Ensure that --generate-link-to-definition is only used with HTML output and is unstable

src/librustdoc/config.rs

@@ -659,6 +659,14 @@ impl Options {
         let nocapture = matches.opt_present("nocapture");
         let generate_link_to_definition = matches.opt_present("generate-link-to-definition");
 
+        if generate_link_to_definition && (show_coverage || output_format != OutputFormat::Html) {
+            diag.struct_err(
+                "--generate-link-to-definition option can only be used with HTML output format",
+            )
+            .emit();
+            return Err(1);
+        }
+
         let (lint_opts, describe_lints, lint_cap) =
             get_cmd_lint_options(matches, error_format, &debugging_opts);
 

src/test/rustdoc-ui/generate-link-to-definition-opt-unstable.rs

@@ -0,0 +1,6 @@
+// This test purpose is to check that the "--generate-link-to-definition"
+// option can only be used on nightly.
+
+// compile-flags: --generate-link-to-definition
+
+pub fn f() {}

src/test/rustdoc-ui/generate-link-to-definition-opt-unstable.stderr

@@ -0,0 +1,2 @@
+error: the `-Z unstable-options` flag must also be passed to enable the flag `generate-link-to-definition`
+

src/test/rustdoc-ui/generate-link-to-definition-opt.rs

@@ -0,0 +1,6 @@
+// This test purpose is to check that the "--generate-link-to-definition"
+// option can only be used with HTML generation.
+
+// compile-flags: -Zunstable-options --generate-link-to-definition --output-format json
+
+pub fn f() {}

src/test/rustdoc-ui/generate-link-to-definition-opt.stderr

@@ -0,0 +1,2 @@
+error: --generate-link-to-definition option can only be used with HTML output format
+

src/test/rustdoc-ui/generate-link-to-definition-opt2.rs

@@ -0,0 +1,6 @@
+// This test purpose is to check that the "--generate-link-to-definition"
+// option can only be used with HTML generation.
+
+// compile-flags: -Zunstable-options --generate-link-to-definition --show-coverage
+
+pub fn f() {}

src/test/rustdoc-ui/generate-link-to-definition-opt2.stderr

@@ -0,0 +1,2 @@
+error: --generate-link-to-definition option can only be used with HTML output format
+