Rollup merge of #130693 - jieyouxu:minicore, r=bjorn3

Add `minicore` test auxiliary and support `//@ add-core-stubs` directive in ui/assembly/codegen tests

Context: [Real cross-compiling tests instead of `#![no_core]` silliness #130375](https://github.com/rust-lang/rust/issues/130375)
MCP: https://github.com/rust-lang/compiler-team/issues/786
Tracking issue: https://github.com/rust-lang/rust/issues/131485

This prototype PR is subject to further changes based on feedback.

### New `minicore` test auxiliary and `//@ add-core-stubs` compiletest directive

This PR introduces a prototype implementation of a `minicore` auxiliary test helper that provides `core` stubs for `#![no_core]` ui/assembly/codegen tests that need to build but not run on both the host platform and the cross-compiled target platform.

Key summary:

- `tests/auxiliary/minicore.rs` contains stub definitions of `core` items intended for consumption by `check-pass`/`build-pass` tests that want the typical prelude items like `Copy` to be stubbed out under `#![no_core]` scenarios, so that the test can be built (not run) for cross-compiled target platforms. Such tests don't want nor need full `-Z build-std` (e.g. `tests/ui/abi/compatibility.rs`).
- `minicore` is intended for `core` items **only**, not `std`- or `alloc`-exclusive items. If stubs for `alloc` or `std` are wanted, they should be provided by an additional directive and test auxiliary, and not be conflated with `minicore` or `core` stubs. This is because a wider range of tests can benefit from `core`-only stubs.

### Implementation

- The `minicore` auxiliary is a single source file `tests/auxiliary/minicore.rs`.
- The path to `minicore` is made avaiable from bootstrap to compiletest via the `--minicore-path` compiletest flag.
- `minicore` is then built on-demand via the `//@ add-core-stubs` compiletest directive, for each test revision for the given target platform (this distinction is important for when host platform != target platform in cross-compilation scenario).
- `minicore` is then made available to the test as an [extern prelude].

[extern prelude]: https://doc.rust-lang.org/reference/names/preludes.html#extern-prelude

### Example usage

```rs
// tests/ui/abi/my-abi-test.rs

//@ check-pass
//@ add-core-stubs
//@ compile-flags: --target i686-unknown-linux-gnu
//@ needs-llvm-components: x86

#![feature(no_core, lang_items)]
#![no_std]
#![no_core]
#![allow(unused, internal_features)]

extern crate minicore;
use minicore::*;

#[lang = "clone"]
pub trait Clone: Sized {      // `Sized` is provided by `minicore`
    fn clone(&self) -> Self;
}
```

### Implementation steps

- [x] 1. Add an initial `minicore` test auxiliary.
- [x] 2. Build `minicore` in bootstrap.
- [x] 3. Setup a `--minicore-path` compiletest cli flag and pass `minicore` build artifact path from bootstrap to compiletest.
- [x] 4. Assert `add-core-stubs` is mutually incompatible with tests that require to be `run`, as the stubs are only good for tests that only need to be built (i.e. no `run-{pass,fail}`).
- [x] 5. Add some self-tests to sanity check the behavior.
- [x] 6. Ensure that `tests/auxiliary/minicore.rs` is input stamped, i.e. modifying `tests/auxiliary/minicore.rs` should invalidate test cache and force the test to be rerun.

### Known limitations

- The current `minicore` is very minimal, because this PR is intended to focus on supporting the test infrastructure first. Further stubs could be added in follow-up PRs and/or on a as-needed basis.

try-job: aarch64-apple
try-job: armhf-gnu
try-job: x86_64-msvc
try-job: test-various
try-job: dist-various-1

8 files changed, 131 insertions, 32 deletions

diff --git a/src/bootstrap/src/core/build_steps/test.rs b/src/bootstrap/src/core/build_steps/test.rs
index 2ad1b39a87c..0a290a697e6 100644
--- a/src/bootstrap/src/core/build_steps/test.rs
+++ b/src/bootstrap/src/core/build_steps/test.rs
@@ -1725,6 +1725,11 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
         cmd.arg("--run-lib-path").arg(builder.sysroot_libdir(compiler, target));
         cmd.arg("--rustc-path").arg(builder.rustc(compiler));
 
+        // Minicore auxiliary lib for `no_core` tests that need `core` stubs in cross-compilation
+        // scenarios.
+        cmd.arg("--minicore-path")
+            .arg(builder.src.join("tests").join("auxiliary").join("minicore.rs"));
+
         let is_rustdoc = suite.ends_with("rustdoc-ui") || suite.ends_with("rustdoc-js");
 
         if mode == "run-make" {
diff --git a/src/tools/compiletest/src/common.rs b/src/tools/compiletest/src/common.rs
index e82b88eef79..e4f2f95a91b 100644
--- a/src/tools/compiletest/src/common.rs
+++ b/src/tools/compiletest/src/common.rs
@@ -392,6 +392,11 @@ pub struct Config {
 
     /// Command for visual diff display, e.g. `diff-tool --color=always`.
     pub diff_command: Option<String>,
+
+    /// Path to minicore aux library, used for `no_core` tests that need `core` stubs in
+    /// cross-compilation scenarios that do not otherwise want/need to `-Zbuild-std`. Used in e.g.
+    /// ABI tests.
+    pub minicore_path: PathBuf,
 }
 
 impl Config {
diff --git a/src/tools/compiletest/src/directive-list.rs b/src/tools/compiletest/src/directive-list.rs
index 4d57907f26f..980b3f6829a 100644
--- a/src/tools/compiletest/src/directive-list.rs
+++ b/src/tools/compiletest/src/directive-list.rs
@@ -3,6 +3,7 @@
 /// a best-effort approximation for diagnostics. Add new headers to this list when needed.
 const KNOWN_DIRECTIVE_NAMES: &[&str] = &[
     // tidy-alphabetical-start
+    "add-core-stubs",
     "assembly-output",
     "aux-bin",
     "aux-build",
diff --git a/src/tools/compiletest/src/header.rs b/src/tools/compiletest/src/header.rs
index bfcdd747eb4..300a03e5f33 100644
--- a/src/tools/compiletest/src/header.rs
+++ b/src/tools/compiletest/src/header.rs
@@ -198,6 +198,9 @@ pub struct TestProps {
     pub no_auto_check_cfg: bool,
     /// Run tests which require enzyme being build
     pub has_enzyme: bool,
+    /// Build and use `minicore` as `core` stub for `no_core` tests in cross-compilation scenarios
+    /// that don't otherwise want/need `-Z build-std`.
+    pub add_core_stubs: bool,
 }
 
 mod directives {
@@ -243,6 +246,7 @@ mod directives {
     pub const LLVM_COV_FLAGS: &'static str = "llvm-cov-flags";
     pub const FILECHECK_FLAGS: &'static str = "filecheck-flags";
     pub const NO_AUTO_CHECK_CFG: &'static str = "no-auto-check-cfg";
+    pub const ADD_CORE_STUBS: &'static str = "add-core-stubs";
     // This isn't a real directive, just one that is probably mistyped often
     pub const INCORRECT_COMPILER_FLAGS: &'static str = "compiler-flags";
 }
@@ -300,6 +304,7 @@ impl TestProps {
             filecheck_flags: vec![],
             no_auto_check_cfg: false,
             has_enzyme: false,
+            add_core_stubs: false,
         }
     }
 
@@ -564,6 +569,8 @@ impl TestProps {
                     }
 
                     config.set_name_directive(ln, NO_AUTO_CHECK_CFG, &mut self.no_auto_check_cfg);
+
+                    self.update_add_core_stubs(ln, config);
                 },
             );
 
@@ -677,6 +684,27 @@ impl TestProps {
     pub fn local_pass_mode(&self) -> Option<PassMode> {
         self.pass_mode
     }
+
+    pub fn update_add_core_stubs(&mut self, ln: &str, config: &Config) {
+        let add_core_stubs = config.parse_name_directive(ln, directives::ADD_CORE_STUBS);
+        if add_core_stubs {
+            if !matches!(config.mode, Mode::Ui | Mode::Codegen | Mode::Assembly) {
+                panic!(
+                    "`add-core-stubs` is currently only supported for ui, codegen and assembly test modes"
+                );
+            }
+
+            // FIXME(jieyouxu): this check is currently order-dependent, but we should probably
+            // collect all directives in one go then perform a validation pass after that.
+            if self.local_pass_mode().is_some_and(|pm| pm == PassMode::Run) {
+                // `minicore` can only be used with non-run modes, because it's `core` prelude stubs
+                // and can't run.
+                panic!("`add-core-stubs` cannot be used to run the test binary");
+            }
+
+            self.add_core_stubs = add_core_stubs;
+        }
+    }
 }
 
 /// If the given line begins with the appropriate comment prefix for a directive,
diff --git a/src/tools/compiletest/src/header/tests.rs b/src/tools/compiletest/src/header/tests.rs
index 2e6effcab98..0e735dc77c4 100644
--- a/src/tools/compiletest/src/header/tests.rs
+++ b/src/tools/compiletest/src/header/tests.rs
@@ -152,6 +152,7 @@ impl ConfigBuilder {
             "--git-repository=",
             "--nightly-branch=",
             "--git-merge-commit-email=",
+            "--minicore-path=",
         ];
         let mut args: Vec<String> = args.iter().map(ToString::to_string).collect();
 
diff --git a/src/tools/compiletest/src/lib.rs b/src/tools/compiletest/src/lib.rs
index ccf8057bf5c..5c06a39c477 100644
--- a/src/tools/compiletest/src/lib.rs
+++ b/src/tools/compiletest/src/lib.rs
@@ -181,7 +181,8 @@ pub fn parse_config(args: Vec<String>) -> Config {
             "compiletest-diff-tool",
             "What custom diff tool to use for displaying compiletest tests.",
             "COMMAND",
-        );
+        )
+        .reqopt("", "minicore-path", "path to minicore aux library", "PATH");
 
     let (argv0, args_) = args.split_first().unwrap();
     if args.len() == 1 || args[1] == "-h" || args[1] == "--help" {
@@ -371,7 +372,10 @@ pub fn parse_config(args: Vec<String>) -> Config {
         git_merge_commit_email: matches.opt_str("git-merge-commit-email").unwrap(),
 
         profiler_runtime: matches.opt_present("profiler-runtime"),
+
         diff_command: matches.opt_str("compiletest-diff-tool"),
+
+        minicore_path: opt_path(matches, "minicore-path"),
     }
 }
 
@@ -409,6 +413,7 @@ pub fn log_config(config: &Config) {
     logv(c, format!("host-linker: {:?}", config.host_linker));
     logv(c, format!("verbose: {}", config.verbose));
     logv(c, format!("format: {:?}", config.format));
+    logv(c, format!("minicore_path: {:?}", config.minicore_path.display()));
     logv(c, "\n".to_string());
 }
 
@@ -885,6 +890,12 @@ fn files_related_to_test(
         related.push(path);
     }
 
+    // `minicore.rs` test auxiliary: we need to make sure tests get rerun if this changes.
+    //
+    // FIXME(jieyouxu): untangle these paths, we should provide both a path to root `tests/` or
+    // `tests/auxiliary/` and the test suite in question. `src_base` is also a terrible name.
+    related.push(config.src_base.parent().unwrap().join("auxiliary").join("minicore.rs"));
+
     related
 }
 
diff --git a/src/tools/compiletest/src/runtest.rs b/src/tools/compiletest/src/runtest.rs
index a8a71c196fc..b337458f943 100644
--- a/src/tools/compiletest/src/runtest.rs
+++ b/src/tools/compiletest/src/runtest.rs
@@ -1150,14 +1150,20 @@ impl<'test> TestCx<'test> {
         }
     }
 
-    /// `root_testpaths` refers to the path of the original test.
-    /// the auxiliary and the test with an aux-build have the same `root_testpaths`.
+    /// `root_testpaths` refers to the path of the original test. the auxiliary and the test with an
+    /// aux-build have the same `root_testpaths`.
     fn compose_and_run_compiler(
         &self,
         mut rustc: Command,
         input: Option<String>,
         root_testpaths: &TestPaths,
     ) -> ProcRes {
+        if self.props.add_core_stubs {
+            let minicore_path = self.build_minicore();
+            rustc.arg("--extern");
+            rustc.arg(&format!("minicore={}", minicore_path.to_str().unwrap()));
+        }
+
         let aux_dir = self.aux_output_dir();
         self.build_all_auxiliary(root_testpaths, &aux_dir, &mut rustc);
 
@@ -1171,6 +1177,37 @@ impl<'test> TestCx<'test> {
         )
     }
 
+    /// Builds `minicore`. Returns the path to the minicore rlib within the base test output
+    /// directory.
+    fn build_minicore(&self) -> PathBuf {
+        let output_file_path = self.output_base_dir().join("libminicore.rlib");
+        let mut rustc = self.make_compile_args(
+            &self.config.minicore_path,
+            TargetLocation::ThisFile(output_file_path.clone()),
+            Emit::None,
+            AllowUnused::Yes,
+            LinkToAux::No,
+            vec![],
+        );
+
+        rustc.args(&["--crate-type", "rlib"]);
+        rustc.arg("-Cpanic=abort");
+
+        let res =
+            self.compose_and_run(rustc, self.config.compile_lib_path.to_str().unwrap(), None, None);
+        if !res.status.success() {
+            self.fatal_proc_rec(
+                &format!(
+                    "auxiliary build of {:?} failed to compile: ",
+                    self.config.minicore_path.display()
+                ),
+                &res,
+            );
+        }
+
+        output_file_path
+    }
+
     /// Builds an aux dependency.
     fn build_auxiliary(
         &self,
@@ -1662,6 +1699,15 @@ impl<'test> TestCx<'test> {
 
         rustc.args(&self.props.compile_flags);
 
+        // FIXME(jieyouxu): we should report a fatal error or warning if user wrote `-Cpanic=` with
+        // something that's not `abort`, however, by moving this last we should override previous
+        // `-Cpanic=`s
+        //
+        // `minicore` requires `#![no_std]` and `#![no_core]`, which means no unwinding panics.
+        if self.props.add_core_stubs {
+            rustc.arg("-Cpanic=abort");
+        }
+
         rustc
     }
 
@@ -1848,34 +1894,6 @@ impl<'test> TestCx<'test> {
         (proc_res, output_path)
     }
 
-    fn compile_test_and_save_assembly(&self) -> (ProcRes, PathBuf) {
-        // This works with both `--emit asm` (as default output name for the assembly)
-        // and `ptx-linker` because the latter can write output at requested location.
-        let output_path = self.output_base_name().with_extension("s");
-        let input_file = &self.testpaths.file;
-
-        // Use the `//@ assembly-output:` directive to determine how to emit assembly.
-        let emit = match self.props.assembly_output.as_deref() {
-            Some("emit-asm") => Emit::Asm,
-            Some("bpf-linker") => Emit::LinkArgsAsm,
-            Some("ptx-linker") => Emit::None, // No extra flags needed.
-            Some(other) => self.fatal(&format!("unknown 'assembly-output' directive: {other}")),
-            None => self.fatal("missing 'assembly-output' directive"),
-        };
-
-        let rustc = self.make_compile_args(
-            input_file,
-            TargetLocation::ThisFile(output_path.clone()),
-            emit,
-            AllowUnused::No,
-            LinkToAux::Yes,
-            Vec::new(),
-        );
-
-        let proc_res = self.compose_and_run_compiler(rustc, None, self.testpaths);
-        (proc_res, output_path)
-    }
-
     fn verify_with_filecheck(&self, output: &Path) -> ProcRes {
         let mut filecheck = Command::new(self.config.llvm_filecheck.as_ref().unwrap());
         filecheck.arg("--input-file").arg(output).arg(&self.testpaths.file);
diff --git a/src/tools/compiletest/src/runtest/assembly.rs b/src/tools/compiletest/src/runtest/assembly.rs
index 430a5534da1..89d7de58c20 100644
--- a/src/tools/compiletest/src/runtest/assembly.rs
+++ b/src/tools/compiletest/src/runtest/assembly.rs
@@ -1,4 +1,6 @@
-use super::TestCx;
+use std::path::PathBuf;
+
+use super::{AllowUnused, Emit, LinkToAux, ProcRes, TargetLocation, TestCx};
 
 impl TestCx<'_> {
     pub(super) fn run_assembly_test(&self) {
@@ -16,4 +18,32 @@ impl TestCx<'_> {
             self.fatal_proc_rec("verification with 'FileCheck' failed", &proc_res);
         }
     }
+
+    fn compile_test_and_save_assembly(&self) -> (ProcRes, PathBuf) {
+        // This works with both `--emit asm` (as default output name for the assembly)
+        // and `ptx-linker` because the latter can write output at requested location.
+        let output_path = self.output_base_name().with_extension("s");
+        let input_file = &self.testpaths.file;
+
+        // Use the `//@ assembly-output:` directive to determine how to emit assembly.
+        let emit = match self.props.assembly_output.as_deref() {
+            Some("emit-asm") => Emit::Asm,
+            Some("bpf-linker") => Emit::LinkArgsAsm,
+            Some("ptx-linker") => Emit::None, // No extra flags needed.
+            Some(other) => self.fatal(&format!("unknown 'assembly-output' directive: {other}")),
+            None => self.fatal("missing 'assembly-output' directive"),
+        };
+
+        let rustc = self.make_compile_args(
+            input_file,
+            TargetLocation::ThisFile(output_path.clone()),
+            emit,
+            AllowUnused::No,
+            LinkToAux::Yes,
+            Vec::new(),
+        );
+
+        let proc_res = self.compose_and_run_compiler(rustc, None, self.testpaths);
+        (proc_res, output_path)
+    }
 }