Compute the right output filenames for rustdoc invocations that request the JSON format

src/cargo/core/compiler/context/compilation_files.rs

@@ -436,10 +436,31 @@ impl<'a, 'cfg: 'a> CompilationFiles<'a, 'cfg> {
     ) -> CargoResult<Arc<Vec<OutputFile>>> {
         let ret = match unit.mode {
             CompileMode::Doc { .. } => {
-                let path = self
-                    .out_dir(unit)
-                    .join(unit.target.crate_name())
-                    .join("index.html");
+                let json_output = if let Some(extra_rustdoc_args) = bcx.extra_args_for(unit) {
+                    extra_rustdoc_args
+                        .iter()
+                        .find(|arg| {
+                            *arg == "--output-format=json"
+                                || *arg == "-wjson"
+                                || *arg == "-w json"
+                                || *arg == "--output-format json"
+                        })
+                        .is_some()
+                } else {
+                    false
+                };
+                // If the output format is JSON, then rustdoc will output
+                // a <crate name>.json file rather than the usual index.html
+                // We compute its expected path here in order to track it as
+                // the correct output file.
+                let path = if json_output {
+                    self.out_dir(unit)
+                        .join(format!("{}.json", unit.target.crate_name()))
+                } else {
+                    self.out_dir(unit)
+                        .join(unit.target.crate_name())
+                        .join("index.html")
+                };
                 vec![OutputFile {
                     path,
                     hardlink: None,

tests/testsuite/doc.rs

@@ -1759,6 +1759,74 @@ fn doc_json_artifacts() {
         .run();
 }
 
+#[cargo_test(nightly, reason = "rustdoc-json")]
+fn doc_json_artifacts_are_cached_correctly() {
+    let p = project().file("src/lib.rs", "").build();
+
+    // The first run should emit the unit as dirty ("fresh": false), while showing
+    // the correct output filenames.
+    p.cargo("rustdoc --lib --message-format=json -- -Zunstable-options --output-format=json")
+        .with_json_contains_unordered(
+            r#"
+{
+    "reason": "compiler-artifact",
+    "package_id": "foo 0.0.1 [..]",
+    "manifest_path": "[ROOT]/foo/Cargo.toml",
+    "target":
+    {
+        "kind": ["lib"],
+        "crate_types": ["lib"],
+        "name": "foo",
+        "src_path": "[ROOT]/foo/src/lib.rs",
+        "edition": "2015",
+        "doc": true,
+        "doctest": true,
+        "test": true
+    },
+    "profile": "{...}",
+    "features": [],
+    "filenames": ["[ROOT]/foo/target/doc/foo.json"],
+    "executable": null,
+    "fresh": false
+}
+
+{"reason":"build-finished","success":true}
+"#,
+        )
+        .run();
+
+    // We then run it again and check that the unit is now marked as fresh.
+    p.cargo("rustdoc --lib --message-format=json -- -Zunstable-options --output-format=json")
+        .with_json_contains_unordered(
+            r#"
+{
+    "reason": "compiler-artifact",
+    "package_id": "foo 0.0.1 [..]",
+    "manifest_path": "[ROOT]/foo/Cargo.toml",
+    "target":
+    {
+        "kind": ["lib"],
+        "crate_types": ["lib"],
+        "name": "foo",
+        "src_path": "[ROOT]/foo/src/lib.rs",
+        "edition": "2015",
+        "doc": true,
+        "doctest": true,
+        "test": true
+    },
+    "profile": "{...}",
+    "features": [],
+    "filenames": ["[ROOT]/foo/target/doc/foo.json"],
+    "executable": null,
+    "fresh": true
+}
+
+{"reason":"build-finished","success":true}
+        "#,
+        )
+        .run();
+}
+
 #[cargo_test]
 fn short_message_format() {
     let p = project().file("src/lib.rs", BAD_INTRA_LINK_LIB).build();