Rollup merge of rust-lang#100681 - CAD97:rustc-print-rustc, r=petrochenkov

Add `rustc --print rustc-path`

Related:

- rust-lang/cargo#10986
- rust-lang/rustup#3035

Goal:

Like the original rust-lang/rustup#2958, the goal is to enable `cargo` to call `rustc` directly, rather than through the `rustup` shim.

Solution:

`cargo` asks `rustc` to tell it what executable to run. Sometime early in compilation, `cargo` will run `$(RUSTC_WRAPPER) $(RUSTC ?: rustc) --print rustc-path`[^1]. Further calls to `rustc` to do execution will use the resolved printed executable path rather than continuing to call the "input `rustc` path," which will allow it to bypass the `rustup` shim.

The cargo side is rust-lang/cargo#10998.

[^1]: This can potentially be combined with other `--print`s, as well!

This is a tiny patch, so I've implemented it as insta-stable; this will need signoff probably from both the compiler and cargo teams. I'm working on the cargo side of the patch currently, but wanted to get this up ASAP.

cc ```@davidlattimore``` ```@bjorn3``` ```@rust-lang/cargo``` ```@rust-lang/compiler``` (sorry for the big ping list 😅)

Author: matthiaskrgr

compiler/rustc_driver/src/lib.rs

@@ -725,6 +725,10 @@ fn print_crate_info(
             | TargetFeatures => {
                 codegen_backend.print(*req, sess);
             }
+            RustcPath => match env::current_exe() {
+                Ok(exe) => println!("{}", exe.display()),
+                Err(_) => early_error(ErrorOutputType::default(), "failed to get rustc path"),
+            },
             // Any output here interferes with Cargo's parsing of other printed output
             NativeStaticLibs => {}
             LinkArgs => {}

compiler/rustc_session/src/config.rs

@@ -545,6 +545,7 @@ pub enum PrintRequest {
     NativeStaticLibs,
     StackProtectorStrategies,
     LinkArgs,
+    RustcPath,
 }
 
 pub enum Input {
@@ -1777,6 +1778,20 @@ fn collect_print_requests(
         cg.target_feature = String::new();
     }
 
+    let gate = |req, opt| {
+        if unstable_opts.unstable_options {
+            req
+        } else {
+            early_error(
+                error_format,
+                &format!(
+                    "the `-Z unstable-options` flag must also be passed to \
+                        enable the {opt} print option",
+                ),
+            );
+        }
+    };
+
     prints.extend(matches.opt_strs("print").into_iter().map(|s| match &*s {
         "crate-name" => PrintRequest::CrateName,
         "file-names" => PrintRequest::FileNames,
@@ -1791,18 +1806,9 @@ fn collect_print_requests(
         "tls-models" => PrintRequest::TlsModels,
         "native-static-libs" => PrintRequest::NativeStaticLibs,
         "stack-protector-strategies" => PrintRequest::StackProtectorStrategies,
-        "target-spec-json" => {
-            if unstable_opts.unstable_options {
-                PrintRequest::TargetSpec
-            } else {
-                early_error(
-                    error_format,
-                    "the `-Z unstable-options` flag must also be passed to \
-                     enable the target-spec-json print option",
-                );
-            }
-        }
+        "target-spec-json" => gate(PrintRequest::TargetSpec, "target-spec-json"),
         "link-args" => PrintRequest::LinkArgs,
+        "rustc-path" => gate(PrintRequest::RustcPath, "rustc-path"),
         req => early_error(error_format, &format!("unknown print request `{req}`")),
     }));
 

src/test/run-make/print-rustc-path/Makefile

@@ -0,0 +1,9 @@
+-include ../../run-make-fulldeps/tools.mk
+
+ifdef IS_WINDOWS
+all:
+	$(RUSTC) -Zunstable-options --print rustc-path | $(CGREP) bin\rustc
+else
+all:
+	$(RUSTC) -Zunstable-options --print rustc-path | $(CGREP) bin/rustc
+endif