Add spec for `Coverage.supported?` and `start(eval: true)`. (#6499)

* Don't emit coverage for eval when eval coverage is disabled.

spec/ruby/library/coverage/result_spec.rb

@@ -106,15 +106,35 @@ describe 'Coverage.result' do
   end
 
   ruby_version_is '3.2' do
-    it 'returns the correct results when eval is used' do
-      Coverage.start
+    it 'indicates support for different features' do
+      Coverage.supported?(:lines).should == true
+    end
+
+    it 'returns the correct results when eval coverage is enabled' do
+      Coverage.supported?(:eval).should == true
+
+      Coverage.start(lines: true, eval: true)
       require @eval_code_file.chomp('.rb')
       result = Coverage.result
 
       result.should == {
-          @eval_code_file => [
-              1, nil, 1, nil, 1, 1, nil, nil, nil, nil, 1
-          ]
+          @eval_code_file => {
+              lines: [1, nil, 1, nil, 1, 1, nil, nil, nil, nil, 1]
+          }
+      }
+    end
+
+    it 'returns the correct results when eval coverage is enabled' do
+      Coverage.supported?(:eval).should == true
+
+      Coverage.start(lines: true, eval: false)
+      require @eval_code_file.chomp('.rb')
+      result = Coverage.result
+
+      result.should == {
+          @eval_code_file => {
+              lines: [1, nil, 1, nil, 1, nil, nil, nil, nil, nil, 1]
+          }
       }
     end
   end

vm_eval.c

@@ -11,6 +11,7 @@
 
 **********************************************************************/
 
+#include "internal/thread.h"
 struct local_var_list {
     VALUE tbl;
 };
@@ -1672,7 +1673,9 @@ eval_make_iseq(VALUE src, VALUE fname, int line, const rb_binding_t *bind,
     rb_iseq_t *iseq = NULL;
     rb_ast_t *ast;
     int isolated_depth = 0;
-    int coverage_enabled = Qtrue;
+
+    // Conditionally enable coverage depending on the current mode:
+    VALUE coverage_enabled = RBOOL(rb_get_coverage_mode() & COVERAGE_TARGET_EVAL);
 
     {
         int depth = 1;