Print the generic parameter along with the variance in dumps.

compiler/rustc_hir_analysis/src/variance/dump.rs

@@ -1,18 +1,36 @@
+use std::fmt::Write;
+
 use rustc_hir::def::DefKind;
-use rustc_hir::def_id::CRATE_DEF_ID;
-use rustc_middle::ty::TyCtxt;
+use rustc_hir::def_id::{LocalDefId, CRATE_DEF_ID};
+use rustc_middle::ty::{GenericArgs, TyCtxt};
 use rustc_span::symbol::sym;
 
+fn format_variances(tcx: TyCtxt<'_>, def_id: LocalDefId) -> String {
+    let variances = tcx.variances_of(def_id);
+    let generics = GenericArgs::identity_for_item(tcx, def_id);
+    // 7 = 2-letter parameter + ": " + 1-letter variance + ", "
+    let mut ret = String::with_capacity(2 + 7 * variances.len());
+    ret.push('[');
+    for (arg, variance) in generics.iter().zip(variances.iter()) {
+        write!(ret, "{arg}: {variance:?}, ").unwrap();
+    }
+    // Remove trailing `, `.
+    if !variances.is_empty() {
+        ret.pop();
+        ret.pop();
+    }
+    ret.push(']');
+    ret
+}
+
 pub(crate) fn variances(tcx: TyCtxt<'_>) {
     if tcx.has_attr(CRATE_DEF_ID, sym::rustc_variance_of_opaques) {
         for id in tcx.hir().items() {
             let DefKind::OpaqueTy = tcx.def_kind(id.owner_id) else { continue };
 
-            let variances = tcx.variances_of(id.owner_id);
-
             tcx.dcx().emit_err(crate::errors::VariancesOf {
                 span: tcx.def_span(id.owner_id),
-                variances: format!("{variances:?}"),
+                variances: format_variances(tcx, id.owner_id.def_id),
             });
         }
     }
@@ -22,11 +40,9 @@ pub(crate) fn variances(tcx: TyCtxt<'_>) {
             continue;
         }
 
-        let variances = tcx.variances_of(id.owner_id);
-
         tcx.dcx().emit_err(crate::errors::VariancesOf {
             span: tcx.def_span(id.owner_id),
-            variances: format!("{variances:?}"),
+            variances: format_variances(tcx, id.owner_id.def_id),
         });
     }
 }

tests/ui/error-codes/E0208.rs

@@ -1,7 +1,7 @@
 #![feature(rustc_attrs)]
 
 #[rustc_variance]
-struct Foo<'a, T> { //~ ERROR [+, o]
+struct Foo<'a, T> { //~ ERROR ['a: +, T: o]
     t: &'a mut T,
 }
 

tests/ui/error-codes/E0208.stderr

@@ -1,4 +1,4 @@
-error: [+, o]
+error: ['a: +, T: o]
   --> $DIR/E0208.rs:4:1
    |
 LL | struct Foo<'a, T> {

tests/ui/impl-trait/capture-lifetime-not-in-hir.rs

@@ -6,13 +6,13 @@ trait Bar<'a> {
 }
 
 fn foo<'a, T: Bar<'a>>() -> impl Into<T::Assoc> {
-    //~^ ERROR [o, o]
+    //~^ ERROR ['a: o, T: o]
     // captures both T and 'a invariantly
     ()
 }
 
 fn foo2<'a, T: Bar<'a>>() -> impl Into<T::Assoc> + 'a {
-    //~^ ERROR [o, o, o]
+    //~^ ERROR ['a: o, T: o, 'a: o]
     // captures both T and 'a invariantly, and also duplicates `'a`
     // i.e. the opaque looks like `impl Into<<T as Bar<'a>>::Assoc> + 'a_duplicated`
     ()

tests/ui/impl-trait/capture-lifetime-not-in-hir.stderr

@@ -1,10 +1,10 @@
-error: [o, o]
+error: ['a: o, T: o]
   --> $DIR/capture-lifetime-not-in-hir.rs:8:29
    |
 LL | fn foo<'a, T: Bar<'a>>() -> impl Into<T::Assoc> {
    |                             ^^^^^^^^^^^^^^^^^^^
 
-error: [o, o, o]
+error: ['a: o, T: o, 'a: o]
   --> $DIR/capture-lifetime-not-in-hir.rs:14:30
    |
 LL | fn foo2<'a, T: Bar<'a>>() -> impl Into<T::Assoc> + 'a {

tests/ui/impl-trait/implicit-capture-late.stderr

@@ -10,7 +10,7 @@ note: lifetime declared here
 LL | fn foo(x: Vec<i32>) -> Box<dyn for<'a> Deref<Target = impl ?Sized>> {
    |                                    ^^
 
-error: [o]
+error: ['a: o]
   --> $DIR/implicit-capture-late.rs:10:55
    |
 LL | fn foo(x: Vec<i32>) -> Box<dyn for<'a> Deref<Target = impl ?Sized>> {

tests/ui/impl-trait/in-trait/variance.rs

@@ -7,14 +7,16 @@ impl<T> Captures<'_> for T {}
 
 trait Foo<'i> {
     fn implicit_capture_early<'a: 'a>() -> impl Sized {}
-    //~^ [o, *, *, o, o]
-    // Self, 'i, 'a, 'i_duplicated, 'a_duplicated
+    //~^ [Self: o, 'i: *, 'a: *, 'a: o, 'i: o]
 
-    fn explicit_capture_early<'a: 'a>() -> impl Sized + Captures<'a> {} //~ [o, *, *, o, o]
+    fn explicit_capture_early<'a: 'a>() -> impl Sized + Captures<'a> {}
+    //~^ [Self: o, 'i: *, 'a: *, 'a: o, 'i: o]
 
-    fn implicit_capture_late<'a>(_: &'a ()) -> impl Sized {} //~ [o, *, o, o]
+    fn implicit_capture_late<'a>(_: &'a ()) -> impl Sized {}
+    //~^ [Self: o, 'i: *, 'a: o, 'i: o]
 
-    fn explicit_capture_late<'a>(_: &'a ()) -> impl Sized + Captures<'a> {} //~ [o, *, o, o]
+    fn explicit_capture_late<'a>(_: &'a ()) -> impl Sized + Captures<'a> {}
+    //~^ [Self: o, 'i: *, 'a: o, 'i: o]
 }
 
 fn main() {}

tests/ui/impl-trait/in-trait/variance.stderr

@@ -1,23 +1,23 @@
-error: [o, *, *, o, o]
+error: [Self: o, 'i: *, 'a: *, 'a: o, 'i: o]
   --> $DIR/variance.rs:9:44
    |
 LL |     fn implicit_capture_early<'a: 'a>() -> impl Sized {}
    |                                            ^^^^^^^^^^
 
-error: [o, *, *, o, o]
-  --> $DIR/variance.rs:13:44
+error: [Self: o, 'i: *, 'a: *, 'a: o, 'i: o]
+  --> $DIR/variance.rs:12:44
    |
 LL |     fn explicit_capture_early<'a: 'a>() -> impl Sized + Captures<'a> {}
    |                                            ^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [o, *, o, o]
+error: [Self: o, 'i: *, 'a: o, 'i: o]
   --> $DIR/variance.rs:15:48
    |
 LL |     fn implicit_capture_late<'a>(_: &'a ()) -> impl Sized {}
    |                                                ^^^^^^^^^^
 
-error: [o, *, o, o]
-  --> $DIR/variance.rs:17:48
+error: [Self: o, 'i: *, 'a: o, 'i: o]
+  --> $DIR/variance.rs:18:48
    |
 LL |     fn explicit_capture_late<'a>(_: &'a ()) -> impl Sized + Captures<'a> {}
    |                                                ^^^^^^^^^^^^^^^^^^^^^^^^^

tests/ui/impl-trait/variance.e2024.stderr

@@ -1,22 +1,22 @@
-error: [*, o]
+error: ['a: *, 'a: o]
   --> $DIR/variance.rs:14:36
    |
 LL | fn not_captured_early<'a: 'a>() -> impl Sized {}
    |                                    ^^^^^^^^^^
 
-error: [*, o]
+error: ['a: *, 'a: o]
   --> $DIR/variance.rs:19:32
    |
 LL | fn captured_early<'a: 'a>() -> impl Sized + Captures<'a> {}
    |                                ^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [o]
+error: ['a: o]
   --> $DIR/variance.rs:21:40
    |
 LL | fn not_captured_late<'a>(_: &'a ()) -> impl Sized {}
    |                                        ^^^^^^^^^^
 
-error: [o]
+error: ['a: o]
   --> $DIR/variance.rs:26:36
    |
 LL | fn captured_late<'a>(_: &'a ()) -> impl Sized + Captures<'a> {}

tests/ui/impl-trait/variance.new.stderr

@@ -1,22 +1,22 @@
-error: [*, o]
+error: ['a: *, 'a: o]
   --> $DIR/variance.rs:14:36
    |
 LL | fn not_captured_early<'a: 'a>() -> impl Sized {}
    |                                    ^^^^^^^^^^
 
-error: [*, o]
+error: ['a: *, 'a: o]
   --> $DIR/variance.rs:19:32
    |
 LL | fn captured_early<'a: 'a>() -> impl Sized + Captures<'a> {}
    |                                ^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [o]
+error: ['a: o]
   --> $DIR/variance.rs:21:40
    |
 LL | fn not_captured_late<'a>(_: &'a ()) -> impl Sized {}
    |                                        ^^^^^^^^^^
 
-error: [o]
+error: ['a: o]
   --> $DIR/variance.rs:26:36
    |
 LL | fn captured_late<'a>(_: &'a ()) -> impl Sized + Captures<'a> {}

tests/ui/impl-trait/variance.old.stderr

@@ -1,10 +1,10 @@
-error: [*]
+error: ['a: *]
   --> $DIR/variance.rs:14:36
    |
 LL | fn not_captured_early<'a: 'a>() -> impl Sized {}
    |                                    ^^^^^^^^^^
 
-error: [*, o]
+error: ['a: *, 'a: o]
   --> $DIR/variance.rs:19:32
    |
 LL | fn captured_early<'a: 'a>() -> impl Sized + Captures<'a> {}
@@ -16,7 +16,7 @@ error: []
 LL | fn not_captured_late<'a>(_: &'a ()) -> impl Sized {}
    |                                        ^^^^^^^^^^
 
-error: [o]
+error: ['a: o]
   --> $DIR/variance.rs:26:36
    |
 LL | fn captured_late<'a>(_: &'a ()) -> impl Sized + Captures<'a> {}

tests/ui/impl-trait/variance.rs

@@ -12,17 +12,17 @@ trait Captures<'a> {}
 impl<T> Captures<'_> for T {}
 
 fn not_captured_early<'a: 'a>() -> impl Sized {}
-//[old]~^ [*]
-//[new]~^^ [*, o]
-//[e2024]~^^^ [*, o]
+//[old]~^ ['a: *]
+//[new]~^^ ['a: *, 'a: o]
+//[e2024]~^^^ ['a: *, 'a: o]
 
-fn captured_early<'a: 'a>() -> impl Sized + Captures<'a> {} //~ [*, o]
+fn captured_early<'a: 'a>() -> impl Sized + Captures<'a> {} //~ ['a: *, 'a: o]
 
 fn not_captured_late<'a>(_: &'a ()) -> impl Sized {}
 //[old]~^ []
-//[new]~^^ [o]
-//[e2024]~^^^ [o]
+//[new]~^^ ['a: o]
+//[e2024]~^^^ ['a: o]
 
-fn captured_late<'a>(_: &'a ()) -> impl Sized + Captures<'a> {} //~ [o]
+fn captured_late<'a>(_: &'a ()) -> impl Sized + Captures<'a> {} //~ ['a: o]
 
 fn main() {}

tests/ui/type-alias-impl-trait/variance.rs

@@ -5,21 +5,21 @@
 trait Captures<'a> {}
 impl<T> Captures<'_> for T {}
 
-type NotCapturedEarly<'a> = impl Sized; //~ [*, o]
+type NotCapturedEarly<'a> = impl Sized; //~ ['a: *, 'a: o]
 //~^ ERROR: unconstrained opaque type
 
-type CapturedEarly<'a> = impl Sized + Captures<'a>; //~ [*, o]
+type CapturedEarly<'a> = impl Sized + Captures<'a>; //~ ['a: *, 'a: o]
 //~^ ERROR: unconstrained opaque type
 
-type NotCapturedLate<'a> = dyn for<'b> Iterator<Item = impl Sized>; //~ [*, o, o]
+type NotCapturedLate<'a> = dyn for<'b> Iterator<Item = impl Sized>; //~ ['a: *, 'b: o, 'a: o]
 //~^ ERROR `impl Trait` cannot capture higher-ranked lifetime from `dyn` type
 //~| ERROR: unconstrained opaque type
 
-type Captured<'a> = dyn for<'b> Iterator<Item = impl Sized + Captures<'a>>; //~ [*, o, o]
+type Captured<'a> = dyn for<'b> Iterator<Item = impl Sized + Captures<'a>>; //~ ['a: *, 'b: o, 'a: o]
 //~^ ERROR `impl Trait` cannot capture higher-ranked lifetime from `dyn` type
 //~| ERROR: unconstrained opaque type
 
-type Bar<'a, 'b: 'b, T> = impl Sized; //~ ERROR [*, *, o, o, o]
+type Bar<'a, 'b: 'b, T> = impl Sized; //~ ERROR ['a: *, 'b: *, T: o, 'a: o, 'b: o]
 //~^ ERROR: unconstrained opaque type
 
 trait Foo<'i> {
@@ -31,24 +31,24 @@ trait Foo<'i> {
 }
 
 impl<'i> Foo<'i> for &'i () {
-    type ImplicitCapture<'a> = impl Sized; //~ [*, *, o, o]
+    type ImplicitCapture<'a> = impl Sized; //~ ['i: *, 'a: *, 'a: o, 'i: o]
     //~^ ERROR: unconstrained opaque type
 
-    type ExplicitCaptureFromHeader<'a> = impl Sized + Captures<'i>; //~ [*, *, o, o]
+    type ExplicitCaptureFromHeader<'a> = impl Sized + Captures<'i>; //~ ['i: *, 'a: *, 'a: o, 'i: o]
     //~^ ERROR: unconstrained opaque type
 
-    type ExplicitCaptureFromGat<'a> = impl Sized + Captures<'a>; //~ [*, *, o, o]
+    type ExplicitCaptureFromGat<'a> = impl Sized + Captures<'a>; //~ ['i: *, 'a: *, 'a: o, 'i: o]
     //~^ ERROR: unconstrained opaque type
 }
 
 impl<'i> Foo<'i> for () {
-    type ImplicitCapture<'a> = impl Sized; //~ [*, *, o, o]
+    type ImplicitCapture<'a> = impl Sized; //~ ['i: *, 'a: *, 'a: o, 'i: o]
     //~^ ERROR: unconstrained opaque type
 
-    type ExplicitCaptureFromHeader<'a> = impl Sized + Captures<'i>; //~ [*, *, o, o]
+    type ExplicitCaptureFromHeader<'a> = impl Sized + Captures<'i>; //~ ['i: *, 'a: *, 'a: o, 'i: o]
     //~^ ERROR: unconstrained opaque type
 
-    type ExplicitCaptureFromGat<'a> = impl Sized + Captures<'a>; //~ [*, *, o, o]
+    type ExplicitCaptureFromGat<'a> = impl Sized + Captures<'a>; //~ ['i: *, 'a: *, 'a: o, 'i: o]
     //~^ ERROR: unconstrained opaque type
 }
 
@@ -59,15 +59,15 @@ impl<'a> Nesting<'a> for &'a () {
     type Output = &'a ();
 }
 type NestedDeeply<'a> =
-    impl Nesting< //~ [*, o]
+    impl Nesting< //~ ['a: *, 'a: o]
         'a,
-        Output = impl Nesting< //~ [*, o]
+        Output = impl Nesting< //~ ['a: *, 'a: o]
             'a,
-            Output = impl Nesting< //~ [*, o]
+            Output = impl Nesting< //~ ['a: *, 'a: o]
                 'a,
-                Output = impl Nesting< //~ [*, o]
+                Output = impl Nesting< //~ ['a: *, 'a: o]
                     'a,
-                    Output = impl Nesting<'a> //~ [*, o]
+                    Output = impl Nesting<'a> //~ ['a: *, 'a: o]
                 >
             >,
         >,

tests/ui/type-alias-impl-trait/variance.stderr

@@ -110,73 +110,73 @@ LL |     type ExplicitCaptureFromGat<'a> = impl Sized + Captures<'a>;
    |
    = note: `ExplicitCaptureFromGat` must be used in combination with a concrete type within the same impl
 
-error: [*, o]
+error: ['a: *, 'a: o]
   --> $DIR/variance.rs:8:29
    |
 LL | type NotCapturedEarly<'a> = impl Sized;
    |                             ^^^^^^^^^^
 
-error: [*, o]
+error: ['a: *, 'a: o]
   --> $DIR/variance.rs:11:26
    |
 LL | type CapturedEarly<'a> = impl Sized + Captures<'a>;
    |                          ^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [*, o, o]
+error: ['a: *, 'b: o, 'a: o]
   --> $DIR/variance.rs:14:56
    |
 LL | type NotCapturedLate<'a> = dyn for<'b> Iterator<Item = impl Sized>;
    |                                                        ^^^^^^^^^^
 
-error: [*, o, o]
+error: ['a: *, 'b: o, 'a: o]
   --> $DIR/variance.rs:18:49
    |
 LL | type Captured<'a> = dyn for<'b> Iterator<Item = impl Sized + Captures<'a>>;
    |                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [*, *, o, o, o]
+error: ['a: *, 'b: *, T: o, 'a: o, 'b: o]
   --> $DIR/variance.rs:22:27
    |
 LL | type Bar<'a, 'b: 'b, T> = impl Sized;
    |                           ^^^^^^^^^^
 
-error: [*, *, o, o]
+error: ['i: *, 'a: *, 'a: o, 'i: o]
   --> $DIR/variance.rs:34:32
    |
 LL |     type ImplicitCapture<'a> = impl Sized;
    |                                ^^^^^^^^^^
 
-error: [*, *, o, o]
+error: ['i: *, 'a: *, 'a: o, 'i: o]
   --> $DIR/variance.rs:37:42
    |
 LL |     type ExplicitCaptureFromHeader<'a> = impl Sized + Captures<'i>;
    |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [*, *, o, o]
+error: ['i: *, 'a: *, 'a: o, 'i: o]
   --> $DIR/variance.rs:40:39
    |
 LL |     type ExplicitCaptureFromGat<'a> = impl Sized + Captures<'a>;
    |                                       ^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [*, *, o, o]
+error: ['i: *, 'a: *, 'a: o, 'i: o]
   --> $DIR/variance.rs:45:32
    |
 LL |     type ImplicitCapture<'a> = impl Sized;
    |                                ^^^^^^^^^^
 
-error: [*, *, o, o]
+error: ['i: *, 'a: *, 'a: o, 'i: o]
   --> $DIR/variance.rs:48:42
    |
 LL |     type ExplicitCaptureFromHeader<'a> = impl Sized + Captures<'i>;
    |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [*, *, o, o]
+error: ['i: *, 'a: *, 'a: o, 'i: o]
   --> $DIR/variance.rs:51:39
    |
 LL |     type ExplicitCaptureFromGat<'a> = impl Sized + Captures<'a>;
    |                                       ^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [*, o]
+error: ['a: *, 'a: o]
   --> $DIR/variance.rs:62:5
    |
 LL | /     impl Nesting<
@@ -188,7 +188,7 @@ LL | |         >,
 LL | |     >;
    | |_____^
 
-error: [*, o]
+error: ['a: *, 'a: o]
   --> $DIR/variance.rs:64:18
    |
 LL |           Output = impl Nesting<
@@ -201,7 +201,7 @@ LL | |             >,
 LL | |         >,
    | |_________^
 
-error: [*, o]
+error: ['a: *, 'a: o]
   --> $DIR/variance.rs:66:22
    |
 LL |               Output = impl Nesting<
@@ -214,7 +214,7 @@ LL | |                 >
 LL | |             >,
    | |_____________^
 
-error: [*, o]
+error: ['a: *, 'a: o]
   --> $DIR/variance.rs:68:26
    |
 LL |                   Output = impl Nesting<
@@ -224,7 +224,7 @@ LL | |                     Output = impl Nesting<'a>
 LL | |                 >
    | |_________________^
 
-error: [*, o]
+error: ['a: *, 'a: o]
   --> $DIR/variance.rs:70:30
    |
 LL |                     Output = impl Nesting<'a>

tests/ui/variance/variance-associated-consts.rs

@@ -10,7 +10,7 @@ trait Trait {
 }
 
 #[rustc_variance]
-struct Foo<T: Trait> { //~ ERROR [o]
+struct Foo<T: Trait> { //~ ERROR [T: o]
     field: [u8; <T as Trait>::Const]
     //~^ ERROR: unconstrained generic constant
 }

tests/ui/variance/variance-associated-consts.stderr

@@ -9,7 +9,7 @@ help: try adding a `where` bound
 LL | struct Foo<T: Trait> where [(); <T as Trait>::Const]: {
    |                      ++++++++++++++++++++++++++++++++
 
-error: [o]
+error: [T: o]
   --> $DIR/variance-associated-consts.rs:13:1
    |
 LL | struct Foo<T: Trait> {

tests/ui/variance/variance-associated-types.rs

@@ -10,12 +10,12 @@ fn method(&'a self) { }
 }
 
 #[rustc_variance]
-struct Foo<'a, T : Trait<'a>> { //~ ERROR [+, +]
+struct Foo<'a, T : Trait<'a>> { //~ ERROR ['a: +, T: +]
     field: (T, &'a ())
 }
 
 #[rustc_variance]
-struct Bar<'a, T : Trait<'a>> { //~ ERROR [o, o]
+struct Bar<'a, T : Trait<'a>> { //~ ERROR ['a: o, T: o]
     field: <T as Trait<'a>>::Type
 }
 

tests/ui/variance/variance-associated-types.stderr

@@ -1,10 +1,10 @@
-error: [+, +]
+error: ['a: +, T: +]
   --> $DIR/variance-associated-types.rs:13:1
    |
 LL | struct Foo<'a, T : Trait<'a>> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [o, o]
+error: ['a: o, T: o]
   --> $DIR/variance-associated-types.rs:18:1
    |
 LL | struct Bar<'a, T : Trait<'a>> {

tests/ui/variance/variance-object-types.rs

@@ -4,7 +4,7 @@
 // For better or worse, associated types are invariant, and hence we
 // get an invariant result for `'a`.
 #[rustc_variance]
-struct Foo<'a> { //~ ERROR [o]
+struct Foo<'a> { //~ ERROR ['a: o]
     x: Box<dyn Fn(i32) -> &'a i32 + 'static>
 }
 

tests/ui/variance/variance-object-types.stderr

@@ -1,4 +1,4 @@
-error: [o]
+error: ['a: o]
   --> $DIR/variance-object-types.rs:7:1
    |
 LL | struct Foo<'a> {

tests/ui/variance/variance-regions-direct.rs

@@ -6,7 +6,7 @@
 // Regions that just appear in normal spots are contravariant:
 
 #[rustc_variance]
-struct Test2<'a, 'b, 'c> { //~ ERROR [+, +, +]
+struct Test2<'a, 'b, 'c> { //~ ERROR ['a: +, 'b: +, 'c: +]
     x: &'a isize,
     y: &'b [isize],
     c: &'c str
@@ -15,7 +15,7 @@ struct Test2<'a, 'b, 'c> { //~ ERROR [+, +, +]
 // Those same annotations in function arguments become covariant:
 
 #[rustc_variance]
-struct Test3<'a, 'b, 'c> { //~ ERROR [-, -, -]
+struct Test3<'a, 'b, 'c> { //~ ERROR ['a: -, 'b: -, 'c: -]
     x: extern "Rust" fn(&'a isize),
     y: extern "Rust" fn(&'b [isize]),
     c: extern "Rust" fn(&'c str),
@@ -24,7 +24,7 @@ struct Test3<'a, 'b, 'c> { //~ ERROR [-, -, -]
 // Mutability induces invariance:
 
 #[rustc_variance]
-struct Test4<'a, 'b:'a> { //~ ERROR [+, o]
+struct Test4<'a, 'b:'a> { //~ ERROR ['a: +, 'b: o]
     x: &'a mut &'b isize,
 }
 
@@ -32,7 +32,7 @@ struct Test4<'a, 'b:'a> { //~ ERROR [+, o]
 // contravariant context:
 
 #[rustc_variance]
-struct Test5<'a, 'b:'a> { //~ ERROR [-, o]
+struct Test5<'a, 'b:'a> { //~ ERROR ['a: -, 'b: o]
     x: extern "Rust" fn(&'a mut &'b isize),
 }
 
@@ -42,14 +42,14 @@ struct Test5<'a, 'b:'a> { //~ ERROR [-, o]
 // argument list occurs in an invariant context.
 
 #[rustc_variance]
-struct Test6<'a, 'b:'a> { //~ ERROR [+, o]
+struct Test6<'a, 'b:'a> { //~ ERROR ['a: +, 'b: o]
     x: &'a mut extern "Rust" fn(&'b isize),
 }
 
 // No uses at all is bivariant:
 
 #[rustc_variance]
-struct Test7<'a> { //~ ERROR [*]
+struct Test7<'a> { //~ ERROR ['a: *]
     //~^ ERROR: `'a` is never used
     x: isize
 }
@@ -57,7 +57,7 @@ struct Test7<'a> { //~ ERROR [*]
 // Try enums too.
 
 #[rustc_variance]
-enum Test8<'a, 'b, 'c:'b> { //~ ERROR [-, +, o]
+enum Test8<'a, 'b, 'c:'b> { //~ ERROR ['a: -, 'b: +, 'c: o]
     Test8A(extern "Rust" fn(&'a isize)),
     Test8B(&'b [isize]),
     Test8C(&'b mut &'c str),

tests/ui/variance/variance-regions-direct.stderr

@@ -6,43 +6,43 @@ LL | struct Test7<'a> {
    |
    = help: consider removing `'a`, referring to it in a field, or using a marker such as `PhantomData`
 
-error: [+, +, +]
+error: ['a: +, 'b: +, 'c: +]
   --> $DIR/variance-regions-direct.rs:9:1
    |
 LL | struct Test2<'a, 'b, 'c> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [-, -, -]
+error: ['a: -, 'b: -, 'c: -]
   --> $DIR/variance-regions-direct.rs:18:1
    |
 LL | struct Test3<'a, 'b, 'c> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [+, o]
+error: ['a: +, 'b: o]
   --> $DIR/variance-regions-direct.rs:27:1
    |
 LL | struct Test4<'a, 'b:'a> {
    | ^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [-, o]
+error: ['a: -, 'b: o]
   --> $DIR/variance-regions-direct.rs:35:1
    |
 LL | struct Test5<'a, 'b:'a> {
    | ^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [+, o]
+error: ['a: +, 'b: o]
   --> $DIR/variance-regions-direct.rs:45:1
    |
 LL | struct Test6<'a, 'b:'a> {
    | ^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [*]
+error: ['a: *]
   --> $DIR/variance-regions-direct.rs:52:1
    |
 LL | struct Test7<'a> {
    | ^^^^^^^^^^^^^^^^
 
-error: [-, +, o]
+error: ['a: -, 'b: +, 'c: o]
   --> $DIR/variance-regions-direct.rs:60:1
    |
 LL | enum Test8<'a, 'b, 'c:'b> {

tests/ui/variance/variance-regions-indirect.rs

@@ -5,7 +5,7 @@
 #![feature(rustc_attrs)]
 
 #[rustc_variance]
-enum Base<'a, 'b, 'c:'b, 'd> { //~ ERROR [-, +, o, *]
+enum Base<'a, 'b, 'c:'b, 'd> { //~ ERROR ['a: -, 'b: +, 'c: o, 'd: *]
     //~^ ERROR: `'d` is never used
     Test8A(extern "Rust" fn(&'a isize)),
     Test8B(&'b [isize]),
@@ -13,25 +13,25 @@ enum Base<'a, 'b, 'c:'b, 'd> { //~ ERROR [-, +, o, *]
 }
 
 #[rustc_variance]
-struct Derived1<'w, 'x:'y, 'y, 'z> { //~ ERROR [*, o, +, -]
+struct Derived1<'w, 'x:'y, 'y, 'z> { //~ ERROR ['w: *, 'x: o, 'y: +, 'z: -]
     //~^ ERROR: `'w` is never used
     f: Base<'z, 'y, 'x, 'w>
 }
 
 #[rustc_variance] // Combine - and + to yield o
-struct Derived2<'a, 'b:'a, 'c> { //~ ERROR [o, o, *]
+struct Derived2<'a, 'b:'a, 'c> { //~ ERROR ['a: o, 'b: o, 'c: *]
     //~^ ERROR: `'c` is never used
     f: Base<'a, 'a, 'b, 'c>
 }
 
 #[rustc_variance] // Combine + and o to yield o (just pay attention to 'a here)
-struct Derived3<'a:'b, 'b, 'c> { //~ ERROR [o, +, *]
+struct Derived3<'a:'b, 'b, 'c> { //~ ERROR ['a: o, 'b: +, 'c: *]
     //~^ ERROR: `'c` is never used
     f: Base<'a, 'b, 'a, 'c>
 }
 
 #[rustc_variance] // Combine + and * to yield + (just pay attention to 'a here)
-struct Derived4<'a, 'b, 'c:'b> { //~ ERROR [-, +, o]
+struct Derived4<'a, 'b, 'c:'b> { //~ ERROR ['a: -, 'b: +, 'c: o]
     f: Base<'a, 'b, 'c, 'a>
 }
 

tests/ui/variance/variance-regions-indirect.stderr

@@ -30,31 +30,31 @@ LL | struct Derived3<'a:'b, 'b, 'c> {
    |
    = help: consider removing `'c`, referring to it in a field, or using a marker such as `PhantomData`
 
-error: [-, +, o, *]
+error: ['a: -, 'b: +, 'c: o, 'd: *]
   --> $DIR/variance-regions-indirect.rs:8:1
    |
 LL | enum Base<'a, 'b, 'c:'b, 'd> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [*, o, +, -]
+error: ['w: *, 'x: o, 'y: +, 'z: -]
   --> $DIR/variance-regions-indirect.rs:16:1
    |
 LL | struct Derived1<'w, 'x:'y, 'y, 'z> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [o, o, *]
+error: ['a: o, 'b: o, 'c: *]
   --> $DIR/variance-regions-indirect.rs:22:1
    |
 LL | struct Derived2<'a, 'b:'a, 'c> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [o, +, *]
+error: ['a: o, 'b: +, 'c: *]
   --> $DIR/variance-regions-indirect.rs:28:1
    |
 LL | struct Derived3<'a:'b, 'b, 'c> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [-, +, o]
+error: ['a: -, 'b: +, 'c: o]
   --> $DIR/variance-regions-indirect.rs:34:1
    |
 LL | struct Derived4<'a, 'b, 'c:'b> {

tests/ui/variance/variance-trait-bounds.rs

@@ -13,24 +13,24 @@ trait Setter<T> {
 }
 
 #[rustc_variance]
-struct TestStruct<U,T:Setter<U>> { //~ ERROR [+, +]
+struct TestStruct<U,T:Setter<U>> { //~ ERROR [U: +, T: +]
     t: T, u: U
 }
 
 #[rustc_variance]
-enum TestEnum<U,T:Setter<U>> { //~ ERROR [*, +]
+enum TestEnum<U,T:Setter<U>> { //~ ERROR [U: *, T: +]
     //~^ ERROR: `U` is never used
     Foo(T)
 }
 
 #[rustc_variance]
-struct TestContraStruct<U,T:Setter<U>> { //~ ERROR [*, +]
+struct TestContraStruct<U,T:Setter<U>> { //~ ERROR [U: *, T: +]
     //~^ ERROR: `U` is never used
     t: T
 }
 
 #[rustc_variance]
-struct TestBox<U,T:Getter<U>+Setter<U>> { //~ ERROR [*, +]
+struct TestBox<U,T:Getter<U>+Setter<U>> { //~ ERROR [U: *, T: +]
     //~^ ERROR: `U` is never used
     t: T
 }

tests/ui/variance/variance-trait-bounds.stderr

@@ -25,25 +25,25 @@ LL | struct TestBox<U,T:Getter<U>+Setter<U>> {
    = help: consider removing `U`, referring to it in a field, or using a marker such as `PhantomData`
    = help: if you intended `U` to be a const parameter, use `const U: /* Type */` instead
 
-error: [+, +]
+error: [U: +, T: +]
   --> $DIR/variance-trait-bounds.rs:16:1
    |
 LL | struct TestStruct<U,T:Setter<U>> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [*, +]
+error: [U: *, T: +]
   --> $DIR/variance-trait-bounds.rs:21:1
    |
 LL | enum TestEnum<U,T:Setter<U>> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [*, +]
+error: [U: *, T: +]
   --> $DIR/variance-trait-bounds.rs:27:1
    |
 LL | struct TestContraStruct<U,T:Setter<U>> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [*, +]
+error: [U: *, T: +]
   --> $DIR/variance-trait-bounds.rs:33:1
    |
 LL | struct TestBox<U,T:Getter<U>+Setter<U>> {

tests/ui/variance/variance-trait-object-bound.rs

@@ -11,7 +11,7 @@
 trait T { fn foo(&self); }
 
 #[rustc_variance]
-struct TOption<'a> { //~ ERROR [+]
+struct TOption<'a> { //~ ERROR ['a: +]
     v: Option<Box<dyn T + 'a>>,
 }
 

tests/ui/variance/variance-trait-object-bound.stderr

@@ -1,4 +1,4 @@
-error: [+]
+error: ['a: +]
   --> $DIR/variance-trait-object-bound.rs:14:1
    |
 LL | struct TOption<'a> {

tests/ui/variance/variance-types-bounds.rs

@@ -4,24 +4,24 @@
 #![feature(rustc_attrs)]
 
 #[rustc_variance]
-struct TestImm<A, B> { //~ ERROR [+, +]
+struct TestImm<A, B> { //~ ERROR [A: +, B: +]
     x: A,
     y: B,
 }
 
 #[rustc_variance]
-struct TestMut<A, B:'static> { //~ ERROR [+, o]
+struct TestMut<A, B:'static> { //~ ERROR [A: +, B: o]
     x: A,
     y: &'static mut B,
 }
 
 #[rustc_variance]
-struct TestIndirect<A:'static, B:'static> { //~ ERROR [+, o]
+struct TestIndirect<A:'static, B:'static> { //~ ERROR [A: +, B: o]
     m: TestMut<A, B>
 }
 
 #[rustc_variance]
-struct TestIndirect2<A:'static, B:'static> { //~ ERROR [o, o]
+struct TestIndirect2<A:'static, B:'static> { //~ ERROR [A: o, B: o]
     n: TestMut<A, B>,
     m: TestMut<B, A>
 }
@@ -35,7 +35,7 @@ trait Setter<A> {
 }
 
 #[rustc_variance]
-struct TestObject<A, R> { //~ ERROR [o, o]
+struct TestObject<A, R> { //~ ERROR [A: o, R: o]
     n: Box<dyn Setter<A>+Send>,
     m: Box<dyn Getter<R>+Send>,
 }

tests/ui/variance/variance-types-bounds.stderr

@@ -1,28 +1,28 @@
-error: [+, +]
+error: [A: +, B: +]
   --> $DIR/variance-types-bounds.rs:7:1
    |
 LL | struct TestImm<A, B> {
    | ^^^^^^^^^^^^^^^^^^^^
 
-error: [+, o]
+error: [A: +, B: o]
   --> $DIR/variance-types-bounds.rs:13:1
    |
 LL | struct TestMut<A, B:'static> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [+, o]
+error: [A: +, B: o]
   --> $DIR/variance-types-bounds.rs:19:1
    |
 LL | struct TestIndirect<A:'static, B:'static> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [o, o]
+error: [A: o, B: o]
   --> $DIR/variance-types-bounds.rs:24:1
    |
 LL | struct TestIndirect2<A:'static, B:'static> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [o, o]
+error: [A: o, R: o]
   --> $DIR/variance-types-bounds.rs:38:1
    |
 LL | struct TestObject<A, R> {

tests/ui/variance/variance-types.rs

@@ -7,32 +7,32 @@
 // not considered bivariant.
 
 #[rustc_variance]
-struct InvariantMut<'a,A:'a,B:'a> { //~ ERROR [+, o, o]
+struct InvariantMut<'a,A:'a,B:'a> { //~ ERROR ['a: +, A: o, B: o]
     t: &'a mut (A,B)
 }
 
 #[rustc_variance]
-struct InvariantCell<A> { //~ ERROR [o]
+struct InvariantCell<A> { //~ ERROR [A: o]
     t: Cell<A>
 }
 
 #[rustc_variance]
-struct InvariantIndirect<A> { //~ ERROR [o]
+struct InvariantIndirect<A> { //~ ERROR [A: o]
     t: InvariantCell<A>
 }
 
 #[rustc_variance]
-struct Covariant<A> { //~ ERROR [+]
+struct Covariant<A> { //~ ERROR [A: +]
     t: A, u: fn() -> A
 }
 
 #[rustc_variance]
-struct Contravariant<A> { //~ ERROR [-]
+struct Contravariant<A> { //~ ERROR [A: -]
     t: fn(A)
 }
 
 #[rustc_variance]
-enum Enum<A,B,C> { //~ ERROR [+, -, o]
+enum Enum<A,B,C> { //~ ERROR [A: +, B: -, C: o]
     Foo(Covariant<A>),
     Bar(Contravariant<B>),
     Zed(Covariant<C>,Contravariant<C>)

tests/ui/variance/variance-types.stderr

@@ -1,34 +1,34 @@
-error: [+, o, o]
+error: ['a: +, A: o, B: o]
   --> $DIR/variance-types.rs:10:1
    |
 LL | struct InvariantMut<'a,A:'a,B:'a> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [o]
+error: [A: o]
   --> $DIR/variance-types.rs:15:1
    |
 LL | struct InvariantCell<A> {
    | ^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [o]
+error: [A: o]
   --> $DIR/variance-types.rs:20:1
    |
 LL | struct InvariantIndirect<A> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [+]
+error: [A: +]
   --> $DIR/variance-types.rs:25:1
    |
 LL | struct Covariant<A> {
    | ^^^^^^^^^^^^^^^^^^^
 
-error: [-]
+error: [A: -]
   --> $DIR/variance-types.rs:30:1
    |
 LL | struct Contravariant<A> {
    | ^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [+, -, o]
+error: [A: +, B: -, C: o]
   --> $DIR/variance-types.rs:35:1
    |
 LL | enum Enum<A,B,C> {