Do not show spans from external crates in missing_trait_methods

Pointing to the missing method definition in external crates will use
paths which depend on the system, and even on the Rust compiler version
used when the iterator is defined in the standard library.

When the trait being implemented and whose method is missing is
external, point only to the trait implementation. The user will be able
to figure out, or even navigate using their IDE, to the proper
definition.

As a side-effect, this will remove a large lintcheck churn at every Rustup 
for this lint.

clippy_lints/src/missing_trait_methods.rs

@@ -1,6 +1,7 @@
 use clippy_utils::diagnostics::span_lint_and_then;
 use clippy_utils::is_lint_allowed;
 use clippy_utils::macros::span_is_local;
+use clippy_utils::source::snippet_opt;
 use rustc_hir::def_id::DefIdSet;
 use rustc_hir::{Impl, Item, ItemKind};
 use rustc_lint::{LateContext, LateLintPass};
@@ -84,7 +85,14 @@ impl<'tcx> LateLintPass<'tcx> for MissingTraitMethods {
                     cx.tcx.def_span(item.owner_id),
                     format!("missing trait method provided by default: `{}`", assoc.name()),
                     |diag| {
-                        diag.span_help(cx.tcx.def_span(assoc.def_id), "implement the method");
+                        if assoc.def_id.is_local() {
+                            diag.span_help(cx.tcx.def_span(assoc.def_id), "implement the method");
+                        } else if let Some(snippet) = snippet_opt(cx, of_trait.trait_ref.path.span) {
+                            diag.help(format!(
+                                "implement the missing `{}` method of the `{snippet}` trait",
+                                assoc.name(),
+                            ));
+                        }
                     },
                 );
             }

tests/ui/missing_trait_methods.rs

@@ -62,3 +62,10 @@ impl MissingMultiple for Partial {}
 //~| missing_trait_methods
 
 fn main() {}
+
+//~v missing_trait_methods
+impl PartialEq<Partial> for Partial {
+    fn eq(&self, other: &Partial) -> bool {
+        todo!()
+    }
+}

tests/ui/missing_trait_methods.stderr

@@ -60,5 +60,13 @@ help: implement the method
 LL |     fn three() {}
    |     ^^^^^^^^^^
 
-error: aborting due to 5 previous errors
+error: missing trait method provided by default: `ne`
+  --> tests/ui/missing_trait_methods.rs:67:1
+   |
+LL | impl PartialEq<Partial> for Partial {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: implement the missing `ne` method of the `PartialEq<Partial>` trait
+
+error: aborting due to 6 previous errors