Fix DiagCtxtInner::reset_err_count.

Several fields were not being reset. Using destructuring makes it much
harder to miss a field.

Author: nnethercote

Diff for: compiler/rustc_errors/src/lib.rs

@@ -78,6 +78,7 @@ use std::fmt;
 use std::hash::Hash;
 use std::io::Write;
 use std::num::NonZeroUsize;
+use std::ops::DerefMut;
 use std::panic;
 use std::path::{Path, PathBuf};
 
@@ -666,22 +667,51 @@ impl DiagCtxt {
     /// tools that want to reuse a `Parser` cleaning the previously emitted diagnostics as well as
     /// the overall count of emitted error diagnostics.
     pub fn reset_err_count(&self) {
+        // Use destructuring so that if a field gets added to `DiagCtxtInner`, it's impossible to
+        // fail to update this method as well.
         let mut inner = self.inner.borrow_mut();
-        inner.stashed_err_count = 0;
-        inner.deduplicated_err_count = 0;
-        inner.deduplicated_warn_count = 0;
-        inner.must_produce_diag = false;
-        inner.has_printed = false;
-        inner.suppressed_expected_diag = false;
-
-        // actually free the underlying memory (which `clear` would not do)
-        inner.err_guars = Default::default();
-        inner.lint_err_guars = Default::default();
-        inner.delayed_bugs = Default::default();
-        inner.taught_diagnostics = Default::default();
-        inner.emitted_diagnostic_codes = Default::default();
-        inner.emitted_diagnostics = Default::default();
-        inner.stashed_diagnostics = Default::default();
+        let DiagCtxtInner {
+            flags: _,
+            err_guars,
+            lint_err_guars,
+            delayed_bugs,
+            stashed_err_count,
+            deduplicated_err_count,
+            deduplicated_warn_count,
+            emitter: _,
+            must_produce_diag,
+            has_printed,
+            suppressed_expected_diag,
+            taught_diagnostics,
+            emitted_diagnostic_codes,
+            emitted_diagnostics,
+            stashed_diagnostics,
+            future_breakage_diagnostics,
+            check_unstable_expect_diagnostics,
+            unstable_expect_diagnostics,
+            fulfilled_expectations,
+            ice_file: _,
+        } = inner.deref_mut();
+
+        // For the `Vec`s and `HashMap`s, we overwrite with an empty container to free the
+        // underlying memory (which `clear` would not do).
+        *err_guars = Default::default();
+        *lint_err_guars = Default::default();
+        *delayed_bugs = Default::default();
+        *stashed_err_count = 0;
+        *deduplicated_err_count = 0;
+        *deduplicated_warn_count = 0;
+        *must_produce_diag = false;
+        *has_printed = false;
+        *suppressed_expected_diag = false;
+        *taught_diagnostics = Default::default();
+        *emitted_diagnostic_codes = Default::default();
+        *emitted_diagnostics = Default::default();
+        *stashed_diagnostics = Default::default();
+        *future_breakage_diagnostics = Default::default();
+        *check_unstable_expect_diagnostics = false;
+        *unstable_expect_diagnostics = Default::default();
+        *fulfilled_expectations = Default::default();
     }
 
     /// Stash a given diagnostic with the given `Span` and [`StashKey`] as the key.