Refactor the two-phase check for impls and impl items

compiler/rustc_middle/src/query/mod.rs

@@ -1137,7 +1137,7 @@ rustc_queries! {
     /// their respective impl (i.e., part of the derive macro)
     query live_symbols_and_ignored_derived_traits(_: ()) -> &'tcx (
         LocalDefIdSet,
-        LocalDefIdMap<Vec<(DefId, DefId)>>
+        LocalDefIdMap<FxIndexSet<(DefId, DefId)>>
     ) {
         arena_cache
         desc { "finding live symbols in crate" }

src/bootstrap/src/utils/exec.rs

@@ -332,16 +332,19 @@ impl Default for CommandOutput {
 
 /// Helper trait to format both Command and BootstrapCommand as a short execution line,
 /// without all the other details (e.g. environment variables).
+#[cfg(feature = "tracing")]
 pub trait FormatShortCmd {
     fn format_short_cmd(&self) -> String;
 }
 
+#[cfg(feature = "tracing")]
 impl FormatShortCmd for BootstrapCommand {
     fn format_short_cmd(&self) -> String {
         self.command.format_short_cmd()
     }
 }
 
+#[cfg(feature = "tracing")]
 impl FormatShortCmd for Command {
     fn format_short_cmd(&self) -> String {
         let program = Path::new(self.get_program());

tests/ui/derives/clone-debug-dead-code.stderr

@@ -40,7 +40,7 @@ LL | struct D { f: () }
    |        |
    |        field in this struct
    |
-   = note: `D` has derived impls for the traits `Clone` and `Debug`, but these are intentionally ignored during dead code analysis
+   = note: `D` has derived impls for the traits `Debug` and `Clone`, but these are intentionally ignored during dead code analysis
 
 error: field `f` is never read
   --> $DIR/clone-debug-dead-code.rs:21:12

tests/ui/deriving/deriving-in-macro.rs

@@ -1,5 +1,6 @@
-//@ run-pass
+//@ check-pass
 #![allow(non_camel_case_types)]
+#![allow(dead_code)]
 
 macro_rules! define_vec {
     () => (

tests/ui/lint/dead-code/alias-type-used-as-generic-arg-in-impl.rs

@@ -0,0 +1,19 @@
+//@ check-pass
+
+#![deny(dead_code)]
+
+struct T<X>(X);
+
+type A<X> = T<X>;
+
+trait Tr {
+    fn foo();
+}
+
+impl<X> Tr for T<A<X>> {
+    fn foo() {}
+}
+
+fn main() {
+   T::<T<()>>::foo();
+}

tests/ui/lint/dead-code/issue-41883.stderr

@@ -29,6 +29,8 @@ error: struct `UnusedStruct` is never constructed
    |
 LL |     struct UnusedStruct;
    |            ^^^^^^^^^^^^
+   |
+   = note: `UnusedStruct` has a derived impl for the trait `Debug`, but this is intentionally ignored during dead code analysis
 
 error: aborting due to 4 previous errors
 

tests/ui/lint/dead-code/multiple-dead-codes-in-the-same-struct.stderr

@@ -56,6 +56,8 @@ warning: struct `Foo` is never constructed
    |
 LL | struct Foo(usize, #[allow(unused)] usize);
    |        ^^^
+   |
+   = note: `Foo` has a derived impl for the trait `Debug`, but this is intentionally ignored during dead code analysis
 
 error: aborting due to 2 previous errors; 2 warnings emitted
 

tests/ui/lint/dead-code/trait-only-used-as-type-bound.rs

@@ -0,0 +1,31 @@
+//@ check-pass
+
+#![deny(dead_code)]
+
+trait UInt: Copy + From<u8> {}
+
+impl UInt for u16 {}
+
+trait Int: Copy {
+    type Unsigned: UInt;
+
+    fn as_unsigned(self) -> Self::Unsigned;
+}
+
+impl Int for i16 {
+    type Unsigned = u16;
+
+    fn as_unsigned(self) -> u16 {
+        self as _
+    }
+}
+
+fn priv_func<T: Int>(x: u8, y: T) -> (T::Unsigned, T::Unsigned) {
+    (T::Unsigned::from(x), y.as_unsigned())
+}
+
+pub fn pub_func(x: u8, y: i16) -> (u16, u16) {
+    priv_func(x, y)
+}
+
+fn main() {}