Teaches Cargo to source all std dependencies from vendored sources in the rust-src
component, making -Zbuild-std compatible with vendored builds.

This was originally landed in https://github.com/rust-lang/cargo/pull/8834
but was backed out for causing breakage in other situations. It works fine
for Firefox's usecase, though.

Most of these changes just add/edit tests for the functionality. Only the
change to src/cargo/core/compiler/standard_lib.rs is important.

diff --git a/src/cargo/core/compiler/standard_lib.rs b/src/cargo/core/compiler/standard_lib.rs
index e0baebd51..547b84147 100644
--- a/src/cargo/core/compiler/standard_lib.rs
+++ b/src/cargo/core/compiler/standard_lib.rs
@@ -12,6 +12,7 @@ use crate::util::errors::CargoResult;
 use crate::Config;
 use std::collections::{HashMap, HashSet};
 use std::env;
+use std::fs;
 use std::path::PathBuf;
 
 use super::BuildConfig;
@@ -74,27 +75,45 @@ pub fn resolve_std<'cfg>(
     }
 
     let src_path = detect_sysroot_src_path(target_data)?;
-    let to_patch = [
-        "rustc-std-workspace-core",
-        "rustc-std-workspace-alloc",
-        "rustc-std-workspace-std",
-    ];
-    let patches = to_patch
-        .iter()
-        .map(|&name| {
-            let source_path = SourceId::for_path(&src_path.join("library").join(name))?;
-            let dep = Dependency::parse(name, None, source_path)?;
+
+    // Special std packages should be pulled from `library/` and should be
+    // prefixed with `rustc-std-workspace-` in certain places.
+    let libs_prefix = "library/";
+    let special_std_prefix = "rustc-std-workspace-";
+    let libs_path = src_path.join(libs_prefix);
+
+    // Crates in rust-src to build. libtest is in some sense the "root" package
+    // of std, as nothing else depends on it, so it must be explicitly added.
+    let mut members = vec![format!("{}test", libs_prefix)];
+
+    // If rust-src contains a "vendor" directory, then patch in all the crates it contains.
+    let vendor_path = src_path.join("vendor");
+    let vendor_dir = fs::read_dir(vendor_path)?;
+    let patches = vendor_dir
+        .into_iter()
+        .map(|entry| {
+            let entry = entry?;
+            let name = entry
+                .file_name()
+                .into_string()
+                .map_err(|_| anyhow::anyhow!("package name wasn't utf8"))?;
+
+            // Remap the rustc-std-workspace crates to the actual rust-src libraries
+            let path = if let Some(real_name) = name.strip_prefix(special_std_prefix) {
+                // Record this crate as something to build in the workspace
+                members.push(format!("{}{}", libs_prefix, real_name));
+                libs_path.join(&name)
+            } else {
+                entry.path()
+            };
+            let source_path = SourceId::for_path(&path)?;
+            let dep = Dependency::parse(&name, None, source_path)?;
             Ok(dep)
         })
         .collect::<CargoResult<Vec<_>>>()?;
+
     let crates_io_url = crate::sources::CRATES_IO_INDEX.parse().unwrap();
     let patch = HashMap::from([(crates_io_url, patches)]);
-    let members = vec![
-        String::from("library/std"),
-        String::from("library/core"),
-        String::from("library/alloc"),
-        String::from("library/test"),
-    ];
     let ws_config = crate::core::WorkspaceConfig::Root(crate::core::WorkspaceRootConfig::new(
         &src_path,
         &Some(members),
diff --git a/tests/testsuite/mock-std/library/test/Cargo.toml b/tests/testsuite/mock-std/library/test/Cargo.toml
index 299db7bfd..eb81bbf5e 100644
--- a/tests/testsuite/mock-std/library/test/Cargo.toml
+++ b/tests/testsuite/mock-std/library/test/Cargo.toml
@@ -10,6 +10,7 @@ std = { path = "../std" }
 panic_unwind = { path = "../panic_unwind" }
 compiler_builtins = { path = "../compiler_builtins" }
 registry-dep-using-std = { version = "*", features = ['mockbuild'] }
+registry-dep-only-used-by-test = { version = "*" }
 
 [features]
 panic-unwind = []
diff --git a/tests/testsuite/mock-std/library/test/src/lib.rs b/tests/testsuite/mock-std/library/test/src/lib.rs
index a112855f5..224b89bb2 100644
--- a/tests/testsuite/mock-std/library/test/src/lib.rs
+++ b/tests/testsuite/mock-std/library/test/src/lib.rs
@@ -7,4 +7,5 @@ extern crate test;
 pub use test::*;
 
 pub fn custom_api() {
+    registry_dep_only_used_by_test::wow_testing_is_so_easy();
 }
diff --git a/tests/testsuite/mock-std/vendor/registry-dep-only-used-by-test/Cargo.toml b/tests/testsuite/mock-std/vendor/registry-dep-only-used-by-test/Cargo.toml
new file mode 100644
index 000000000..31ba65a98
--- /dev/null
+++ b/tests/testsuite/mock-std/vendor/registry-dep-only-used-by-test/Cargo.toml
@@ -0,0 +1,9 @@
+[package]
+name = "registry-dep-only-used-by-test"
+version = "1.0.0"
+authors = ["Alex Crichton <alex@alexcrichton.com>"]
+edition = "2018"
+
+[dependencies]
+
+[features]
diff --git a/tests/testsuite/mock-std/vendor/registry-dep-only-used-by-test/src/lib.rs b/tests/testsuite/mock-std/vendor/registry-dep-only-used-by-test/src/lib.rs
new file mode 100644
index 000000000..a68d2aeef
--- /dev/null
+++ b/tests/testsuite/mock-std/vendor/registry-dep-only-used-by-test/src/lib.rs
@@ -0,0 +1,2 @@
+pub fn wow_testing_is_so_easy() {
+}
\ No newline at end of file
diff --git a/tests/testsuite/mock-std/vendor/registry-dep-using-alloc/Cargo.toml b/tests/testsuite/mock-std/vendor/registry-dep-using-alloc/Cargo.toml
new file mode 100644
index 000000000..f7e4ab232
--- /dev/null
+++ b/tests/testsuite/mock-std/vendor/registry-dep-using-alloc/Cargo.toml
@@ -0,0 +1,12 @@
+[package]
+name = "registry-dep-using-alloc"
+version = "1.0.0"
+authors = ["Alex Crichton <alex@alexcrichton.com>"]
+edition = "2018"
+
+[dependencies]
+rustc-std-workspace-alloc = { version = "*", optional = true }
+rustc-std-workspace-core = { version = "*", optional = true }
+
+[features]
+mockbuild = ["rustc-std-workspace-alloc", "rustc-std-workspace-core"]
\ No newline at end of file
diff --git a/tests/testsuite/mock-std/vendor/registry-dep-using-alloc/src/lib.rs b/tests/testsuite/mock-std/vendor/registry-dep-using-alloc/src/lib.rs
new file mode 100644
index 000000000..b9ab30339
--- /dev/null
+++ b/tests/testsuite/mock-std/vendor/registry-dep-using-alloc/src/lib.rs
@@ -0,0 +1,9 @@
+#[cfg(feature = "mockbuild")]
+pub fn custom_api() {
+}
+
+#[cfg(not(feature = "mockbuild"))]
+pub fn non_sysroot_api() {
+    core::custom_api();
+    alloc::custom_api();
+}
\ No newline at end of file
diff --git a/tests/testsuite/mock-std/vendor/registry-dep-using-core/Cargo.toml b/tests/testsuite/mock-std/vendor/registry-dep-using-core/Cargo.toml
new file mode 100644
index 000000000..befb83a63
--- /dev/null
+++ b/tests/testsuite/mock-std/vendor/registry-dep-using-core/Cargo.toml
@@ -0,0 +1,11 @@
+[package]
+name = "registry-dep-using-core"
+version = "1.0.0"
+authors = ["Alex Crichton <alex@alexcrichton.com>"]
+edition = "2018"
+
+[dependencies]
+rustc-std-workspace-core = { version = "*", optional = true }
+
+[features]
+mockbuild = ["rustc-std-workspace-core"]
\ No newline at end of file
diff --git a/tests/testsuite/mock-std/vendor/registry-dep-using-core/src/lib.rs b/tests/testsuite/mock-std/vendor/registry-dep-using-core/src/lib.rs
new file mode 100644
index 000000000..f9dbac0f4
--- /dev/null
+++ b/tests/testsuite/mock-std/vendor/registry-dep-using-core/src/lib.rs
@@ -0,0 +1,8 @@
+#[cfg(feature = "mockbuild")]
+pub fn custom_api() {
+}
+
+#[cfg(not(feature = "mockbuild"))]
+pub fn non_sysroot_api() {
+    core::custom_api();
+}
\ No newline at end of file
diff --git a/tests/testsuite/mock-std/vendor/registry-dep-using-std/Cargo.toml b/tests/testsuite/mock-std/vendor/registry-dep-using-std/Cargo.toml
new file mode 100644
index 000000000..71ef0a42f
--- /dev/null
+++ b/tests/testsuite/mock-std/vendor/registry-dep-using-std/Cargo.toml
@@ -0,0 +1,11 @@
+[package]
+name = "registry-dep-using-std"
+version = "1.0.0"
+authors = ["Alex Crichton <alex@alexcrichton.com>"]
+edition = "2018"
+
+[dependencies]
+rustc-std-workspace-std = { version = "*", optional = true }
+
+[features]
+mockbuild = ["rustc-std-workspace-std"]
\ No newline at end of file
diff --git a/tests/testsuite/mock-std/vendor/registry-dep-using-std/src/lib.rs b/tests/testsuite/mock-std/vendor/registry-dep-using-std/src/lib.rs
new file mode 100644
index 000000000..f3af39178
--- /dev/null
+++ b/tests/testsuite/mock-std/vendor/registry-dep-using-std/src/lib.rs
@@ -0,0 +1,8 @@
+#[cfg(feature = "mockbuild")]
+pub fn custom_api() {
+}
+
+#[cfg(not(feature = "mockbuild"))]
+pub fn non_sysroot_api() {
+    std::custom_api();
+}
\ No newline at end of file
diff --git a/tests/testsuite/mock-std/vendor/rustc-std-workspace-alloc/Cargo.toml b/tests/testsuite/mock-std/vendor/rustc-std-workspace-alloc/Cargo.toml
new file mode 100644
index 000000000..4465a08a8
--- /dev/null
+++ b/tests/testsuite/mock-std/vendor/rustc-std-workspace-alloc/Cargo.toml
@@ -0,0 +1 @@
+this file shouldn't be read 
\ No newline at end of file
diff --git a/tests/testsuite/mock-std/vendor/rustc-std-workspace-core/Cargo.toml b/tests/testsuite/mock-std/vendor/rustc-std-workspace-core/Cargo.toml
new file mode 100644
index 000000000..4465a08a8
--- /dev/null
+++ b/tests/testsuite/mock-std/vendor/rustc-std-workspace-core/Cargo.toml
@@ -0,0 +1 @@
+this file shouldn't be read 
\ No newline at end of file
diff --git a/tests/testsuite/mock-std/vendor/rustc-std-workspace-std/Cargo.toml b/tests/testsuite/mock-std/vendor/rustc-std-workspace-std/Cargo.toml
new file mode 100644
index 000000000..4465a08a8
--- /dev/null
+++ b/tests/testsuite/mock-std/vendor/rustc-std-workspace-std/Cargo.toml
@@ -0,0 +1 @@
+this file shouldn't be read 
\ No newline at end of file
diff --git a/tests/testsuite/standard_lib.rs b/tests/testsuite/standard_lib.rs
index d3be303ea..486a9b4e0 100644
--- a/tests/testsuite/standard_lib.rs
+++ b/tests/testsuite/standard_lib.rs
@@ -15,71 +15,18 @@ struct Setup {
 }
 
 fn setup() -> Setup {
-    // Our mock sysroot requires a few packages from crates.io, so make sure
-    // they're "published" to crates.io. Also edit their code a bit to make sure
-    // that they have access to our custom crates with custom apis.
+    // Register a version of one of the std dependencies that doesn't compile.
+    // This ensures that the mock-std's vendor is actually being used.
     Package::new("registry-dep-using-core", "1.0.0")
         .file(
             "src/lib.rs",
             "
-                #![no_std]
-
-                #[cfg(feature = \"mockbuild\")]
-                pub fn custom_api() {
-                }
-
-                #[cfg(not(feature = \"mockbuild\"))]
-                pub fn non_sysroot_api() {
-                    core::custom_api();
-                }
+               don't compile me bro!!
             ",
         )
         .add_dep(Dependency::new("rustc-std-workspace-core", "*").optional(true))
         .feature("mockbuild", &["rustc-std-workspace-core"])
         .publish();
-    Package::new("registry-dep-using-alloc", "1.0.0")
-        .file(
-            "src/lib.rs",
-            "
-                #![no_std]
-
-                extern crate alloc;
-
-                #[cfg(feature = \"mockbuild\")]
-                pub fn custom_api() {
-                }
-
-                #[cfg(not(feature = \"mockbuild\"))]
-                pub fn non_sysroot_api() {
-                    core::custom_api();
-                    alloc::custom_api();
-                }
-            ",
-        )
-        .add_dep(Dependency::new("rustc-std-workspace-core", "*").optional(true))
-        .add_dep(Dependency::new("rustc-std-workspace-alloc", "*").optional(true))
-        .feature(
-            "mockbuild",
-            &["rustc-std-workspace-core", "rustc-std-workspace-alloc"],
-        )
-        .publish();
-    Package::new("registry-dep-using-std", "1.0.0")
-        .file(
-            "src/lib.rs",
-            "
-                #[cfg(feature = \"mockbuild\")]
-                pub fn custom_api() {
-                }
-
-                #[cfg(not(feature = \"mockbuild\"))]
-                pub fn non_sysroot_api() {
-                    std::custom_api();
-                }
-            ",
-        )
-        .add_dep(Dependency::new("rustc-std-workspace-std", "*").optional(true))
-        .feature("mockbuild", &["rustc-std-workspace-std"])
-        .publish();
 
     let p = ProjectBuilder::new(paths::root().join("rustc-wrapper"))
         .file(
@@ -335,6 +282,81 @@ fn depend_same_as_std() {
 fn test() {
     let setup = setup();
 
+    // Our mock sysroot requires a few packages from crates.io, so make sure
+    // they're "published" to crates.io. Also edit their code a bit to make sure
+    // that they have access to our custom crates with custom apis.
+    Package::new("registry-dep-using-core", "1.0.0")
+        .file(
+            "src/lib.rs",
+            "
+                #![no_std]
+
+                #[cfg(feature = \"mockbuild\")]
+                pub fn custom_api() {
+                }
+
+                #[cfg(not(feature = \"mockbuild\"))]
+                pub fn non_sysroot_api() {
+                    core::custom_api();
+                }
+            ",
+        )
+        .add_dep(Dependency::new("rustc-std-workspace-core", "*").optional(true))
+        .feature("mockbuild", &["rustc-std-workspace-core"])
+        .publish();
+    Package::new("registry-dep-using-alloc", "1.0.0")
+        .file(
+            "src/lib.rs",
+            "
+                #![no_std]
+
+                extern crate alloc;
+
+                #[cfg(feature = \"mockbuild\")]
+                pub fn custom_api() {
+                }
+
+                #[cfg(not(feature = \"mockbuild\"))]
+                pub fn non_sysroot_api() {
+                    core::custom_api();
+                    alloc::custom_api();
+                }
+            ",
+        )
+        .add_dep(Dependency::new("rustc-std-workspace-core", "*").optional(true))
+        .add_dep(Dependency::new("rustc-std-workspace-alloc", "*").optional(true))
+        .feature(
+            "mockbuild",
+            &["rustc-std-workspace-core", "rustc-std-workspace-alloc"],
+        )
+        .publish();
+    Package::new("registry-dep-using-std", "1.0.0")
+        .file(
+            "src/lib.rs",
+            "
+                #[cfg(feature = \"mockbuild\")]
+                pub fn custom_api() {
+                }
+
+                #[cfg(not(feature = \"mockbuild\"))]
+                pub fn non_sysroot_api() {
+                    std::custom_api();
+                }
+            ",
+        )
+        .add_dep(Dependency::new("rustc-std-workspace-std", "*").optional(true))
+        .feature("mockbuild", &["rustc-std-workspace-std"])
+        .publish();
+    Package::new("registry-dep-only-used-by-test", "1.0.0")
+        .file(
+            "src/lib.rs",
+            "
+                pub fn wow_testing_is_so_easy() {
+                }
+            ",
+        )
+        .publish();
+
     let p = project()
         .file(
             "src/lib.rs",