Add support for nounused --extern flag

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

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

compiler/rustc_interface/src/tests.rs

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

compiler/rustc_metadata/src/creader.rs

@@ -907,6 +907,10 @@ impl<'a> CrateLoader<'a> {
                 // Don't worry about pathless `--extern foo` sysroot references
                 continue;
             }
+            if entry.nounused_dep {
+                // We're not worried about this one
+                continue;
+            }
             let name_interned = Symbol::intern(name);
             if self.used_extern_options.contains(&name_interned) {
                 continue;

compiler/rustc_session/src/config.rs

@@ -474,6 +474,11 @@ pub struct ExternEntry {
     /// This can be disabled with the `noprelude` option like
     /// `--extern noprelude:name`.
     pub add_prelude: bool,
+    /// The extern entry shouldn't be considered for unused dependency warnings.
+    ///
+    /// `--extern nounused:std=/path/to/lib/libstd.rlib`. This is used to
+    /// suppress `unused-crate-dependencies` warnings.
+    pub nounused_dep: bool,
 }
 
 #[derive(Clone, Debug)]
@@ -512,7 +517,7 @@ 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, nounused_dep: false }
     }
 
     pub fn files(&self) -> Option<impl Iterator<Item = &CanonicalizedPath>> {
@@ -2131,6 +2136,7 @@ pub fn parse_externs(
 
         let mut is_private_dep = false;
         let mut add_prelude = true;
+        let mut nounused_dep = false;
         if let Some(opts) = options {
             if !is_unstable_enabled {
                 early_error(
@@ -2152,6 +2158,7 @@ pub fn parse_externs(
                             );
                         }
                     }
+                    "nounused" => nounused_dep = true,
                     _ => early_error(error_format, &format!("unknown --extern option `{opt}`")),
                 }
             }
@@ -2160,6 +2167,8 @@ 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 `nounused`
+        entry.nounused_dep |= nounused_dep;
         // If any flag is missing `noprelude`, then add to the prelude.
         entry.add_prelude |= add_prelude;
     }

src/test/ui/extern-flag/no-nounused.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_nounused`
+}

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

@@ -0,0 +1,10 @@
+error: external crate `somedep` unused in `no_nounused`: remove the dependency or add `use somedep as _;`
+  --> $DIR/no-nounused.rs:5:1
+   |
+LL | fn main() {
+   | ^
+   |
+   = note: requested on the command line with `-D unused-crate-dependencies`
+
+error: aborting due to previous error
+

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

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