diagnostics: avoid mismatch between variance index and hir generic

This happens because variances are constructed from ty generics,
and ty generics are always constructed with lifetimes first.

See compiler/rustc_hir_analysis/src/collect/generics_of.rs:248-269

Fixes #83556

Author: notriddle

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -1755,20 +1755,41 @@ fn check_variances_for_type_defn<'tcx>(
             .collect::<FxHashSet<_>>()
     });
 
+    let ty_generics = tcx.generics_of(item.owner_id);
+
     for (index, _) in variances.iter().enumerate() {
         let parameter = Parameter(index as u32);
 
         if constrained_parameters.contains(&parameter) {
             continue;
         }
 
-        let param = &hir_generics.params[index];
+        let ty_param = &ty_generics.params[index];
+        let mut hir_param = &hir_generics.params[index];
+
+        if ty_param.name != hir_param.name.ident().name {
+            // valid programs always have lifetimes before types in the generic parameter list
+            // ty_generics are normalized to be in this required order, and variances are built
+            // from ty generics, not from hir generics. but we need hir generics to get
+            // a span out
+            //
+            // if they aren't in the same order, then the user has written invalid code, and already
+            // got an error about it (or I'm wrong about this)
+            tcx.sess
+                .delay_span_bug(hir_param.span, "hir generics and ty generics in different order");
+            for hp in hir_generics.params {
+                if hp.name.ident().name == ty_param.name {
+                    hir_param = hp;
+                    break;
+                }
+            }
+        }
 
-        match param.name {
+        match hir_param.name {
             hir::ParamName::Error => {}
             _ => {
                 let has_explicit_bounds = explicitly_bounded_params.contains(&parameter);
-                report_bivariance(tcx, param, has_explicit_bounds);
+                report_bivariance(tcx, hir_param, has_explicit_bounds);
             }
         }
     }

tests/ui/generics/issue-83556.rs

@@ -0,0 +1,5 @@
+struct Foo<T, 'a>(&'a ());
+//~^ ERROR lifetime parameters must be declared prior to
+//~| ERROR parameter `T` is never used
+
+fn main() {}

tests/ui/generics/issue-83556.stderr

@@ -0,0 +1,18 @@
+error: lifetime parameters must be declared prior to type and const parameters
+  --> $DIR/issue-83556.rs:1:15
+   |
+LL | struct Foo<T, 'a>(&'a ());
+   |           ----^^- help: reorder the parameters: lifetimes, then consts and types: `<'a, T>`
+
+error[E0392]: parameter `T` is never used
+  --> $DIR/issue-83556.rs:1:12
+   |
+LL | struct Foo<T, 'a>(&'a ());
+   |            ^ unused parameter
+   |
+   = help: consider removing `T`, referring to it in a field, or using a marker such as `PhantomData`
+   = help: if you intended `T` to be a const parameter, use `const T: usize` instead
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0392`.