coverage: All counter terms in an unused function are zero

This is currently handled automatically by the fact that codegen doesn't visit
coverage statements in unused functions, but that will no longer be the case
when unused IDs are identified by a separate query instead.

compiler/rustc_codegen_llvm/src/coverageinfo/map_data.rs

@@ -117,7 +117,7 @@ impl<'tcx> FunctionCoverageCollector<'tcx> {
         // (By construction, expressions can only refer to other expressions
         // that have lower IDs, so one pass is sufficient.)
         for (id, expression) in self.function_coverage_info.expressions.iter_enumerated() {
-            if !self.expressions_seen.contains(id) {
+            if !self.is_used || !self.expressions_seen.contains(id) {
                 // If an expression was not seen, it must have been optimized away,
                 // so any operand that refers to it can be replaced with zero.
                 zero_expressions.insert(id);
@@ -238,7 +238,7 @@ impl<'tcx> FunctionCoverage<'tcx> {
     }
 
     fn is_zero_term(&self, term: CovTerm) -> bool {
-        is_zero_term(&self.counters_seen, &self.zero_expressions, term)
+        !self.is_used || is_zero_term(&self.counters_seen, &self.zero_expressions, term)
     }
 }
 

compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs

@@ -538,8 +538,7 @@ fn add_unused_function_coverage<'tcx>(
         }),
     );
 
-    // An unused function's mappings will automatically be rewritten to map to
-    // zero, because none of its counters/expressions are marked as seen.
+    // An unused function's mappings will all be rewritten to map to zero.
     let function_coverage = FunctionCoverageCollector::unused(instance, function_coverage_info);
 
     cx.coverage_cx().function_coverage_map.borrow_mut().insert(instance, function_coverage);