Introduce an option for disabling deduplication of diagnostics

src/librustc_errors/lib.rs

@@ -329,6 +329,8 @@ pub struct HandlerFlags {
     /// show macro backtraces even for non-local macros.
     /// (rustc: see `-Z external-macro-backtrace`)
     pub external_macro_backtrace: bool,
+    /// If true, identical diagnostics are reported only once.
+    pub deduplicate_diagnostics: bool,
 }
 
 impl Drop for HandlerInner {
@@ -736,16 +738,16 @@ impl HandlerInner {
             self.emitted_diagnostic_codes.insert(code.clone());
         }
 
-        let diagnostic_hash = {
+        let already_emitted = |this: &mut Self| {
             use std::hash::Hash;
             let mut hasher = StableHasher::new();
             diagnostic.hash(&mut hasher);
-            hasher.finish()
+            let diagnostic_hash = hasher.finish();
+            !this.emitted_diagnostics.insert(diagnostic_hash)
         };
 
-        // Only emit the diagnostic if we haven't already emitted an equivalent
-        // one:
-        if self.emitted_diagnostics.insert(diagnostic_hash) {
+        // Only emit the diagnostic if we haven't already emitted an equivalent one.
+        if !(self.flags.deduplicate_diagnostics && already_emitted(self)) {
             self.emitter.emit_diagnostic(diagnostic);
             if diagnostic.is_error() {
                 self.deduplicated_err_count += 1;

src/librustc_session/options.rs

@@ -946,4 +946,6 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
     insert_sideeffect: bool = (false, parse_bool, [TRACKED],
         "fix undefined behavior when a thread doesn't eventually make progress \
          (such as entering an empty infinite loop) by inserting llvm.sideeffect"),
+    deduplicate_diagnostics: Option<bool> = (None, parse_opt_bool, [UNTRACKED],
+        "deduplicate identical diagnostics"),
 }

src/librustc_session/session.rs

@@ -943,12 +943,11 @@ pub fn build_session_with_source_map(
     let cap_lints_allow = sopts.lint_cap.map_or(false, |cap| cap == lint::Allow);
 
     let can_emit_warnings = !(warnings_allow || cap_lints_allow);
-
     let treat_err_as_bug = sopts.debugging_opts.treat_err_as_bug;
     let dont_buffer_diagnostics = sopts.debugging_opts.dont_buffer_diagnostics;
     let report_delayed_bugs = sopts.debugging_opts.report_delayed_bugs;
-
     let external_macro_backtrace = sopts.debugging_opts.external_macro_backtrace;
+    let deduplicate_diagnostics = sopts.debugging_opts.deduplicate_diagnostics.unwrap_or(true);
 
     let write_dest = match diagnostics_output {
         DiagnosticOutput::Default => None,
@@ -964,7 +963,7 @@ pub fn build_session_with_source_map(
             report_delayed_bugs,
             dont_buffer_diagnostics,
             external_macro_backtrace,
-            ..Default::default()
+            deduplicate_diagnostics,
         },
     );
 

src/test/ui/consts/miri_unleashed/mutable_const2.stderr

@@ -10,7 +10,7 @@ error: internal compiler error: mutable allocation in constant
 LL | const MUTABLE_BEHIND_RAW: *mut i32 = &UnsafeCell::new(42) as *const _ as *mut _;
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:345:17
+thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:347:17
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
 
 error: internal compiler error: unexpected panic