Rollup merge of rust-lang#123794 - oli-obk:define_opaque_types2, r=lcnr

More DefineOpaqueTypes::Yes

This accepts more code on stable. It is now possible to have match arms return a function item `foo::<ConcreteType>` and a function item `foo::<OpaqueTypeInDefiningScope>` in another, and that will constrain `OpaqueTypeInDefiningScope` to have the hidden type `ConcreteType`. So the following function will now compile, but on master it errors with a type mismatch on the second match arm

```rust
// The function item whose generic params we want to merge.
fn foo<T>(t: T) -> T { t }
// Helper ensuring we can constrain `T` on `F` without explicitly specifying it
fn bind<T, F: FnOnce(T) -> T>(_: T, f: F) -> F { f }

fn k() -> impl Sized {
    let x = match true {
        true => {
            // `f` is `FnDef(foo, [infer_var])`
            let f = foo;
            // Get a value of an opaque type on stable
            let t = k();
            // this returns `FnDef(foo, [k::return])`
            bind(t, f)
        }
        false => foo::<()>,
    };
    todo!()
}
```

r? `@compiler-errors`

cc rust-lang#116652

compiler/rustc_hir_typeck/src/coercion.rs

@@ -1139,7 +1139,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         // are the same function and their parameters have a LUB.
                         match self.commit_if_ok(|_| {
                             self.at(cause, self.param_env).lub(
-                                DefineOpaqueTypes::No,
+                                DefineOpaqueTypes::Yes,
                                 prev_ty,
                                 new_ty,
                             )

tests/ui/fn/fn_def_coercion.rs

@@ -0,0 +1,58 @@
+//! Test that coercing between function items of the same function,
+//! but with different generic args succeeds in typeck, but then fails
+//! in borrowck when the lifetimes can't actually be merged.
+
+fn foo<T>(t: T) -> T {
+    t
+}
+
+fn f<'a, 'b, 'c: 'a + 'b>(a: &'a (), b: &'b (), c: &'c ()) {
+    let mut x = foo::<&'a ()>; //~ ERROR: lifetime may not live long enough
+    x = foo::<&'b ()>; //~ ERROR: lifetime may not live long enough
+    x = foo::<&'c ()>;
+    x(a);
+    x(b);
+    x(c);
+}
+
+fn g<'a, 'b, 'c: 'a + 'b>(a: &'a (), b: &'b (), c: &'c ()) {
+    let x = foo::<&'c ()>;
+    let _: &'c () = x(a); //~ ERROR lifetime may not live long enough
+}
+
+fn h<'a, 'b, 'c: 'a + 'b>(a: &'a (), b: &'b (), c: &'c ()) {
+    let x = foo::<&'a ()>;
+    let _: &'a () = x(c);
+}
+
+fn i<'a, 'b, 'c: 'a + 'b>(a: &'a (), b: &'b (), c: &'c ()) {
+    let mut x = foo::<&'c ()>;
+    x = foo::<&'b ()>; //~ ERROR lifetime may not live long enough
+    x = foo::<&'a ()>; //~ ERROR lifetime may not live long enough
+    x(a);
+    x(b);
+    x(c);
+}
+
+fn j<'a, 'b, 'c: 'a + 'b>(a: &'a (), b: &'b (), c: &'c ()) {
+    let x = match true {
+        true => foo::<&'b ()>,  //~ ERROR lifetime may not live long enough
+        false => foo::<&'a ()>, //~ ERROR lifetime may not live long enough
+    };
+    x(a);
+    x(b);
+    x(c);
+}
+
+fn k<'a, 'b, 'c: 'a + 'b>(a: &'a (), b: &'b (), c: &'c ()) {
+    let x = match true {
+        true => foo::<&'c ()>,
+        false => foo::<&'a ()>, //~ ERROR lifetime may not live long enough
+    };
+
+    x(a);
+    x(b); //~ ERROR lifetime may not live long enough
+    x(c);
+}
+
+fn main() {}

tests/ui/fn/fn_def_coercion.stderr

@@ -0,0 +1,154 @@
+error: lifetime may not live long enough
+  --> $DIR/fn_def_coercion.rs:10:17
+   |
+LL | fn f<'a, 'b, 'c: 'a + 'b>(a: &'a (), b: &'b (), c: &'c ()) {
+   |      --  -- lifetime `'b` defined here
+   |      |
+   |      lifetime `'a` defined here
+LL |     let mut x = foo::<&'a ()>;
+   |                 ^^^^^^^^^^^^^ assignment requires that `'a` must outlive `'b`
+   |
+   = help: consider adding the following bound: `'a: 'b`
+   = note: requirement occurs because of a function pointer to `foo`
+   = note: the function `foo` is invariant over the parameter `T`
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
+
+error: lifetime may not live long enough
+  --> $DIR/fn_def_coercion.rs:11:5
+   |
+LL | fn f<'a, 'b, 'c: 'a + 'b>(a: &'a (), b: &'b (), c: &'c ()) {
+   |      --  -- lifetime `'b` defined here
+   |      |
+   |      lifetime `'a` defined here
+LL |     let mut x = foo::<&'a ()>;
+LL |     x = foo::<&'b ()>;
+   |     ^^^^^^^^^^^^^^^^^ assignment requires that `'b` must outlive `'a`
+   |
+   = help: consider adding the following bound: `'b: 'a`
+   = note: requirement occurs because of a function pointer to `foo`
+   = note: the function `foo` is invariant over the parameter `T`
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
+
+help: `'a` and `'b` must be the same: replace one with the other
+
+error: lifetime may not live long enough
+  --> $DIR/fn_def_coercion.rs:20:12
+   |
+LL | fn g<'a, 'b, 'c: 'a + 'b>(a: &'a (), b: &'b (), c: &'c ()) {
+   |      --      -- lifetime `'c` defined here
+   |      |
+   |      lifetime `'a` defined here
+LL |     let x = foo::<&'c ()>;
+LL |     let _: &'c () = x(a);
+   |            ^^^^^^ type annotation requires that `'a` must outlive `'c`
+   |
+   = help: consider adding the following bound: `'a: 'c`
+
+error: lifetime may not live long enough
+  --> $DIR/fn_def_coercion.rs:30:5
+   |
+LL | fn i<'a, 'b, 'c: 'a + 'b>(a: &'a (), b: &'b (), c: &'c ()) {
+   |      --  -- lifetime `'b` defined here
+   |      |
+   |      lifetime `'a` defined here
+LL |     let mut x = foo::<&'c ()>;
+LL |     x = foo::<&'b ()>;
+   |     ^^^^^^^^^^^^^^^^^ assignment requires that `'b` must outlive `'a`
+   |
+   = help: consider adding the following bound: `'b: 'a`
+   = note: requirement occurs because of a function pointer to `foo`
+   = note: the function `foo` is invariant over the parameter `T`
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
+
+error: lifetime may not live long enough
+  --> $DIR/fn_def_coercion.rs:31:5
+   |
+LL | fn i<'a, 'b, 'c: 'a + 'b>(a: &'a (), b: &'b (), c: &'c ()) {
+   |      --  -- lifetime `'b` defined here
+   |      |
+   |      lifetime `'a` defined here
+...
+LL |     x = foo::<&'a ()>;
+   |     ^^^^^^^^^^^^^^^^^ assignment requires that `'a` must outlive `'b`
+   |
+   = help: consider adding the following bound: `'a: 'b`
+   = note: requirement occurs because of a function pointer to `foo`
+   = note: the function `foo` is invariant over the parameter `T`
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
+
+help: `'a` and `'b` must be the same: replace one with the other
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: lifetime may not live long enough
+  --> $DIR/fn_def_coercion.rs:39:17
+   |
+LL | fn j<'a, 'b, 'c: 'a + 'b>(a: &'a (), b: &'b (), c: &'c ()) {
+   |      --  -- lifetime `'b` defined here
+   |      |
+   |      lifetime `'a` defined here
+LL |     let x = match true {
+LL |         true => foo::<&'b ()>,
+   |                 ^^^^^^^^^^^^^ assignment requires that `'b` must outlive `'a`
+   |
+   = help: consider adding the following bound: `'b: 'a`
+   = note: requirement occurs because of a function pointer to `foo`
+   = note: the function `foo` is invariant over the parameter `T`
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
+
+error: lifetime may not live long enough
+  --> $DIR/fn_def_coercion.rs:40:18
+   |
+LL | fn j<'a, 'b, 'c: 'a + 'b>(a: &'a (), b: &'b (), c: &'c ()) {
+   |      --  -- lifetime `'b` defined here
+   |      |
+   |      lifetime `'a` defined here
+...
+LL |         false => foo::<&'a ()>,
+   |                  ^^^^^^^^^^^^^ assignment requires that `'a` must outlive `'b`
+   |
+   = help: consider adding the following bound: `'a: 'b`
+   = note: requirement occurs because of a function pointer to `foo`
+   = note: the function `foo` is invariant over the parameter `T`
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
+
+help: `'a` and `'b` must be the same: replace one with the other
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: lifetime may not live long enough
+  --> $DIR/fn_def_coercion.rs:50:18
+   |
+LL | fn k<'a, 'b, 'c: 'a + 'b>(a: &'a (), b: &'b (), c: &'c ()) {
+   |      --      -- lifetime `'c` defined here
+   |      |
+   |      lifetime `'a` defined here
+...
+LL |         false => foo::<&'a ()>,
+   |                  ^^^^^^^^^^^^^ assignment requires that `'a` must outlive `'c`
+   |
+   = help: consider adding the following bound: `'a: 'c`
+   = note: requirement occurs because of a function pointer to `foo`
+   = note: the function `foo` is invariant over the parameter `T`
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
+
+error: lifetime may not live long enough
+  --> $DIR/fn_def_coercion.rs:54:5
+   |
+LL | fn k<'a, 'b, 'c: 'a + 'b>(a: &'a (), b: &'b (), c: &'c ()) {
+   |      --  -- lifetime `'b` defined here
+   |      |
+   |      lifetime `'a` defined here
+...
+LL |     x(b);
+   |     ^^^^ argument requires that `'b` must outlive `'a`
+   |
+   = help: consider adding the following bound: `'b: 'a`
+
+help: the following changes may resolve your lifetime errors
+   |
+   = help: add bound `'a: 'c`
+   = help: add bound `'b: 'a`
+
+error: aborting due to 9 previous errors
+

tests/ui/fn/fn_def_opaque_coercion.rs

@@ -0,0 +1,69 @@
+//! Test that coercing between function items of the same function,
+//! but with different generic args works.
+
+//@check-pass
+
+#![feature(type_alias_impl_trait)]
+
+fn foo<T>(t: T) -> T {
+    t
+}
+
+type F = impl Sized;
+
+fn f(a: F) {
+    let mut x = foo::<F>;
+    x = foo::<()>;
+    x(a);
+    x(());
+}
+
+type G = impl Sized;
+
+fn g(a: G) {
+    let x = foo::<()>;
+    let _: () = x(a);
+}
+
+type H = impl Sized;
+
+fn h(a: H) {
+    let x = foo::<H>;
+    let _: H = x(());
+}
+
+type I = impl Sized;
+
+fn i(a: I) {
+    let mut x = foo::<()>;
+    x = foo::<I>;
+    x(a);
+    x(());
+}
+
+type J = impl Sized;
+
+fn j(a: J) {
+    let x = match true {
+        true => foo::<J>,
+        false => foo::<()>,
+    };
+    x(a);
+    x(());
+}
+
+fn k() -> impl Sized {
+    fn bind<T, F: FnOnce(T) -> T>(_: T, f: F) -> F {
+        f
+    }
+    let x = match true {
+        true => {
+            let f = foo;
+            bind(k(), f)
+        }
+        false => foo::<()>,
+    };
+    todo!()
+}
+
+fn main() {}