Auto merge of #115690 - ShE3py:Z-treat-err-as-bug, r=petrochenkov

Allow `-Z treat-err-as-bug=0`

Makes `-Z treat-err-as-bug=0` behave as if the option wasn't present instead of asking the value to be ⩾ 1. This enables a quick on/off of the option, as you only need to change one character instead of removing the whole `-Z`.

Also update some text, e.g.
```bash
$ rustc -Z help | grep treat-err-as-bug
    -Z                      treat-err-as-bug=val -- treat error number `val` that occurs as bug
```
where the value could be interpreted as an error code instead of an ordinal.

1 files changed, 5 insertions, 7 deletions

diff --git a/compiler/rustc_errors/src/lib.rs b/compiler/rustc_errors/src/lib.rs
index 990bd2d1cc9..b747a62b864 100644
--- a/compiler/rustc_errors/src/lib.rs
+++ b/compiler/rustc_errors/src/lib.rs
@@ -519,7 +519,7 @@ pub struct HandlerFlags {
     /// If false, warning-level lints are suppressed.
     /// (rustc: see `--allow warnings` and `--cap-lints`)
     pub can_emit_warnings: bool,
-    /// If true, error-level diagnostics are upgraded to bug-level.
+    /// If Some, the Nth error-level diagnostic is upgraded to bug-level.
     /// (rustc: see `-Z treat-err-as-bug`)
     pub treat_err_as_bug: Option<NonZeroUsize>,
     /// If true, immediately emit diagnostics that would otherwise be buffered.
@@ -1719,19 +1719,17 @@ impl HandlerInner {
             match (
                 self.err_count() + self.lint_err_count,
                 self.delayed_bug_count(),
-                self.flags.treat_err_as_bug.map(|c| c.get()).unwrap_or(0),
+                self.flags.treat_err_as_bug.map(|c| c.get()).unwrap(),
             ) {
                 (1, 0, 1) => panic!("aborting due to `-Z treat-err-as-bug=1`"),
                 (0, 1, 1) => panic!("aborting due delayed bug with `-Z treat-err-as-bug=1`"),
-                (count, delayed_count, as_bug) => {
+                (count, delayed_count, val) => {
                     if delayed_count > 0 {
                         panic!(
-                            "aborting after {count} errors and {delayed_count} delayed bugs due to `-Z treat-err-as-bug={as_bug}`",
+                            "aborting after {count} errors and {delayed_count} delayed bugs due to `-Z treat-err-as-bug={val}`",
                         )
                     } else {
-                        panic!(
-                            "aborting after {count} errors due to `-Z treat-err-as-bug={as_bug}`",
-                        )
+                        panic!("aborting after {count} errors due to `-Z treat-err-as-bug={val}`")
                     }
                 }
             }