Auto merge of #118882 - compiler-errors:normalized-sig-wf, r=<try>

Check normalized call signature for WF in mir typeck

Unfortunately we don't check that the built-in implementations for `Fn*` traits are actually well-formed in the same way that we do for user-provided impls.

Essentially, when checking a call terminator, we end up with a signature that references an unnormalized `<[closure] as FnOnce<...>>::Output` in its output. That output type, due to the built-in impl, doesn't follow the expected rule that `WF(ty)` implies `WF(normalized(ty))`. We fix this by also checking the normalized signature here.

**See** boxy's detailed and useful explanation comment which explains this in more detail: #114936 (comment)

Fixes #114936
Fixes #118876

r? types
cc `@BoxyUwU` `@lcnr`

Author: bors

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -1413,6 +1413,18 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                     ConstraintCategory::Boring,
                 );
                 let sig = self.normalize(sig, term_location);
+                // HACK(#114936): `WF(sig)` does not imply `WF(normalized(sig))`
+                // with built-in `Fn` implementations, since the impl may not be
+                // well-formed itself.
+                self.prove_predicates(
+                    sig.inputs_and_output.iter().map(|ty| {
+                        ty::Binder::dummy(ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(
+                            ty.into(),
+                        )))
+                    }),
+                    term_location.to_locations(),
+                    ConstraintCategory::Boring,
+                );
                 self.check_call_dest(body, term, &sig, *destination, *target, term_location);
 
                 // The ordinary liveness rules will ensure that all

tests/ui/higher-ranked/trait-bounds/issue-59311.rs

@@ -17,6 +17,7 @@ where
     v.t(|| {});
     //~^ ERROR: higher-ranked lifetime error
     //~| ERROR: higher-ranked lifetime error
+    //~| ERROR: higher-ranked lifetime error
 }
 
 fn main() {}

tests/ui/higher-ranked/trait-bounds/issue-59311.stderr

@@ -6,6 +6,15 @@ LL |     v.t(|| {});
    |
    = note: could not prove `{closure@$DIR/issue-59311.rs:17:9: 17:11} well-formed`
 
+error: higher-ranked lifetime error
+  --> $DIR/issue-59311.rs:17:5
+   |
+LL |     v.t(|| {});
+   |     ^^^^^^^^^^
+   |
+   = note: could not prove `{closure@$DIR/issue-59311.rs:17:9: 17:11} well-formed`
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
 error: higher-ranked lifetime error
   --> $DIR/issue-59311.rs:17:9
    |
@@ -14,5 +23,5 @@ LL |     v.t(|| {});
    |
    = note: could not prove `for<'a> &'a V: 'static`
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 

tests/ui/nll/check-normalized-sig-for-wf.rs

@@ -0,0 +1,27 @@
+// <https://github.com/rust-lang/rust/issues/114936>
+fn whoops(
+    s: String,
+    f: impl for<'s> FnOnce(&'s str) -> (&'static str, [&'static &'s (); 0]),
+) -> &'static str
+{
+    f(&s).0
+    //~^ ERROR `s` does not live long enough
+}
+
+// <https://github.com/rust-lang/rust/issues/118876>
+fn extend<T>(input: &T) -> &'static T {
+    struct Bounded<'a, 'b: 'static, T>(&'a T, [&'b (); 0]);
+    let n: Box<dyn FnOnce(&T) -> Bounded<'static, '_, T>> = Box::new(|x| Bounded(x, []));
+    n(input).0
+    //~^ ERROR borrowed data escapes outside of function
+}
+
+// <https://github.com/rust-lang/rust/issues/118876>
+fn extend_mut<'a, T>(input: &'a mut T) -> &'static mut T {
+    struct Bounded<'a, 'b: 'static, T>(&'a mut T, [&'b (); 0]);
+    let mut n: Box<dyn FnMut(&mut T) -> Bounded<'static, '_, T>> = Box::new(|x| Bounded(x, []));
+    n(input).0
+    //~^ ERROR borrowed data escapes outside of function
+}
+
+fn main() {}

tests/ui/nll/check-normalized-sig-for-wf.stderr

@@ -0,0 +1,47 @@
+error[E0597]: `s` does not live long enough
+  --> $DIR/check-normalized-sig-for-wf.rs:7:7
+   |
+LL |     s: String,
+   |     - binding `s` declared here
+...
+LL |     f(&s).0
+   |     --^^-
+   |     | |
+   |     | borrowed value does not live long enough
+   |     argument requires that `s` is borrowed for `'static`
+LL |
+LL | }
+   | - `s` dropped here while still borrowed
+
+error[E0521]: borrowed data escapes outside of function
+  --> $DIR/check-normalized-sig-for-wf.rs:15:5
+   |
+LL | fn extend<T>(input: &T) -> &'static T {
+   |              -----  - let's call the lifetime of this reference `'1`
+   |              |
+   |              `input` is a reference that is only valid in the function body
+...
+LL |     n(input).0
+   |     ^^^^^^^^
+   |     |
+   |     `input` escapes the function body here
+   |     argument requires that `'1` must outlive `'static`
+
+error[E0521]: borrowed data escapes outside of function
+  --> $DIR/check-normalized-sig-for-wf.rs:23:5
+   |
+LL | fn extend_mut<'a, T>(input: &'a mut T) -> &'static mut T {
+   |               --     ----- `input` is a reference that is only valid in the function body
+   |               |
+   |               lifetime `'a` defined here
+...
+LL |     n(input).0
+   |     ^^^^^^^^
+   |     |
+   |     `input` escapes the function body here
+   |     argument requires that `'a` must outlive `'static`
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0521, E0597.
+For more information about an error, try `rustc --explain E0521`.

tests/ui/nll/missing-universe-cause-issue-114907.rs

@@ -33,6 +33,8 @@ fn main() {
     accept(callback);
     //~^ ERROR mismatched types
     //~| ERROR mismatched types
+    //~| ERROR mismatched types
+    //~| ERROR implementation of `FnOnce` is not general enough
     //~| ERROR implementation of `FnOnce` is not general enough
     //~| ERROR implementation of `FnOnce` is not general enough
     //~| ERROR higher-ranked subtype error

tests/ui/nll/missing-universe-cause-issue-114907.stderr

@@ -63,6 +63,40 @@ help: consider specifying the type of the closure parameters
 LL |     let callback = |_: &_| {};
    |                    ~~~~~~~
 
+error: implementation of `FnOnce` is not general enough
+  --> $DIR/missing-universe-cause-issue-114907.rs:33:5
+   |
+LL |     accept(callback);
+   |     ^^^^^^^^^^^^^^^^ implementation of `FnOnce` is not general enough
+   |
+   = note: closure with signature `fn(&'2 ())` must implement `FnOnce<(&'1 (),)>`, for any lifetime `'1`...
+   = note: ...but it actually implements `FnOnce<(&'2 (),)>`, for some specific lifetime `'2`
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error[E0308]: mismatched types
+  --> $DIR/missing-universe-cause-issue-114907.rs:33:5
+   |
+LL |     accept(callback);
+   |     ^^^^^^^^^^^^^^^^ one type is more general than the other
+   |
+   = note: expected trait `for<'a> FnOnce(&'a ())`
+              found trait `FnOnce(&())`
+note: this closure does not fulfill the lifetime requirements
+  --> $DIR/missing-universe-cause-issue-114907.rs:32:20
+   |
+LL |     let callback = |_| {};
+   |                    ^^^
+note: the lifetime requirement is introduced here
+  --> $DIR/missing-universe-cause-issue-114907.rs:20:21
+   |
+LL | struct Handshake<R: Role> {
+   |                     ^^^^
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+help: consider specifying the type of the closure parameters
+   |
+LL |     let callback = |_: &_| {};
+   |                    ~~~~~~~
+
 error: higher-ranked subtype error
   --> $DIR/missing-universe-cause-issue-114907.rs:33:21
    |
@@ -77,6 +111,6 @@ LL |     accept(callback);
    |
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
-error: aborting due to 6 previous errors
+error: aborting due to 8 previous errors
 
 For more information about this error, try `rustc --explain E0308`.