Only assert for unstable expectation ids after conversion (RFC 2383)

This ICE was reported by `@matthiaskrgr`. A big THANK YOU to him. See `rust#94953`

compiler/rustc_errors/src/lib.rs

@@ -426,6 +426,13 @@ struct HandlerInner {
 
     future_breakage_diagnostics: Vec<Diagnostic>,
 
+    /// The [`unstable_expect_diagnostics`] should be empty when this struct is
+    /// dropped. However, it can have values if the compilation is stopped early
+    /// or is only partially executed. To avoid ICEs, like in rust#94953 we only
+    /// check if [`unstable_expect_diagnostics`] is empty, if the expectation ids
+    /// have been converted.
+    check_unstable_expect_diagnostics: bool,
+
     /// Expected [`Diagnostic`]s store a [`LintExpectationId`] as part of
     /// the lint level. [`LintExpectationId`]s created early during the compilation
     /// (before `HirId`s have been defined) are not stable and can therefore not be
@@ -497,10 +504,12 @@ impl Drop for HandlerInner {
             );
         }
 
-        assert!(
-            self.unstable_expect_diagnostics.is_empty(),
-            "all diagnostics with unstable expectations should have been converted",
-        );
+        if self.check_unstable_expect_diagnostics {
+            assert!(
+                self.unstable_expect_diagnostics.is_empty(),
+                "all diagnostics with unstable expectations should have been converted",
+            );
+        }
     }
 }
 
@@ -574,6 +583,7 @@ impl Handler {
                 emitted_diagnostics: Default::default(),
                 stashed_diagnostics: Default::default(),
                 future_breakage_diagnostics: Vec::new(),
+                check_unstable_expect_diagnostics: false,
                 unstable_expect_diagnostics: Vec::new(),
                 fulfilled_expectations: Default::default(),
             }),
@@ -988,12 +998,13 @@ impl Handler {
         &self,
         unstable_to_stable: &FxHashMap<LintExpectationId, LintExpectationId>,
     ) {
-        let diags = std::mem::take(&mut self.inner.borrow_mut().unstable_expect_diagnostics);
+        let mut inner = self.inner.borrow_mut();
+        let diags = std::mem::take(&mut inner.unstable_expect_diagnostics);
+        inner.check_unstable_expect_diagnostics = true;
         if diags.is_empty() {
             return;
         }
 
-        let mut inner = self.inner.borrow_mut();
         for mut diag in diags.into_iter() {
             diag.update_unstable_expectation_id(unstable_to_stable);
 

src/test/ui/lint/rfc-2383-lint-reason/no_ice_for_partial_compiler_runs.rs

@@ -0,0 +1,16 @@
+// This ensures that ICEs like rust#94953 don't happen
+// check-pass
+// compile-flags: -Z unpretty=expanded
+
+#![feature(lint_reasons)]
+
+// This `expect` will create an expectation with an unstable expectation id
+#[expect(while_true)]
+fn create_early_lint_pass_expectation() {
+    // `while_true` is an early lint
+    while true {}
+}
+
+fn main() {
+    create_early_lint_pass_expectation();
+}

src/test/ui/lint/rfc-2383-lint-reason/no_ice_for_partial_compiler_runs.stdout

@@ -0,0 +1,20 @@
+#![feature(prelude_import)]
+#![no_std]
+// This ensures that ICEs like rust#94953 don't happen
+// check-pass
+// compile-flags: -Z unpretty=expanded
+
+#![feature(lint_reasons)]
+#[prelude_import]
+use ::std::prelude::rust_2015::*;
+#[macro_use]
+extern crate std;
+
+// This `expect` will create an expectation with an unstable expectation id
+#[expect(while_true)]
+fn create_early_lint_pass_expectation() {
+    // `while_true` is an early lint
+    while true {}
+}
+
+fn main() { create_early_lint_pass_expectation(); }