())
+}
+
+// Test our handling of cases where there is a generic parameter list in the
+// source, but only synthetic generic parameters
+async fn run2< >(_: &(), foo: impl Foo) -> std::io::Result<()> {
+    let bar = foo.bar();
+    assert_is_send(&bar);
+//~^ ERROR: `<impl Foo as Foo>::Bar` cannot be sent between threads safely
+
+    Ok(())
+}
+
+fn assert_is_send<T: Send>(_: &T) {}
+
+fn main() {}

src/test/ui/suggestions/issue-79843-impl-trait-with-missing-bounds-on-async-fn.stderr

@@ -0,0 +1,33 @@
+error[E0277]: `<impl Foo as Foo>::Bar` cannot be sent between threads safely
+  --> $DIR/issue-79843-impl-trait-with-missing-bounds-on-async-fn.rs:14:20
+   |
+LL |     assert_is_send(&bar);
+   |                    ^^^^ `<impl Foo as Foo>::Bar` cannot be sent between threads safely
+...
+LL | fn assert_is_send<T: Send>(_: &T) {}
+   |                      ---- required by this bound in `assert_is_send`
+   |
+   = help: the trait `Send` is not implemented for `<impl Foo as Foo>::Bar`
+help: introduce a type parameter with a trait bound instead of using `impl Trait`
+   |
+LL | async fn run<F: Foo>(_: &(), foo: F) -> std::io::Result<()> where <F as Foo>::Bar: Send {
+   |             ^^^^^^^^              ^                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0277]: `<impl Foo as Foo>::Bar` cannot be sent between threads safely
+  --> $DIR/issue-79843-impl-trait-with-missing-bounds-on-async-fn.rs:24:20
+   |
+LL |     assert_is_send(&bar);
+   |                    ^^^^ `<impl Foo as Foo>::Bar` cannot be sent between threads safely
+...
+LL | fn assert_is_send<T: Send>(_: &T) {}
+   |                      ---- required by this bound in `assert_is_send`
+   |
+   = help: the trait `Send` is not implemented for `<impl Foo as Foo>::Bar`
+help: introduce a type parameter with a trait bound instead of using `impl Trait`
+   |
+LL | async fn run2<F: Foo>(_: &(), foo: F) -> std::io::Result<()> where <F as Foo>::Bar: Send {
+   |              ^^^^^^^^              ^                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.