Add support for sysroot --extern flag

This adds `sysroot` to the set of extern flags:
`--extern sysroot:core=/path/to/core/libcore.rlib`.

The primary effect of this flag is to suppress `unused-crate-dependencies`
warnings relating to the crate.

It's used for build environments where it's desirable for the build
system to explicitly specify dependencies on sysroot crates rather than
letting rustc find them implicitly. In such cases, these dependencies
are likely to be added unconditionally, so there's no way that a user
could act on an unused dependency warning.

Author: jsgf

compiler/rustc_interface/src/tests.rs

@@ -66,6 +66,7 @@ where
         location: ExternLocation::ExactPaths(locations),
         is_private_dep: false,
         add_prelude: true,
+        sysroot_dep: false,
     }
 }
 

compiler/rustc_metadata/src/creader.rs

@@ -908,6 +908,10 @@ impl<'a> CrateLoader<'a> {
                 // Don't worry about pathless `--extern foo` sysroot references
                 continue;
             }
+            if entry.sysroot_dep {
+                // If it's an explicit sysroot dependency, let it slide
+                continue;
+            }
             let name_interned = Symbol::intern(name);
             if self.used_extern_options.contains(&name_interned) {
                 continue;

compiler/rustc_session/src/config.rs

@@ -477,6 +477,14 @@ pub struct ExternEntry {
     /// This can be disabled with the `noprelude` option like
     /// `--extern noprelude:name`.
     pub add_prelude: bool,
+    /// The extern entry is an explicitly listed sysroot crate.
+    ///
+    /// `--extern sysroot:std=/path/to/lib/libstd.rlib`
+    /// This is useful for when the build system is using explicit sysroot
+    /// dependencies rather than allowing rustc to find them implicitly. This is
+    /// primarily used to suppress `unused-crate-dependencies` warnings, since
+    /// these dependencies are added unconditionally.
+    pub sysroot_dep: bool,
 }
 
 #[derive(Clone, Debug)]
@@ -536,7 +544,7 @@ impl Externs {
 
 impl ExternEntry {
     fn new(location: ExternLocation) -> ExternEntry {
-        ExternEntry { location, is_private_dep: false, add_prelude: false }
+        ExternEntry { location, is_private_dep: false, add_prelude: false, sysroot_dep: false }
     }
 
     pub fn files(&self) -> Option<impl Iterator<Item = &CanonicalizedPath>> {
@@ -2186,6 +2194,7 @@ pub fn parse_externs(
 
         let mut is_private_dep = false;
         let mut add_prelude = true;
+        let mut sysroot_dep = false;
         if let Some(opts) = options {
             if !is_unstable_enabled {
                 early_error(
@@ -2207,6 +2216,7 @@ pub fn parse_externs(
                             );
                         }
                     }
+                    "sysroot" => sysroot_dep = true,
                     _ => early_error(error_format, &format!("unknown --extern option `{opt}`")),
                 }
             }
@@ -2215,6 +2225,8 @@ pub fn parse_externs(
         // Crates start out being not private, and go to being private `priv`
         // is specified.
         entry.is_private_dep |= is_private_dep;
+        // likewise `sysroot`
+        entry.sysroot_dep |= sysroot_dep;
         // If any flag is missing `noprelude`, then add to the prelude.
         entry.add_prelude |= add_prelude;
     }

src/test/ui/extern-flag/no-sysroot.rs

@@ -0,0 +1,6 @@
+// aux-crate:somedep=somedep.rs
+// compile-flags: -Zunstable-options -Dunused-crate-dependencies
+// edition:2018
+
+fn main() { //~ ERROR external crate `somedep` unused in `no_sysroot`
+}

src/test/ui/extern-flag/no-sysroot.stderr

@@ -0,0 +1,11 @@
+error: external crate `somedep` unused in `no_sysroot`: remove the dependency or add `use somedep as _;`
+  --> $DIR/no-sysroot.rs:5:1
+   |
+LL | fn main() {
+   | ^
+   |
+   = note: requested on the command line with `-D unused-crate-dependencies`
+   = help: remove unnecessary dependency `somedep`
+
+error: aborting due to previous error
+

src/test/ui/extern-flag/sysroot.rs

@@ -0,0 +1,7 @@
+// check-pass
+// aux-crate:sysroot:somedep=somedep.rs
+// compile-flags: -Zunstable-options -Dunused-crate-dependencies
+// edition:2018
+
+fn main() {
+}