Use the right level with -Ztreat-err-as-bug.

Errors in `DiagCtxtInner::emit_diagnostic` are never set to
`Level::Bug`, because the condition never succeeds, because
`self.treat_err_as_bug()` is called *before* the error counts are
incremented.

This commit switches to `self.treat_next_err_as_bug()`, fixing the
problem. This changes the error message output to actually say "internal
compiler error".

Author: nnethercote

compiler/rustc_errors/src/lib.rs

@@ -1256,7 +1256,7 @@ impl DiagCtxtInner {
     }
 
     fn emit_diagnostic(&mut self, mut diagnostic: Diagnostic) -> Option<ErrorGuaranteed> {
-        if matches!(diagnostic.level, Error | Fatal) && self.treat_err_as_bug() {
+        if matches!(diagnostic.level, Error | Fatal) && self.treat_next_err_as_bug() {
             diagnostic.level = Bug;
         }
 

tests/rustdoc-ui/ice-bug-report-url.stderr

@@ -1,4 +1,4 @@
-error: expected one of `->`, `where`, or `{`, found `<eof>`
+error: internal compiler error: expected one of `->`, `where`, or `{`, found `<eof>`
   --> $DIR/ice-bug-report-url.rs:14:10
    |
 LL | fn wrong()

tests/ui/consts/const-eval/const-eval-query-stack.stderr

@@ -1,4 +1,4 @@
-error[E0080]: evaluation of constant value failed
+error: internal compiler error[E0080]: evaluation of constant value failed
   --> $DIR/const-eval-query-stack.rs:16:16
    |
 LL | const X: i32 = 1 / 0;

tests/ui/impl-trait/issues/issue-86800.stderr

@@ -4,7 +4,7 @@ error: unconstrained opaque type
 LL | type TransactionFuture<'__, O> = impl '__ + Future<Output = TransactionResult<O>>;
    |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-error[E0792]: expected generic lifetime parameter, found `'_`
+error: internal compiler error[E0792]: expected generic lifetime parameter, found `'_`
   --> $DIR/issue-86800.rs:39:5
    |
 LL | type TransactionFuture<'__, O> = impl '__ + Future<Output = TransactionResult<O>>;

tests/ui/panics/default-backtrace-ice.stderr

@@ -1,4 +1,4 @@
-error[E0425]: cannot find value `missing_ident` in this scope
+error: internal compiler error[E0425]: cannot find value `missing_ident` in this scope
   --> $DIR/default-backtrace-ice.rs:21:13
    |
 LL | fn main() { missing_ident; }

tests/ui/treat-err-as-bug/err.stderr

@@ -1,4 +1,4 @@
-error[E0080]: could not evaluate static initializer
+error: internal compiler error[E0080]: could not evaluate static initializer
   --> $DIR/err.rs:11:21
    |
 LL | pub static C: u32 = 0 - 1;