Check FromIterator trait impl in prelude collision check.

compiler/rustc_span/src/symbol.rs

@@ -155,6 +155,7 @@ symbols! {
         FormatSpec,
         Formatter,
         From,
+        FromIterator,
         Future,
         FxHashMap,
         FxHashSet,

compiler/rustc_typeck/src/check/method/prelude2021.rs

@@ -4,11 +4,13 @@ use hir::ItemKind;
 use rustc_ast::Mutability;
 use rustc_errors::Applicability;
 use rustc_hir as hir;
+use rustc_middle::ty::subst::InternalSubsts;
 use rustc_middle::ty::{Ref, Ty};
 use rustc_session::lint::builtin::RUST_2021_PRELUDE_COLLISIONS;
 use rustc_span::symbol::kw::Underscore;
 use rustc_span::symbol::{sym, Ident};
 use rustc_span::Span;
+use rustc_trait_selection::infer::InferCtxtExt;
 
 use crate::check::{
     method::probe::{self, Pick},
@@ -206,6 +208,25 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return;
         }
 
+        // For from_iter, check if the type actually implements FromIterator.
+        // If we know it does not, we don't need to warn.
+        if method_name.name == sym::from_iter {
+            if let Some(trait_def_id) = self.tcx.get_diagnostic_item(sym::FromIterator) {
+                if !self
+                    .infcx
+                    .type_implements_trait(
+                        trait_def_id,
+                        self_ty,
+                        InternalSubsts::empty(),
+                        self.param_env,
+                    )
+                    .may_apply()
+                {
+                    return;
+                }
+            }
+        }
+
         // No need to lint if this is an inherent method called on a specific type, like `Vec::foo(...)`,
         // since such methods take precedence over trait methods.
         if matches!(pick.kind, probe::PickKind::InherentImplPick) {

library/core/src/iter/traits/collect.rs

@@ -89,6 +89,7 @@
                over elements of type `{A}`",
     label = "value of type `{Self}` cannot be built from `std::iter::Iterator<Item={A}>`"
 )]
+#[rustc_diagnostic_item = "FromIterator"]
 pub trait FromIterator<A>: Sized {
     /// Creates a value from an iterator.
     ///

src/test/ui/rust-2021/future-prelude-collision-unneeded.rs

@@ -11,8 +11,48 @@ impl S {
     }
 }
 
-// See https://github.com/rust-lang/rust/issues/86633
+struct X;
+
+trait Hey {
+    fn from_iter(_: i32) -> Self;
+}
+
+impl Hey for X {
+    fn from_iter(_: i32) -> Self {
+        X
+    }
+}
+
+struct Y<T>(T);
+
+impl Hey for Y<i32> {
+    fn from_iter(_: i32) -> Self {
+        Y(0)
+    }
+}
+
+struct Z<T>(T);
+
+impl Hey for Z<i32> {
+    fn from_iter(_: i32) -> Self {
+        Z(0)
+    }
+}
+
+impl std::iter::FromIterator<u32> for Z<u32> {
+    fn from_iter<T: IntoIterator<Item = u32>>(_: T) -> Self {
+        todo!()
+    }
+}
+
 fn main() {
+    // See https://github.com/rust-lang/rust/issues/86633
     let s = S;
     let s2 = s.try_into();
+
+    // See https://github.com/rust-lang/rust/issues/86902
+    X::from_iter(1);
+    Y::from_iter(1);
+    Y::<i32>::from_iter(1);
+    Z::<i32>::from_iter(1);
 }