polymorphize: remove predicate logic

This commit removes all logic which marks parameters as used based on
their presence in predicates - given rust-lang/rust#75675, this will
enable more polymorphization and avoid the symbol clashes that predicate
logic previously sidestepped.

Signed-off-by: David Wood <david@davidtw.co>

compiler/rustc_monomorphize/src/polymorphize.rs

@@ -89,9 +89,6 @@ fn unused_generic_params(tcx: TyCtxt<'_>, def_id: DefId) -> FiniteBitSet<u32> {
     };
     let mut vis = MarkUsedGenericParams { tcx, def_id, unused_parameters: &mut unused_parameters };
     vis.visit_body(body);
-    debug!(?unused_parameters, "(after visitor)");
-
-    mark_used_by_predicates(tcx, def_id, &mut unused_parameters);
     debug!(?unused_parameters, "(end)");
 
     // Emit errors for debugging and testing if enabled.
@@ -161,44 +158,6 @@ fn mark_used_by_default_parameters<'tcx>(
     }
 }
 
-/// Search the predicates on used generic parameters for any unused generic parameters, and mark
-/// those as used.
-#[instrument(level = "debug", skip(tcx, def_id))]
-fn mark_used_by_predicates<'tcx>(
-    tcx: TyCtxt<'tcx>,
-    def_id: DefId,
-    unused_parameters: &mut FiniteBitSet<u32>,
-) {
-    let def_id = tcx.closure_base_def_id(def_id);
-    let predicates = tcx.explicit_predicates_of(def_id);
-
-    let mut current_unused_parameters = FiniteBitSet::new_empty();
-    // Run to a fixed point to support `where T: Trait<U>, U: Trait<V>`, starting with an empty
-    // bit set so that this is skipped if all parameters are already used.
-    while current_unused_parameters != *unused_parameters {
-        debug!(?current_unused_parameters, ?unused_parameters);
-        current_unused_parameters = *unused_parameters;
-
-        for (predicate, _) in predicates.predicates {
-            // Consider all generic params in a predicate as used if any other parameter in the
-            // predicate is used.
-            let any_param_used = {
-                let mut vis = HasUsedGenericParams { tcx, unused_parameters };
-                predicate.visit_with(&mut vis).is_break()
-            };
-
-            if any_param_used {
-                let mut vis = MarkUsedGenericParams { tcx, def_id, unused_parameters };
-                predicate.visit_with(&mut vis);
-            }
-        }
-    }
-
-    if let Some(parent) = predicates.parent {
-        mark_used_by_predicates(tcx, parent, unused_parameters);
-    }
-}
-
 /// Emit errors for the function annotated by `#[rustc_polymorphize_error]`, labelling each generic
 /// parameter which was unused.
 #[instrument(level = "debug", skip(tcx, generics))]

src/test/ui/polymorphization/predicates.rs

@@ -12,6 +12,7 @@ fn bar<I>() {
 
 #[rustc_polymorphize_error]
 fn foo<I, T>(_: I)
+//~^ ERROR item has unused generic parameters
 where
     I: Iterator<Item = T>,
 {
@@ -20,6 +21,7 @@ where
 
 #[rustc_polymorphize_error]
 fn baz<I, T>(_: I)
+//~^ ERROR item has unused generic parameters
 where
     std::iter::Repeat<I>: Iterator<Item = T>,
 {
@@ -40,6 +42,7 @@ where
     #[rustc_polymorphize_error]
     fn next(&mut self) -> Option<Self::Item> {
         self.find(|_| true)
+        //~^ ERROR item has unused generic parameters
     }
 }
 
@@ -53,6 +56,7 @@ impl Baz<u32> for u16 {}
 
 #[rustc_polymorphize_error]
 fn quux<A, B, C: Default>() -> usize
+//~^ ERROR item has unused generic parameters
 where
     A: Baz<B>,
     B: Baz<C>,
@@ -69,6 +73,7 @@ impl Foobar<u32, u32> for () {}
 
 #[rustc_polymorphize_error]
 fn foobar<F, G>() -> usize
+//~^ ERROR item has unused generic parameters
 where
     (): Foobar<F, G>,
 {

src/test/ui/polymorphization/predicates.stderr

@@ -1,8 +1,45 @@
+error: item has unused generic parameters
+  --> $DIR/predicates.rs:14:4
+   |
+LL | fn foo<I, T>(_: I)
+   |    ^^^    - generic parameter `T` is unused
+
+error: item has unused generic parameters
+  --> $DIR/predicates.rs:23:4
+   |
+LL | fn baz<I, T>(_: I)
+   |    ^^^    - generic parameter `T` is unused
+
+error: item has unused generic parameters
+  --> $DIR/predicates.rs:44:19
+   |
+LL | impl<'a, I, T: 'a, E> Iterator for Foo<'a, I, E>
+   |          -         - generic parameter `E` is unused
+   |          |
+   |          generic parameter `I` is unused
+...
+LL |         self.find(|_| true)
+   |                   ^^^^^^^^
+
+error: item has unused generic parameters
+  --> $DIR/predicates.rs:58:4
+   |
+LL | fn quux<A, B, C: Default>() -> usize
+   |    ^^^^ -  - generic parameter `B` is unused
+   |         |
+   |         generic parameter `A` is unused
+
+error: item has unused generic parameters
+  --> $DIR/predicates.rs:75:4
+   |
+LL | fn foobar<F, G>() -> usize
+   |    ^^^^^^ - generic parameter `F` is unused
+
 error: item has unused generic parameters
   --> $DIR/predicates.rs:9:4
    |
 LL | fn bar<I>() {
    |    ^^^ - generic parameter `I` is unused
 
-error: aborting due to previous error
+error: aborting due to 6 previous errors