Note invariance reason for FnDef types

compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -330,14 +330,14 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                     ty::RawPtr(ty_mut) => {
                         assert_eq!(ty_mut.mutbl, rustc_hir::Mutability::Mut);
                         (
-                            format!("a mutable pointer to {}", ty_mut.ty),
+                            format!("a mutable pointer to `{}`", ty_mut.ty),
                             "mutable pointers are invariant over their type parameter".to_string(),
                         )
                     }
                     ty::Ref(_, inner_ty, mutbl) => {
                         assert_eq!(*mutbl, rustc_hir::Mutability::Mut);
                         (
-                            format!("a mutable reference to {}", inner_ty),
+                            format!("a mutable reference to `{}`", inner_ty),
                             "mutable references are invariant over their type parameter"
                                 .to_string(),
                         )
@@ -351,10 +351,21 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                         let adt_desc = adt.descr();
 
                         let desc = format!(
-                            "the type {ty}, which makes the generic argument {generic_arg} invariant"
+                            "the type `{ty}`, which makes the generic argument `{generic_arg}` invariant"
                         );
                         let note = format!(
-                            "the {adt_desc} {base_ty} is invariant over the parameter {base_generic_arg}"
+                            "the {adt_desc} `{base_ty}` is invariant over the parameter `{base_generic_arg}`"
+                        );
+                        (desc, note)
+                    }
+                    ty::FnDef(def_id, _) => {
+                        let name = self.infcx.tcx.item_name(*def_id);
+                        let identity_substs =
+                            InternalSubsts::identity_for_item(self.infcx.tcx, *def_id);
+                        let desc = format!("a function pointer to `{name}`");
+                        let note = format!(
+                            "the function `{name}` is invariant over the parameter `{}`",
+                            identity_substs[param_index as usize]
                         );
                         (desc, note)
                     }

src/test/ui/associated-types/cache/project-fn-ret-invariant.krisskross.nll.stderr

@@ -10,8 +10,8 @@ LL |     (a, b)
    |     ^^^^^^ function was supposed to return data with lifetime `'b` but it is returning data with lifetime `'a`
    |
    = help: consider adding the following bound: `'a: 'b`
-   = note: requirement occurs because of the type Type<'_>, which makes the generic argument '_ invariant
-   = note: the struct Type<'a> is invariant over the parameter 'a
+   = note: requirement occurs because of the type `Type<'_>`, which makes the generic argument `'_` invariant
+   = note: the struct `Type<'a>` is invariant over the parameter `'a`
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
@@ -26,8 +26,8 @@ LL |     (a, b)
    |     ^^^^^^ function was supposed to return data with lifetime `'a` but it is returning data with lifetime `'b`
    |
    = help: consider adding the following bound: `'b: 'a`
-   = note: requirement occurs because of the type Type<'_>, which makes the generic argument '_ invariant
-   = note: the struct Type<'a> is invariant over the parameter 'a
+   = note: requirement occurs because of the type `Type<'_>`, which makes the generic argument `'_` invariant
+   = note: the struct `Type<'a>` is invariant over the parameter `'a`
    = 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

src/test/ui/associated-types/cache/project-fn-ret-invariant.oneuse.nll.stderr

@@ -10,8 +10,8 @@ LL |     let a = bar(f, x);
    |             ^^^^^^^^^ argument requires that `'a` must outlive `'b`
    |
    = help: consider adding the following bound: `'a: 'b`
-   = note: requirement occurs because of the type Type<'_>, which makes the generic argument '_ invariant
-   = note: the struct Type<'a> is invariant over the parameter 'a
+   = note: requirement occurs because of the type `Type<'_>`, which makes the generic argument `'_` invariant
+   = note: the struct `Type<'a>` is invariant over the parameter `'a`
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
@@ -26,8 +26,8 @@ LL |     let b = bar(f, y);
    |             ^^^^^^^^^ argument requires that `'b` must outlive `'a`
    |
    = help: consider adding the following bound: `'b: 'a`
-   = note: requirement occurs because of the type Type<'_>, which makes the generic argument '_ invariant
-   = note: the struct Type<'a> is invariant over the parameter 'a
+   = note: requirement occurs because of the type `Type<'_>`, which makes the generic argument `'_` invariant
+   = note: the struct `Type<'a>` is invariant over the parameter `'a`
    = 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

src/test/ui/associated-types/cache/project-fn-ret-invariant.transmute.nll.stderr

@@ -7,8 +7,8 @@ LL | fn baz<'a, 'b>(x: Type<'a>) -> Type<'static> {
 LL |     bar(foo, x)
    |     ^^^^^^^^^^^ returning this value requires that `'a` must outlive `'static`
    |
-   = note: requirement occurs because of the type Type<'_>, which makes the generic argument '_ invariant
-   = note: the struct Type<'a> is invariant over the parameter 'a
+   = note: requirement occurs because of the type `Type<'_>`, which makes the generic argument `'_` invariant
+   = note: the struct `Type<'a>` is invariant over the parameter `'a`
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: aborting due to previous error

src/test/ui/c-variadic/variadic-ffi-4.stderr

@@ -8,8 +8,8 @@ LL | pub unsafe extern "C" fn no_escape0<'f>(_: usize, ap: ...) -> VaListImpl<'f
 LL |     ap
    |     ^^ function was supposed to return data with lifetime `'1` but it is returning data with lifetime `'f`
    |
-   = note: requirement occurs because of the type VaListImpl<'_>, which makes the generic argument '_ invariant
-   = note: the struct VaListImpl<'f> is invariant over the parameter 'f
+   = note: requirement occurs because of the type `VaListImpl<'_>`, which makes the generic argument `'_` invariant
+   = note: the struct `VaListImpl<'f>` is invariant over the parameter `'f`
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
@@ -22,8 +22,8 @@ LL | pub unsafe extern "C" fn no_escape0<'f>(_: usize, ap: ...) -> VaListImpl<'f
 LL |     ap
    |     ^^ function was supposed to return data with lifetime `'f` but it is returning data with lifetime `'1`
    |
-   = note: requirement occurs because of the type VaListImpl<'_>, which makes the generic argument '_ invariant
-   = note: the struct VaListImpl<'f> is invariant over the parameter 'f
+   = note: requirement occurs because of the type `VaListImpl<'_>`, which makes the generic argument `'_` invariant
+   = note: the struct `VaListImpl<'f>` is invariant over the parameter `'f`
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
@@ -34,8 +34,8 @@ LL | pub unsafe extern "C" fn no_escape1(_: usize, ap: ...) -> VaListImpl<'stati
 LL |     ap
    |     ^^ returning this value requires that `'1` must outlive `'static`
    |
-   = note: requirement occurs because of the type VaListImpl<'_>, which makes the generic argument '_ invariant
-   = note: the struct VaListImpl<'f> is invariant over the parameter 'f
+   = note: requirement occurs because of the type `VaListImpl<'_>`, which makes the generic argument `'_` invariant
+   = note: the struct `VaListImpl<'f>` is invariant over the parameter `'f`
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
@@ -57,8 +57,8 @@ LL | pub unsafe extern "C" fn no_escape3(_: usize, mut ap0: &mut VaListImpl, mut
 LL |     *ap0 = ap1;
    |     ^^^^ assignment requires that `'1` must outlive `'2`
    |
-   = note: requirement occurs because of the type VaListImpl<'_>, which makes the generic argument '_ invariant
-   = note: the struct VaListImpl<'f> is invariant over the parameter 'f
+   = note: requirement occurs because of the type `VaListImpl<'_>`, which makes the generic argument `'_` invariant
+   = note: the struct `VaListImpl<'f>` is invariant over the parameter `'f`
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
@@ -71,8 +71,8 @@ LL | pub unsafe extern "C" fn no_escape3(_: usize, mut ap0: &mut VaListImpl, mut
 LL |     *ap0 = ap1;
    |     ^^^^ assignment requires that `'2` must outlive `'1`
    |
-   = note: requirement occurs because of the type VaListImpl<'_>, which makes the generic argument '_ invariant
-   = note: the struct VaListImpl<'f> is invariant over the parameter 'f
+   = note: requirement occurs because of the type `VaListImpl<'_>`, which makes the generic argument `'_` invariant
+   = note: the struct `VaListImpl<'f>` is invariant over the parameter `'f`
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
@@ -85,7 +85,7 @@ LL | pub unsafe extern "C" fn no_escape4(_: usize, mut ap0: &mut VaListImpl, mut
 LL |     ap0 = &mut ap1;
    |     ^^^^^^^^^^^^^^ assignment requires that `'1` must outlive `'2`
    |
-   = note: requirement occurs because of a mutable reference to VaListImpl<'_>
+   = note: requirement occurs because of a mutable reference to `VaListImpl<'_>`
    = note: mutable references are invariant over their type parameter
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
@@ -99,7 +99,7 @@ LL | pub unsafe extern "C" fn no_escape4(_: usize, mut ap0: &mut VaListImpl, mut
 LL |     ap0 = &mut ap1;
    |     ^^^^^^^^^^^^^^ assignment requires that `'2` must outlive `'1`
    |
-   = note: requirement occurs because of a mutable reference to VaListImpl<'_>
+   = note: requirement occurs because of a mutable reference to `VaListImpl<'_>`
    = note: mutable references are invariant over their type parameter
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
@@ -127,8 +127,8 @@ LL | pub unsafe extern "C" fn no_escape5(_: usize, mut ap0: &mut VaListImpl, mut
 LL |     *ap0 = ap1.clone();
    |            ^^^^^^^^^^^ argument requires that `'1` must outlive `'2`
    |
-   = note: requirement occurs because of the type VaListImpl<'_>, which makes the generic argument '_ invariant
-   = note: the struct VaListImpl<'f> is invariant over the parameter 'f
+   = note: requirement occurs because of the type `VaListImpl<'_>`, which makes the generic argument `'_` invariant
+   = note: the struct `VaListImpl<'f>` is invariant over the parameter `'f`
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
@@ -141,8 +141,8 @@ LL | pub unsafe extern "C" fn no_escape5(_: usize, mut ap0: &mut VaListImpl, mut
 LL |     *ap0 = ap1.clone();
    |            ^^^^^^^^^^^ argument requires that `'2` must outlive `'1`
    |
-   = note: requirement occurs because of the type VaListImpl<'_>, which makes the generic argument '_ invariant
-   = note: the struct VaListImpl<'f> is invariant over the parameter 'f
+   = note: requirement occurs because of the type `VaListImpl<'_>`, which makes the generic argument `'_` invariant
+   = note: the struct `VaListImpl<'f>` is invariant over the parameter `'f`
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: aborting due to 11 previous errors

src/test/ui/hr-subtype/hr-subtype.free_inv_x_vs_free_inv_y.nll.stderr

@@ -13,8 +13,8 @@ LL | | fn(Inv<'y>)) }
    | |______________- in this macro invocation
    |
    = help: consider adding the following bound: `'x: 'y`
-   = note: requirement occurs because of the type Inv<'_>, which makes the generic argument '_ invariant
-   = note: the struct Inv<'a> is invariant over the parameter 'a
+   = note: requirement occurs because of the type `Inv<'_>`, which makes the generic argument `'_` invariant
+   = note: the struct `Inv<'a>` is invariant over the parameter `'a`
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
    = note: this error originates in the macro `check` (in Nightly builds, run with -Z macro-backtrace for more info)
 
@@ -33,8 +33,8 @@ LL | | fn(Inv<'y>)) }
    | |______________- in this macro invocation
    |
    = help: consider adding the following bound: `'x: 'y`
-   = note: requirement occurs because of the type Inv<'_>, which makes the generic argument '_ invariant
-   = note: the struct Inv<'a> is invariant over the parameter 'a
+   = note: requirement occurs because of the type `Inv<'_>`, which makes the generic argument `'_` invariant
+   = note: the struct `Inv<'a>` is invariant over the parameter `'a`
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
    = note: this error originates in the macro `check` (in Nightly builds, run with -Z macro-backtrace for more info)
 

src/test/ui/match/match-ref-mut-invariance.nll.stderr

@@ -9,7 +9,7 @@ LL |         match self.0 { ref mut x => x }
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ associated function was supposed to return data with lifetime `'b` but it is returning data with lifetime `'a`
    |
    = help: consider adding the following bound: `'a: 'b`
-   = note: requirement occurs because of a mutable reference to &i32
+   = note: requirement occurs because of a mutable reference to `&i32`
    = note: mutable references are invariant over their type parameter
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 

src/test/ui/match/match-ref-mut-let-invariance.nll.stderr

@@ -10,7 +10,7 @@ LL |         x
    |         ^ associated function was supposed to return data with lifetime `'b` but it is returning data with lifetime `'a`
    |
    = help: consider adding the following bound: `'a: 'b`
-   = note: requirement occurs because of a mutable reference to &i32
+   = note: requirement occurs because of a mutable reference to `&i32`
    = note: mutable references are invariant over their type parameter
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 

src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-no-bound.stderr

@@ -52,8 +52,8 @@ LL | |     });
    | |______`cell_a` escapes the function body here
    |        argument requires that `'a` must outlive `'static`
    |
-   = note: requirement occurs because of the type Cell<&'_#10r u32>, which makes the generic argument &'_#10r u32 invariant
-   = note: the struct Cell<T> is invariant over the parameter T
+   = note: requirement occurs because of the type `Cell<&'_#10r u32>`, which makes the generic argument `&'_#10r u32` invariant
+   = note: the struct `Cell<T>` is invariant over the parameter `T`
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: aborting due to previous error

src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-wrong-bound.stderr

@@ -52,8 +52,8 @@ LL | |     });
    | |______`cell_a` escapes the function body here
    |        argument requires that `'a` must outlive `'static`
    |
-   = note: requirement occurs because of the type Cell<&'_#11r u32>, which makes the generic argument &'_#11r u32 invariant
-   = note: the struct Cell<T> is invariant over the parameter T
+   = note: requirement occurs because of the type `Cell<&'_#11r u32>`, which makes the generic argument `&'_#11r u32` invariant
+   = note: the struct `Cell<T>` is invariant over the parameter `T`
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: aborting due to previous error

src/test/ui/nll/issue-95272.rs

@@ -0,0 +1,17 @@
+#![feature(nll)]
+
+use std::cell::Cell;
+
+fn check<'a, 'b>(x: Cell<&'a ()>, y: Cell<&'b ()>)
+where
+    'a: 'b,
+{
+}
+
+fn test<'a, 'b>(x: Cell<&'a ()>, y: Cell<&'b ()>) {
+    let f = check;
+    //~^ ERROR lifetime may not live long enough
+    f(x, y);
+}
+
+fn main() {}

src/test/ui/nll/issue-95272.stderr

@@ -0,0 +1,17 @@
+error: lifetime may not live long enough
+  --> $DIR/issue-95272.rs:12:13
+   |
+LL | fn test<'a, 'b>(x: Cell<&'a ()>, y: Cell<&'b ()>) {
+   |         --  -- lifetime `'b` defined here
+   |         |
+   |         lifetime `'a` defined here
+LL |     let f = check;
+   |             ^^^^^ assignment requires that `'a` must outlive `'b`
+   |
+   = help: consider adding the following bound: `'a: 'b`
+   = note: requirement occurs because of a function pointer to `check`
+   = note: the function `check` is invariant over the parameter `'a`
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
+
+error: aborting due to previous error
+

src/test/ui/nll/type-check-pointer-coercions.stderr

@@ -34,7 +34,7 @@ LL |     x
    |     ^ function was supposed to return data with lifetime `'a` but it is returning data with lifetime `'b`
    |
    = help: consider adding the following bound: `'b: 'a`
-   = note: requirement occurs because of a mutable pointer to &i32
+   = note: requirement occurs because of a mutable pointer to `&i32`
    = note: mutable pointers are invariant over their type parameter
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
@@ -50,7 +50,7 @@ LL |     x
    |     ^ function was supposed to return data with lifetime `'b` but it is returning data with lifetime `'a`
    |
    = help: consider adding the following bound: `'a: 'b`
-   = note: requirement occurs because of a mutable pointer to &i32
+   = note: requirement occurs because of a mutable pointer to `&i32`
    = note: mutable pointers are invariant over their type parameter
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance