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.

Author: bors

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}`")
                     }
                 }
             }

compiler/rustc_session/src/options.rs

@@ -20,7 +20,7 @@ use std::collections::BTreeMap;
 
 use std::collections::hash_map::DefaultHasher;
 use std::hash::Hasher;
-use std::num::NonZeroUsize;
+use std::num::{IntErrorKind, NonZeroUsize};
 use std::path::PathBuf;
 use std::str;
 
@@ -387,7 +387,7 @@ mod desc {
         "`all` (default), `except-unused-generics`, `except-unused-functions`, or `off`";
     pub const parse_instrument_xray: &str = "either a boolean (`yes`, `no`, `on`, `off`, etc), or a comma separated list of settings: `always` or `never` (mutually exclusive), `ignore-loops`, `instruction-threshold=N`, `skip-entry`, `skip-exit`";
     pub const parse_unpretty: &str = "`string` or `string=string`";
-    pub const parse_treat_err_as_bug: &str = "either no value or a number bigger than 0";
+    pub const parse_treat_err_as_bug: &str = "either no value or a non-negative number";
     pub const parse_trait_solver: &str =
         "one of the supported solver modes (`classic`, `next`, or `next-coherence`)";
     pub const parse_lto: &str =
@@ -986,10 +986,16 @@ mod parse {
 
     pub(crate) fn parse_treat_err_as_bug(slot: &mut Option<NonZeroUsize>, v: Option<&str>) -> bool {
         match v {
-            Some(s) => {
-                *slot = s.parse().ok();
-                slot.is_some()
-            }
+            Some(s) => match s.parse() {
+                Ok(val) => {
+                    *slot = Some(val);
+                    true
+                }
+                Err(e) => {
+                    *slot = None;
+                    e.kind() == &IntErrorKind::Zero
+                }
+            },
             None => {
                 *slot = NonZeroUsize::new(1);
                 true
@@ -1846,7 +1852,8 @@ written to standard error output)"),
     trap_unreachable: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "generate trap instructions for unreachable intrinsics (default: use target setting, usually yes)"),
     treat_err_as_bug: Option<NonZeroUsize> = (None, parse_treat_err_as_bug, [TRACKED],
-        "treat error number `val` that occurs as bug"),
+        "treat the `val`th error that occurs as bug (default if not specified: 0 - don't treat errors as bugs. \
+        default if specified without a value: 1 - treat the first error as bug)"),
     trim_diagnostic_paths: bool = (true, parse_bool, [UNTRACKED],
         "in diagnostics, use heuristics to shorten paths referring to items"),
     tune_cpu: Option<String> = (None, parse_opt_string, [TRACKED],