rustc_{errors,session}: add delay_good_path_bug

The first use case of this detection of regression for trimmed paths
computation, that is in the case of rustc, which should be computed only
in case of errors or warnings.

Our current user of this method is deeply nested, being a side effect
from `Display` formatting on lots of rustc types. So taking only the
caller to the error message is not enough - we should collect the
traceback instead.

compiler/rustc_session/src/session.rs

@@ -442,6 +442,24 @@ impl Session {
     pub fn delay_span_bug<S: Into<MultiSpan>>(&self, sp: S, msg: &str) {
         self.diagnostic().delay_span_bug(sp, msg)
     }
+
+    /// Used for code paths of expensive computations that should only take place when
+    /// warnings or errors are emitted. If no messages are emitted ("good path"), then
+    /// it's likely a bug.
+    pub fn delay_good_path_bug(&self, msg: &str) {
+        if self.opts.debugging_opts.print_type_sizes
+            || self.opts.debugging_opts.query_dep_graph
+            || self.opts.debugging_opts.dump_mir.is_some()
+            || self.opts.debugging_opts.unpretty.is_some()
+            || self.opts.output_types.contains_key(&OutputType::Mir)
+            || std::env::var_os("RUSTC_LOG").is_some()
+        {
+            return;
+        }
+
+        self.diagnostic().delay_good_path_bug(msg)
+    }
+
     pub fn note_without_error(&self, msg: &str) {
         self.diagnostic().note_without_error(msg)
     }