Auto merge of #86118 - spastorino:tait-soundness-bug, r=nikomatsakis

Create different inference variables for different defining uses of TAITs

Fixes #73481

r? `@nikomatsakis`
cc `@oli-obk`

src/test/ui/type-alias-impl-trait/multiple-def-uses-in-one-fn-pass.rs

@@ -0,0 +1,12 @@
+// check-pass
+#![feature(min_type_alias_impl_trait)]
+
+type X<A: ToString + Clone, B: ToString + Clone> = impl ToString;
+
+fn f<A: ToString + Clone, B: ToString + Clone>(a: A, b: B) -> (X<A, B>, X<A, B>) {
+    (a.clone(), a)
+}
+
+fn main() {
+    println!("{}", <X<_, _> as ToString>::to_string(&f(42_i32, String::new()).1));
+}

src/test/ui/type-alias-impl-trait/multiple-def-uses-in-one-fn.rs

@@ -0,0 +1,16 @@
+// https://github.com/rust-lang/rust/issues/73481
+// This test used to cause unsoundness, since one of the two possible
+// resolutions was chosen at random instead of erroring due to conflicts.
+
+#![feature(min_type_alias_impl_trait)]
+
+type X<A, B> = impl Into<&'static A>;
+//~^ ERROR the trait bound `&'static B: From<&A>` is not satisfied
+
+fn f<A, B: 'static>(a: &'static A, b: B) -> (X<A, B>, X<B, A>) {
+    (a, a)
+}
+
+fn main() {
+    println!("{}", <X<_, _> as Into<&String>>::into(f(&[1isize, 2, 3], String::new()).1));
+}

src/test/ui/type-alias-impl-trait/multiple-def-uses-in-one-fn.stderr

@@ -0,0 +1,15 @@
+error[E0277]: the trait bound `&'static B: From<&A>` is not satisfied
+  --> $DIR/multiple-def-uses-in-one-fn.rs:7:16
+   |
+LL | type X<A, B> = impl Into<&'static A>;
+   |                ^^^^^^^^^^^^^^^^^^^^^ the trait `From<&A>` is not implemented for `&'static B`
+   |
+   = note: required because of the requirements on the impl of `Into<&'static B>` for `&A`
+help: consider introducing a `where` bound, but there might be an alternative better way to express this requirement
+   |
+LL | fn f<A, B: 'static>(a: &'static A, b: B) -> (X<A, B>, X<B, A>) where &'static B: From<&A> {
+   |                                                                ^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/type-alias-impl-trait/multiple-def-uses-in-one-fn2.rs

@@ -0,0 +1,16 @@
+// https://github.com/rust-lang/rust/issues/73481
+// This test used to cause unsoundness, since one of the two possible
+// resolutions was chosen at random instead of erroring due to conflicts.
+
+#![feature(min_type_alias_impl_trait)]
+
+type X<A: ToString + Clone, B: ToString + Clone> = impl ToString;
+//~^ ERROR could not find defining uses
+
+fn f<A: ToString + Clone, B: ToString + Clone>(a: A, b: B) -> (X<A, B>, X<B, A>) {
+    (a.clone(), a)
+}
+
+fn main() {
+    println!("{}", <X<_, _> as ToString>::to_string(&f(42_i32, String::new()).1));
+}

src/test/ui/type-alias-impl-trait/multiple-def-uses-in-one-fn2.stderr

@@ -0,0 +1,8 @@
+error: could not find defining uses
+  --> $DIR/multiple-def-uses-in-one-fn2.rs:7:52
+   |
+LL | type X<A: ToString + Clone, B: ToString + Clone> = impl ToString;
+   |                                                    ^^^^^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/type-alias-impl-trait/multiple-def-uses-in-one-fn3.rs

@@ -0,0 +1,18 @@
+// https://github.com/rust-lang/rust/issues/73481
+// This test used to cause unsoundness, since one of the two possible
+// resolutions was chosen at random instead of erroring due to conflicts.
+
+#![feature(min_type_alias_impl_trait)]
+
+type X<A: ToString + Clone, B: ToString + Clone> = impl ToString;
+
+fn f<A: ToString + Clone, B: ToString + Clone>(a: A, b: B) -> (X<A, B>, X<B, A>) {
+    (a, b)
+}
+
+fn g<A: ToString + Clone, B: ToString + Clone>(a: A, b: B) -> (X<A, B>, X<A, B>) {
+    (a, b)
+    //~^ ERROR mismatched types
+}
+
+fn main() {}

src/test/ui/type-alias-impl-trait/multiple-def-uses-in-one-fn3.stderr

@@ -0,0 +1,18 @@
+error[E0308]: mismatched types
+  --> $DIR/multiple-def-uses-in-one-fn3.rs:14:9
+   |
+LL | fn g<A: ToString + Clone, B: ToString + Clone>(a: A, b: B) -> (X<A, B>, X<A, B>) {
+   |      -                    - found type parameter
+   |      |
+   |      expected type parameter
+LL |     (a, b)
+   |         ^ expected type parameter `A`, found type parameter `B`
+   |
+   = note: expected type parameter `A`
+              found type parameter `B`
+   = note: a type parameter was expected, but a different one was found; you might be missing a type parameter or trait bound
+   = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.