Implement warning for unused dependencies.

This will print a diagnostic for crates which are mentioned as `--extern`
arguments on the command line, but are never referenced from the source.

This diagnostic is controlled by `-Wunused-crate-dependencies` or
`#![warn(unused_crate_dependencies)]` and is "allow" by default.

There are cases where certain crates need to be linked in but are not
directly referenced - for example if they are providing symbols for C
linkage. In this case the warning can be suppressed with
`use needed_crate as _;`.

Thanks to @petrochenkov for simplified core.

Resolves issue #57274

src/test/ui/unused-crate-deps/auxiliary/bar.rs

@@ -0,0 +1 @@
+pub const BAR: &str = "bar";

src/test/ui/unused-crate-deps/auxiliary/foo.rs

@@ -0,0 +1,5 @@
+// edition:2018
+// aux-crate:bar=bar.rs
+
+pub const FOO: &str = "foo";
+pub use bar::BAR;

src/test/ui/unused-crate-deps/libfib.rs

@@ -0,0 +1,21 @@
+// Test warnings for a library crate
+
+// check-pass
+// aux-crate:bar=bar.rs
+// compile-flags:--crate-type lib -Wunused-crate-dependencies
+
+pub fn fib(n: u32) -> Vec<u32> {
+//~^ WARNING external crate `bar` unused in
+let mut prev = 0;
+    let mut cur = 1;
+    let mut v = vec![];
+
+    for _ in 0..n {
+        v.push(prev);
+        let n = prev + cur;
+        prev = cur;
+        cur = n;
+    }
+
+    v
+}

src/test/ui/unused-crate-deps/libfib.stderr

@@ -0,0 +1,10 @@
+warning: external crate `bar` unused in `libfib`: remove the dependency or add `use bar as _;`
+  --> $DIR/libfib.rs:7:1
+   |
+LL | pub fn fib(n: u32) -> Vec<u32> {
+   | ^
+   |
+   = note: requested on the command line with `-W unused-crate-dependencies`
+
+warning: 1 warning emitted
+

src/test/ui/unused-crate-deps/suppress.rs

@@ -0,0 +1,11 @@
+// Suppress by using crate
+
+// edition:2018
+// check-pass
+// aux-crate:bar=bar.rs
+
+#![warn(unused_crate_dependencies)]
+
+use bar as _;
+
+fn main() {}

src/test/ui/unused-crate-deps/unused-aliases.rs

@@ -0,0 +1,13 @@
+// Warn about unused aliased for the crate
+
+// edition:2018
+// check-pass
+// aux-crate:bar=bar.rs
+// aux-crate:barbar=bar.rs
+
+#![warn(unused_crate_dependencies)]
+//~^ WARNING external crate `barbar` unused in
+
+use bar as _;
+
+fn main() {}

src/test/ui/unused-crate-deps/unused-aliases.stderr

@@ -0,0 +1,14 @@
+warning: external crate `barbar` unused in `unused_aliases`: remove the dependency or add `use barbar as _;`
+  --> $DIR/unused-aliases.rs:8:1
+   |
+LL | #![warn(unused_crate_dependencies)]
+   | ^
+   |
+note: the lint level is defined here
+  --> $DIR/unused-aliases.rs:8:9
+   |
+LL | #![warn(unused_crate_dependencies)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+warning: 1 warning emitted
+

src/test/ui/unused-crate-deps/use_extern_crate_2015.rs

@@ -0,0 +1,13 @@
+// Suppress by using crate
+
+// edition:2015
+// check-pass
+// aux-crate:bar=bar.rs
+
+#![warn(unused_crate_dependencies)]
+
+extern crate bar;
+
+fn main() {
+    println!("bar {}", bar::BAR);
+}

src/test/ui/unused-crate-deps/warn-attr.rs

@@ -0,0 +1,10 @@
+// Check for unused crate dep, no path
+
+// edition:2018
+// check-pass
+// aux-crate:bar=bar.rs
+
+#![warn(unused_crate_dependencies)]
+//~^ WARNING external crate `bar` unused in
+
+fn main() {}

src/test/ui/unused-crate-deps/warn-attr.stderr

@@ -0,0 +1,14 @@
+warning: external crate `bar` unused in `warn_attr`: remove the dependency or add `use bar as _;`
+  --> $DIR/warn-attr.rs:7:1
+   |
+LL | #![warn(unused_crate_dependencies)]
+   | ^
+   |
+note: the lint level is defined here
+  --> $DIR/warn-attr.rs:7:9
+   |
+LL | #![warn(unused_crate_dependencies)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+warning: 1 warning emitted
+

src/test/ui/unused-crate-deps/warn-cmdline-static.rs

@@ -0,0 +1,10 @@
+// Check for unused crate dep, no path
+
+// edition:2018
+// check-pass
+// compile-flags: -Wunused-crate-dependencies
+// aux-crate:bar=bar.rs
+// no-prefer-dynamic
+
+fn main() {}
+//~^ WARNING external crate `bar` unused in

src/test/ui/unused-crate-deps/warn-cmdline-static.stderr

@@ -0,0 +1,10 @@
+warning: external crate `bar` unused in `warn_cmdline_static`: remove the dependency or add `use bar as _;`
+  --> $DIR/warn-cmdline-static.rs:9:1
+   |
+LL | fn main() {}
+   | ^
+   |
+   = note: requested on the command line with `-W unused-crate-dependencies`
+
+warning: 1 warning emitted
+

src/test/ui/unused-crate-deps/warn-cmdline.rs

@@ -0,0 +1,9 @@
+// Check for unused crate dep, no path
+
+// edition:2018
+// check-pass
+// compile-flags: -Wunused-crate-dependencies
+// aux-crate:bar=bar.rs
+
+fn main() {}
+//~^ WARNING external crate `bar` unused in

src/test/ui/unused-crate-deps/warn-cmdline.stderr

@@ -0,0 +1,10 @@
+warning: external crate `bar` unused in `warn_cmdline`: remove the dependency or add `use bar as _;`
+  --> $DIR/warn-cmdline.rs:8:1
+   |
+LL | fn main() {}
+   | ^
+   |
+   = note: requested on the command line with `-W unused-crate-dependencies`
+
+warning: 1 warning emitted
+