Rollup merge of #123055 - onur-ozkan:miri-rustdoc, r=RalfJung

enable cargo miri test doctests

This was the cleanest solution that came to my mind so far.

cc `@RalfJung`

Resolves #123028

README.md

@@ -505,6 +505,8 @@ binaries, and as such worth documenting:
 * `MIRI_LOCAL_CRATES` is set by `cargo-miri` to tell the Miri driver which
   crates should be given special treatment in diagnostics, in addition to the
   crate currently being compiled.
+* `MIRI_ORIG_RUSTDOC` is set and read by different phases of `cargo-miri` to remember the
+  value of `RUSTDOC` from before it was overwritten.
 * `MIRI_VERBOSE` when set to any value tells the various `cargo-miri` phases to
   perform verbose logging.
 * `MIRI_HOST_SYSROOT` is set by bootstrap to tell `cargo-miri` which sysroot to use for *host*

cargo-miri/src/phases.rs

@@ -202,6 +202,9 @@ pub fn phase_cargo_miri(mut args: impl Iterator<Item = String>) {
     cmd.env("MIRI_BE_RUSTC", "target"); // we better remember to *unset* this in the other phases!
 
     // Set rustdoc to us as well, so we can run doctests.
+    if let Some(orig_rustdoc) = env::var_os("RUSTDOC") {
+        cmd.env("MIRI_ORIG_RUSTDOC", orig_rustdoc);
+    }
     cmd.env("RUSTDOC", &cargo_miri_path);
 
     cmd.env("MIRI_LOCAL_CRATES", local_crates(&metadata));
@@ -581,9 +584,10 @@ pub fn phase_rustdoc(mut args: impl Iterator<Item = String>) {
     let verbose = std::env::var("MIRI_VERBOSE")
         .map_or(0, |verbose| verbose.parse().expect("verbosity flag must be an integer"));
 
-    // phase_cargo_miri sets the RUSTDOC env var to ourselves, so we can't use that here;
-    // just default to a straight-forward invocation for now:
-    let mut cmd = Command::new("rustdoc");
+    // phase_cargo_miri sets the RUSTDOC env var to ourselves, and puts a backup
+    // of the old value into MIRI_ORIG_RUSTDOC. So that's what we have to invoke now.
+    let rustdoc = env::var("MIRI_ORIG_RUSTDOC").unwrap_or("rustdoc".to_string());
+    let mut cmd = Command::new(rustdoc);
 
     let extern_flag = "--extern";
     let runtool_flag = "--runtool";