Improve test suite for -Zbuild-std

This commit is aimed directly at rust-lang/wg-cargo-std-aware#33 and in
general making the `-Zbuild-std` tests more robust. The main change here
is that a new source tree is checked in, `tests/testsuite/mock-std`,
which mirrors rust-lang/rust's own tree for libstd. This mock tree is as
empty as it can be, ideally duplicating almost nothing but for not
requiring duplication of Cargo metadata about patches and such.

The end result here looks like:

* All `-Zbuild-std` tests are now run in parallel
* All tests run much more quickly since they're compiling tiny crates
  instead of actually compiling libstd/libcore
* No tests require network access
* We verify that crates have access to the "custom" libraries
  that we build

Coverage of tests is not currently expanded, but it's hoped that we
could add that shortly afterwards. Coverage has actually gone down
slightly since the custom target test was commented out temporarily and
the full integration test of running `-Zbuild-std` isn't run on CI any
more.

Closes rust-lang/wg-cargo-std-aware#33

Author: alexcrichton

src/cargo/core/compiler/standard_lib.rs

@@ -7,6 +7,7 @@ use crate::core::{Dependency, PackageId, PackageSet, Resolve, SourceId, Workspac
 use crate::ops::{self, Packages};
 use crate::util::errors::CargoResult;
 use std::collections::{HashMap, HashSet};
+use std::env;
 use std::path::PathBuf;
 
 /// Parse the `-Zbuild-std` flag.
@@ -148,6 +149,10 @@ pub fn generate_std_roots<'a>(
 }
 
 fn detect_sysroot_src_path(ws: &Workspace<'_>) -> CargoResult<PathBuf> {
+    if let Some(s) = env::var_os("__CARGO_TESTS_ONLY_SRC_ROOT") {
+        return Ok(s.into());
+    }
+
     // NOTE: This is temporary until we figure out how to acquire the source.
     // If we decide to keep the sysroot probe, then BuildConfig will need to
     // be restructured so that the TargetInfo is created earlier and passed

tests/testsuite/mock-std/Cargo.toml

@@ -0,0 +1,9 @@
+[workspace]
+members = [
+  "src/libtest",
+]
+
+[patch.crates-io]
+rustc-std-workspace-std = { path = 'src/tools/rustc-std-workspace-std' }
+rustc-std-workspace-core = { path = 'src/tools/rustc-std-workspace-core' }
+rustc-std-workspace-alloc = { path = 'src/tools/rustc-std-workspace-alloc' }

tests/testsuite/mock-std/src/liballoc/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "alloc"
+version = "0.1.0"
+authors = ["Alex Crichton <alex@alexcrichton.com>"]
+edition = "2018"
+
+[lib]
+path = "lib.rs"
+
+[dependencies]
+core = { path = "../libcore" }

tests/testsuite/mock-std/src/liballoc/lib.rs

@@ -0,0 +1 @@
+pub fn custom_api() {}

tests/testsuite/mock-std/src/libcompiler_builtins/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "compiler_builtins"
+version = "0.1.0"
+authors = ["Alex Crichton <alex@alexcrichton.com>"]
+edition = "2018"
+
+[lib]
+path = "lib.rs"
+
+[dependencies]
+core = { path = "../libcore" }

tests/testsuite/mock-std/src/libcompiler_builtins/lib.rs

@@ -0,0 +1 @@
+// intentionally blank

tests/testsuite/mock-std/src/libcore/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "core"
+version = "0.1.0"
+authors = ["Alex Crichton <alex@alexcrichton.com>"]
+edition = "2018"
+
+[lib]
+path = "lib.rs"

tests/testsuite/mock-std/src/libcore/build.rs

@@ -0,0 +1,23 @@
+//! This build script is basically the whole hack that makes this entire "mock
+//! std" feature work. Here we print out `rustc-link-search` pointing to the
+//! sysroot of the actual compiler itself, and that way we can indeed implicitly
+//! pull in those crates, but only via `extern crate`. That means that we can
+//! build tiny shim core/std/etc crates while they actually load all the various
+//! language/library details from the actual crates, meaning that instead of
+//! literally compiling libstd we compile just our own tiny shims.
+
+use std::process::Command;
+use std::env;
+
+fn main() {
+    let output = Command::new("rustc")
+        .arg("--print")
+        .arg("sysroot")
+        .output()
+        .unwrap();
+    assert!(output.status.success());
+    let stdout = String::from_utf8(output.stdout).unwrap();
+    let stdout = stdout.trim();
+    let host = env::var("HOST").unwrap();
+    println!("cargo:rustc-link-search={}/lib/rustlib/{}/lib", stdout, host);
+}

tests/testsuite/mock-std/src/libcore/lib.rs

@@ -0,0 +1,4 @@
+#![no_std]
+pub use core::*;
+
+pub fn custom_api() {}

tests/testsuite/mock-std/src/libpanic_unwind/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "panic_unwind"
+version = "0.1.0"
+authors = ["Alex Crichton <alex@alexcrichton.com>"]
+edition = "2018"
+
+[lib]
+path = "lib.rs"
+
+[dependencies]
+core = { path = "../libcore" }