unused-lifetimes: don't warn about lifetimes originating from expande…

…d code

previously, we would warn like this:

````
warning: lifetime parameter `'s` never used
 --> /tmp/unusedlif/code.rs:6:62
  |
5 | #[derive(Clone)]
  |          - help: elide the unused lifetime
6 | struct ShimMethod4<T: Trait2 + 'static>(pub &'static dyn for<'s> Fn(&'s mut T::As));
  |                                                              ^^
  |
  = note: requested on the command line with `-W unused-lifetimes`
````

Fixes #104432

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -2244,19 +2244,23 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
                 }
                 None => {
                     debug!(?param.ident, ?param.ident.span);
-
                     let deletion_span = deletion_span();
-                    self.r.lint_buffer.buffer_lint_with_diagnostic(
-                        lint::builtin::UNUSED_LIFETIMES,
-                        param.id,
-                        param.ident.span,
-                        &format!("lifetime parameter `{}` never used", param.ident),
-                        lint::BuiltinLintDiagnostics::SingleUseLifetime {
-                            param_span: param.ident.span,
-                            use_span: None,
-                            deletion_span,
-                        },
-                    );
+                    // the give lifetime originates from expanded code so we won't be able to remove it #104432
+                    let lifetime_only_in_expanded_code =
+                        deletion_span.map(|sp| sp.in_derive_expansion()).unwrap_or(true);
+                    if !lifetime_only_in_expanded_code {
+                        self.r.lint_buffer.buffer_lint_with_diagnostic(
+                            lint::builtin::UNUSED_LIFETIMES,
+                            param.id,
+                            param.ident.span,
+                            &format!("lifetime parameter `{}` never used", param.ident),
+                            lint::BuiltinLintDiagnostics::SingleUseLifetime {
+                                param_span: param.ident.span,
+                                use_span: None,
+                                deletion_span,
+                            },
+                        );
+                    }
                 }
             }
         }

tests/ui/lifetimes/issue-104432-unused-lifetimes-in-expansion.rs

@@ -0,0 +1,12 @@
+// check-pass
+
+#![deny(unused_lifetimes)]
+trait Trait2 {
+    type As;
+}
+
+// we should not warn about an unused lifetime about code generated from this proc macro here
+#[derive(Clone)]
+struct ShimMethod4<T: Trait2 + 'static>(pub &'static dyn for<'s> Fn(&'s mut T::As));
+
+pub fn main() {}