Rollup of 4 pull requests

compiler/rustc_session/src/config.rs

@@ -2152,6 +2152,12 @@ fn collect_print_requests(
     prints.extend(matches.opt_strs("print").into_iter().map(|req| {
         let (req, out) = split_out_file_name(&req);
 
+        if out.is_some() && !unstable_opts.unstable_options {
+            handler.early_error(
+                "the `-Z unstable-options` flag must also be passed to \
+                 enable the path print option",
+            );
+        }
         let kind = match PRINT_KINDS.iter().find(|&&(name, _)| name == req) {
             Some((_, PrintKind::TargetSpec)) => {
                 if unstable_opts.unstable_options {

src/doc/rustc/src/command-line-arguments.md

@@ -260,10 +260,6 @@ The valid types of print values are:
   This returns rustc's minimum supported deployment target if no `*_DEPLOYMENT_TARGET` variable
   is present in the environment, or otherwise returns the variable's parsed value.
 
-A filepath may optionally be specified for each requested information kind, in
-the format `--print KIND=PATH`, just like for `--emit`. When a path is
-specified, information will be written there instead of to stdout.
-
 [conditional compilation]: ../reference/conditional-compilation.html
 [deployment target]: https://developer.apple.com/library/archive/documentation/DeveloperTools/Conceptual/cross_development/Configuring/configuring.html
 

src/doc/unstable-book/src/compiler-flags/path-options.md

@@ -0,0 +1,11 @@
+# `--print` Options
+
+The behavior of the `--print` flag can be modified by optionally be specifiying a filepath
+for each requested information kind, in the format `--print KIND=PATH`, just like for
+`--emit`. When a path is specified, information will be written there instead of to stdout.
+
+This is unstable feature, so you have to provide `-Zunstable-options` to enable it.
+
+## Examples
+
+`rustc main.rs -Z unstable-options --print cfg=cfgs.txt`