feat(embedded): Hack in code fence support

This is to allow us to get feedback on the design proposed
[on zulip](https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/Embedding.20cargo.20manifests.20in.20rust.20source/near/391427092)
to verify we want to make an RFC for this syntax.

src/cargo/util/toml/embedded.rs

@@ -15,19 +15,72 @@ pub fn expand_manifest(
     path: &std::path::Path,
     config: &Config,
 ) -> CargoResult<String> {
-    let comment = extract_comment(content)?.unwrap_or_default();
-    let manifest = match extract_manifest(&comment)? {
-        Some(manifest) => Some(manifest),
-        None => {
-            tracing::trace!("failed to extract manifest");
-            None
+    let source = split_source(content)?;
+    if let Some(frontmatter) = source.frontmatter {
+        match source.info {
+            Some("cargo") => {}
+            None => {
+                anyhow::bail!("frontmatter is missing an infostring; specify `cargo` for embedding a manifest");
+            }
+            Some(other) => {
+                if let Some(remainder) = other.strip_prefix("cargo,") {
+                    anyhow::bail!("cargo does not support frontmatter infostring attributes like `{remainder}` at this time")
+                } else {
+                    anyhow::bail!("frontmatter infostring `{other}` is unsupported by cargo; specify `cargo` for embedding a manifest")
+                }
+            }
+        }
+
+        // HACK: until rustc has native support for this syntax, we have to remove it from the
+        // source file
+        use std::fmt::Write as _;
+        let hash = crate::util::hex::short_hash(&path.to_string_lossy());
+        let mut rel_path = std::path::PathBuf::new();
+        rel_path.push("target");
+        rel_path.push(&hash[0..2]);
+        rel_path.push(&hash[2..]);
+        let target_dir = config.home().join(rel_path);
+        let hacked_path = target_dir
+            .join(
+                path.file_name()
+                    .expect("always a name for embedded manifests"),
+            )
+            .into_path_unlocked();
+        let mut hacked_source = String::new();
+        if let Some(shebang) = source.shebang {
+            writeln!(hacked_source, "{shebang}")?;
         }
+        writeln!(hacked_source)?; // open
+        for _ in 0..frontmatter.lines().count() {
+            writeln!(hacked_source)?;
+        }
+        writeln!(hacked_source)?; // close
+        writeln!(hacked_source, "{}", source.content)?;
+        if let Some(parent) = hacked_path.parent() {
+            cargo_util::paths::create_dir_all(parent)?;
+        }
+        cargo_util::paths::write_if_changed(&hacked_path, hacked_source)?;
+
+        let manifest = expand_manifest_(&frontmatter, &hacked_path, config)
+            .with_context(|| format!("failed to parse manifest at {}", path.display()))?;
+        let manifest = toml::to_string_pretty(&manifest)?;
+        Ok(manifest)
+    } else {
+        // Legacy doc-comment support; here only for transitional purposes
+        let comment = extract_comment(content)?.unwrap_or_default();
+        let manifest = match extract_manifest(&comment)? {
+            Some(manifest) => Some(manifest),
+            None => {
+                tracing::trace!("failed to extract manifest");
+                None
+            }
+        }
+        .unwrap_or_default();
+        let manifest = expand_manifest_(&manifest, path, config)
+            .with_context(|| format!("failed to parse manifest at {}", path.display()))?;
+        let manifest = toml::to_string_pretty(&manifest)?;
+        Ok(manifest)
     }
-    .unwrap_or_default();
-    let manifest = expand_manifest_(&manifest, path, config)
-        .with_context(|| format!("failed to parse manifest at {}", path.display()))?;
-    let manifest = toml::to_string_pretty(&manifest)?;
-    Ok(manifest)
 }
 
 fn expand_manifest_(
@@ -59,11 +112,8 @@ fn expand_manifest_(
             anyhow::bail!("`package.{key}` is not allowed in embedded manifests")
         }
     }
-    let bin_path = path
-        .file_name()
-        .ok_or_else(|| anyhow::format_err!("no file name"))?
-        .to_string_lossy()
-        .into_owned();
+    // HACK: Using an absolute path while `hacked_path` is in use
+    let bin_path = path.to_string_lossy().into_owned();
     let file_stem = path
         .file_stem()
         .ok_or_else(|| anyhow::format_err!("no file name"))?
@@ -150,6 +200,72 @@ fn sanitize_name(name: &str) -> String {
     name
 }
 
+struct Source<'s> {
+    shebang: Option<&'s str>,
+    info: Option<&'s str>,
+    frontmatter: Option<&'s str>,
+    content: &'s str,
+}
+
+fn split_source(input: &str) -> CargoResult<Source<'_>> {
+    let mut source = Source {
+        shebang: None,
+        info: None,
+        frontmatter: None,
+        content: input,
+    };
+
+    if source.content.starts_with("#![") {
+        return Ok(source);
+    }
+    if source.content.starts_with("#!") {
+        let (shebang, content) = source
+            .content
+            .split_once('\n')
+            .unwrap_or((source.content, ""));
+        source.shebang = Some(shebang);
+        source.content = content;
+    }
+
+    let tick_end = source
+        .content
+        .char_indices()
+        .find_map(|(i, c)| (c != '`').then_some(i))
+        .unwrap_or(source.content.len());
+    let (fence_pattern, rest) = match tick_end {
+        0 => {
+            return Ok(source);
+        }
+        1 | 2 => {
+            anyhow::bail!("found {tick_end} backticks in rust frontmatter, expected at least 3")
+        }
+        _ => source.content.split_at(tick_end),
+    };
+    let (info, content) = rest.split_once("\n").unwrap_or((rest, ""));
+    if !info.is_empty() {
+        source.info = Some(info.trim_end());
+    }
+    source.content = content;
+
+    let Some((frontmatter, content)) = source.content.split_once(fence_pattern) else {
+        anyhow::bail!("no closing `{fence_pattern}` found for frontmatter");
+    };
+    source.frontmatter = Some(frontmatter);
+    source.content = content;
+
+    let (line, content) = source
+        .content
+        .split_once("\n")
+        .unwrap_or((source.content, ""));
+    let line = line.trim();
+    if !line.is_empty() {
+        anyhow::bail!("unexpected trailing content on closing fence: `{line}`");
+    }
+    source.content = content;
+
+    Ok(source)
+}
+
 /// Locates a "code block manifest" in Rust source.
 fn extract_comment(input: &str) -> CargoResult<Option<String>> {
     let mut doc_fragments = Vec::new();
@@ -487,7 +603,7 @@ mod test_expand {
         snapbox::assert_eq(
             r#"[[bin]]
 name = "test-"
-path = "test.rs"
+path = "/home/me/test.rs"
 
 [package]
 autobenches = false
@@ -514,7 +630,7 @@ strip = true
         snapbox::assert_eq(
             r#"[[bin]]
 name = "test-"
-path = "test.rs"
+path = "/home/me/test.rs"
 
 [dependencies]
 time = "0.1.25"