Codegen/profile options in configuration

[jedbrown]

Problem

I'd like to use mdbook test on a book that uses #![feature(autodiff)] (an approved MCP being implemented to hopefully land on nightly later this year). This feature requires -C lto=fat. I hacked mdBook with the following and it works, but I'd like a change that can be upstreamed.

diff --git i/src/book/mod.rs w/src/book/mod.rs
index c0ab8a5..409482f 100644
--- i/src/book/mod.rs
+++ w/src/book/mod.rs
@@ -319,7 +319,11 @@ impl MDBook {
                 tmpf.write_all(ch.content.as_bytes())?;
 
                 let mut cmd = Command::new("rustdoc");
-                cmd.arg(&path).arg("--test").args(&library_args);
+                cmd.arg(&path)
+                    .arg("--test")
+                    .args(&library_args)
+                    .arg("-C")
+                    .arg("lto=fat");
 
                 if let Some(edition) = self.config.rust.edition {
                     match edition {

Proposed Solution

I see rustdoc is invoked directly, not via cargo, so the concept of cargo profiles does not directly apply. The narrowest solution would be to support the following as a special case

[rust]
lto = "fat"

More generally, we could have

[rust.codegen]
lto = "fat"

where all the key-values are passed along with rustdoc -C (--codegen). A more comprehensive approach might be to invoke cargo rustdoc and thereby be able to use cargo profiles.

Which of these (or an alternative) would the maintainers prefer?

Notes

No response