Remove InferCtxt::err_count_on_creation.

It's no longer used meaningfully.

This also means `DiagCtxtHandle::err_count_excluding_lint_errs` can be
removed.

compiler/rustc_errors/src/lib.rs

@@ -925,18 +925,6 @@ pub fn emit_stashed_diagnostics(&self) -> Option<ErrorGuaranteed> {
         self.inner.borrow_mut().emit_stashed_diagnostics()
     }
 
-    /// This excludes lint errors, and delayed bugs.
-    #[inline]
-    pub fn err_count_excluding_lint_errs(&self) -> usize {
-        let inner = self.inner.borrow();
-        inner.err_guars.len()
-            + inner
-                .stashed_diagnostics
-                .values()
-                .filter(|(diag, guar)| guar.is_some() && diag.is_lint.is_none())
-                .count()
-    }
-
     /// This excludes delayed bugs.
     #[inline]
     pub fn err_count(&self) -> usize {

compiler/rustc_infer/src/infer/at.rs

@@ -82,7 +82,6 @@ pub fn fork_with_intercrate(&self, intercrate: bool) -> Self {
             reported_trait_errors: self.reported_trait_errors.clone(),
             reported_signature_mismatch: self.reported_signature_mismatch.clone(),
             tainted_by_errors: self.tainted_by_errors.clone(),
-            err_count_on_creation: self.err_count_on_creation,
             universe: self.universe.clone(),
             intercrate,
             next_trait_solver: self.next_trait_solver,

compiler/rustc_infer/src/infer/mod.rs

@@ -279,27 +279,14 @@ pub struct InferCtxt<'tcx> {
     pub reported_signature_mismatch: RefCell<FxHashSet<(Span, Option<Span>)>>,
 
     /// When an error occurs, we want to avoid reporting "derived"
-    /// errors that are due to this original failure. Normally, we
-    /// handle this with the `err_count_on_creation` count, which
-    /// basically just tracks how many errors were reported when we
-    /// started type-checking a fn and checks to see if any new errors
-    /// have been reported since then. Not great, but it works.
-    ///
-    /// However, when errors originated in other passes -- notably
-    /// resolve -- this heuristic breaks down. Therefore, we have this
-    /// auxiliary flag that one can set whenever one creates a
-    /// type-error that is due to an error in a prior pass.
+    /// errors that are due to this original failure. We have this
+    /// flag that one can set whenever one creates a type-error that
+    /// is due to an error in a prior pass.
     ///
     /// Don't read this flag directly, call `is_tainted_by_errors()`
     /// and `set_tainted_by_errors()`.
     tainted_by_errors: Cell<Option<ErrorGuaranteed>>,
 
-    /// Track how many errors were reported when this infcx is created.
-    /// If the number of errors increases, that's also a sign (like
-    /// `tainted_by_errors`) to avoid reporting certain kinds of errors.
-    // FIXME(matthewjasper) Merge into `tainted_by_errors`
-    err_count_on_creation: usize,
-
     /// What is the innermost universe we have created? Starts out as
     /// `UniverseIndex::root()` but grows from there as we enter
     /// universal quantifiers.
@@ -656,7 +643,6 @@ pub fn build(&mut self) -> InferCtxt<'tcx> {
             reported_trait_errors: Default::default(),
             reported_signature_mismatch: Default::default(),
             tainted_by_errors: Cell::new(None),
-            err_count_on_creation: tcx.dcx().err_count_excluding_lint_errs(),
             universe: Cell::new(ty::UniverseIndex::ROOT),
             intercrate,
             next_trait_solver,