diagnostics: Fix ICE building a trait ref in method suggestions

compiler/rustc_hir_typeck/src/method/suggest.rs

@@ -4796,7 +4796,24 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let hir::Node::Expr(rcvr) = self.tcx.hir_node(hir_id) else {
             return false;
         };
-        let trait_ref = ty::TraitRef::new(self.tcx, trait_def_id, rcvr_ty.into_iter());
+        // The trait may have generic parameters beyond `Self` (e.g. `Borrow<Borrowed>`), and
+        // `rcvr_ty` may even be unknown. We only ever know the receiver type (the `Self` arg),
+        // so fill `Self` from `rcvr_ty` when available and the remaining parameters with fresh
+        // inference variables; building a `TraitRef` with a partial arg list would otherwise trip
+        // `debug_assert_args_compatible` and ICE. See #157189.
+        let trait_ref = ty::TraitRef::new_from_args(
+            self.tcx,
+            trait_def_id,
+            ty::GenericArgs::for_item(self.tcx, trait_def_id, |param, _| {
+                if param.index == 0
+                    && let Some(rcvr_ty) = rcvr_ty
+                {
+                    rcvr_ty.into()
+                } else {
+                    self.var_for_def(rcvr.span, param)
+                }
+            }),
+        );
         let trait_pred = ty::Binder::dummy(ty::TraitPredicate {
             trait_ref,
             polarity: ty::PredicatePolarity::Positive,

tests/ui/methods/method-suggestion-trait-with-extra-generics-no-ice.rs

@@ -0,0 +1,24 @@
+//! Regression test for #157189.
+//!
+//! When a method call fails to resolve, the "trait which provides `<method>` is
+//! implemented but not in scope" diagnostic probes all traits for a method of the
+//! same name. Here `.borrow()` matches `std::borrow::Borrow::borrow`, and `Borrow`
+//! has a generic parameter (`Borrowed`) besides `Self`. Building the trait
+//! reference for the diagnostic used to pass only the receiver type as the single
+//! argument, which mismatched the trait's generics and ICEd in
+//! `debug_assert_args_compatible`. It should just report the error.
+
+trait Foo {
+    extern "C" fn borrow(&self);
+}
+
+struct Bar;
+
+fn main() {
+    let foo: Box<dyn Fn(bool) -> usize> = Box::new(Bar);
+    //~^ ERROR expected a `Fn(bool)` closure, found `Bar`
+    foo.borrow();
+    //~^ ERROR no method named `borrow` found
+    foo.take()
+    //~^ ERROR `Box<dyn Fn(bool) -> usize>` is not an iterator
+}

tests/ui/methods/method-suggestion-trait-with-extra-generics-no-ice.stderr

@@ -0,0 +1,65 @@
+error[E0599]: no method named `borrow` found for struct `Box<dyn Fn(bool) -> usize>` in the current scope
+  --> $DIR/method-suggestion-trait-with-extra-generics-no-ice.rs:20:9
+   |
+LL |     foo.borrow();
+   |         ^^^^^^
+   |
+  --> $SRC_DIR/core/src/borrow.rs:LL:COL
+   |
+   = note: the method is available for `Box<dyn Fn(bool) -> usize>` here
+   |
+   = help: items from traits can only be used if the trait is in scope
+help: use parentheses to call this trait object
+   |
+LL |     foo(/* bool */).borrow();
+   |        ++++++++++++
+help: trait `Borrow` which provides `borrow` is implemented but not in scope; perhaps you want to import it
+   |
+LL + use std::borrow::Borrow;
+   |
+help: there is a method `borrow_mut` with a similar name
+   |
+LL |     foo.borrow_mut();
+   |               ++++
+
+error[E0277]: expected a `Fn(bool)` closure, found `Bar`
+  --> $DIR/method-suggestion-trait-with-extra-generics-no-ice.rs:18:43
+   |
+LL |     let foo: Box<dyn Fn(bool) -> usize> = Box::new(Bar);
+   |                                           ^^^^^^^^^^^^^ expected an `Fn(bool)` closure, found `Bar`
+   |
+help: the trait `Fn(bool)` is not implemented for `Bar`
+  --> $DIR/method-suggestion-trait-with-extra-generics-no-ice.rs:15:1
+   |
+LL | struct Bar;
+   | ^^^^^^^^^^
+   = note: required for the cast from `Box<Bar>` to `Box<dyn Fn(bool) -> usize>`
+
+error[E0599]: `Box<dyn Fn(bool) -> usize>` is not an iterator
+  --> $DIR/method-suggestion-trait-with-extra-generics-no-ice.rs:22:9
+   |
+LL |     foo.take()
+   |         ^^^^
+   |         |
+   |         this is an associated function, not a method
+   |         `Box<dyn Fn(bool) -> usize>` is not an iterator
+   |
+   = note: found the following associated functions; to be used as methods, functions must have a `self` parameter
+   = note: the candidate is defined in an impl for the type `Box<T, A>`
+   = note: the following trait bounds were not satisfied:
+           `dyn Fn(bool) -> usize: Iterator`
+           which is required by `Box<dyn Fn(bool) -> usize>: Iterator`
+           `Box<dyn Fn(bool) -> usize>: Iterator`
+           which is required by `&mut Box<dyn Fn(bool) -> usize>: Iterator`
+           `dyn Fn(bool) -> usize: Iterator`
+           which is required by `&mut dyn Fn(bool) -> usize: Iterator`
+help: use associated function syntax instead
+   |
+LL -     foo.take()
+LL +     Box::<dyn Fn(bool) -> usize>::take(foo)
+   |
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0277, E0599.
+For more information about an error, try `rustc --explain E0277`.