Replace ToJson with serde

src/test/rustdoc-ui/coverage/html.rs

@@ -0,0 +1,4 @@
+// compile-flags:-Z unstable-options --output-format html --show-coverage
+
+/// Foo
+pub struct Xo;

src/test/rustdoc-ui/coverage/html.stderr

@@ -0,0 +1,2 @@
+error: html output format isn't supported for the --show-coverage option
+

src/test/rustdoc-ui/coverage/json.rs

@@ -1,5 +1,5 @@
 // build-pass
-// compile-flags:-Z unstable-options --show-coverage
+// compile-flags:-Z unstable-options --output-format json --show-coverage
 
 pub mod foo {
     /// Hello!

src/test/rustdoc-ui/coverage/json.stdout

@@ -1,7 +1 @@
-+-------------------------------------+------------+------------+------------+
-| File                                | Documented |      Total | Percentage |
-+-------------------------------------+------------+------------+------------+
-| ...test/rustdoc-ui/coverage/json.rs |          7 |         13 |      53.8% |
-+-------------------------------------+------------+------------+------------+
-| Total                               |          7 |         13 |      53.8% |
-+-------------------------------------+------------+------------+------------+
+{"$DIR/json.rs":{"total":13,"with_docs":7}}

src/test/rustdoc-ui/coverage/output-format-fail-check.rs

@@ -1,5 +1,4 @@
 // compile-flags:-Z unstable-options --output-format
-// should-fail
 
 /// toudoum!
 pub struct SomeStruct;

src/test/rustdoc-ui/coverage/output-format-fail-check.stderr

@@ -0,0 +1,6 @@
+warning: the 'output-format' flag is considered deprecated
+   |
+   = warning: see issue #44136 <https://github.com/rust-lang/rust/issues/44136> for more information
+
+error: too many file operands
+