Rollup merge of #160576 - GuillaumeGomez:rustdoc-standalone-md, r=Urgau,fmease

[rustdoc] Create output file after we checked that the standalone markdown file is valid

This PR makes the creation of the output (HTML) file after we checked that the input markdown is valid to prevent the output file content to be truncated ([`File::create` doc](https://doc.rust-lang.org/nightly/std/fs/struct.File.html#method.create)) in any case.

r? @Urgau

Author: JonathanBrouwer

src/librustdoc/markdown.rs

@@ -69,13 +69,14 @@ pub(crate) fn render_and_write(
     let playground_url = options.markdown_playground_url.or(options.playground_url);
     let playground = playground_url.map(|url| markdown::Playground { crate_name: None, url });
 
-    let mut out =
-        File::create(&output).map_err(|e| format!("{output}: {e}", output = output.display()))?;
-
     let (metadata, text) = extract_leading_metadata(&input_str);
     if metadata.is_empty() {
         return Err("invalid markdown file: no initial lines starting with `# ` or `%`".to_owned());
     }
+
+    let mut out =
+        File::create(&output).map_err(|e| format!("{output}: {e}", output = output.display()))?;
+
     let title = metadata[0];
 
     let error_codes = ErrorCodes::from(options.unstable_features.is_nightly_build());

tests/run-make/rustdoc/markdown-without-title/rmake.rs

@@ -0,0 +1,45 @@
+// When rustdoc gets a markdown file as input, we want to ensure that if the markdown is invalid,
+// the output file won't be truncated in case this markdown is invalid.
+
+//@ needs-target-std
+
+use run_make_support::{path, rfs, rustdoc};
+
+fn main() {
+    let output_content = "output";
+    let base_file_name = "input";
+
+    let out_dir = path("out");
+    rfs::create_dir(&out_dir);
+
+    // We create the file that should be created by rustdoc and add some content
+    // into it that we will check is still there once rustdoc failed.
+    let output = out_dir.join(format!("{base_file_name}.html"));
+    rfs::write(&output, output_content);
+
+    // We create an "invalid" markdown file (ie no title).
+    let md_file = format!("{base_file_name}.md");
+    rfs::write(&md_file, "Markdown without a title");
+
+    // We run the failing rustdoc.
+    rustdoc()
+        .input(&md_file)
+        .out_dir(&out_dir)
+        .run_fail()
+        .assert_exit_code(1)
+        .assert_stderr_contains(
+            "error: invalid markdown file: no initial lines starting with `# ` or `%`",
+        );
+
+    // Shouldn't have changed.
+    assert_eq!(rfs::read_to_string(&output), output_content);
+
+    // We update the input markdown to make it valid for rustdoc.
+    rfs::write(&md_file, "# a title\n\nMarkdown with a title");
+
+    // We run rustdoc successfully.
+    rustdoc().input(&md_file).out_dir(&out_dir).run();
+
+    // Should have changed.
+    assert_ne!(rfs::read_to_string(output), output_content);
+}