From 939a224ce39fbc422e14d8255616fc587b845b8e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Esteban=20K=C3=BCber?= <esteban@kuber.com.ar>
Date: Fri, 20 Oct 2023 22:11:01 +0000
Subject: [PATCH] Point at assoc fn definition on type param divergence

When the number of type parameters in the associated function of an impl
and its trait differ, we now *always* point at the trait one, even if it
comes from a foreign crate. When it is local, we point at the specific
params, when it is foreign, we point at the whole associated item.

Fix #69944.
---
 .../src/check/compare_impl_item.rs            | 24 ++++---------------
 tests/ui/typeck/issue-36708.stderr            |  7 +++++-
 2 files changed, 11 insertions(+), 20 deletions(-)

diff --git a/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs b/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
index a1470cc69c306..74e2b157dd0a4 100644
--- a/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
+++ b/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
@@ -1557,38 +1557,24 @@ fn compare_number_of_generics<'tcx>(
                 DiagnosticId::Error("E0049".into()),
             );
 
-            let mut suffix = None;
-
+            let msg =
+                format!("expected {trait_count} {kind} parameter{}", pluralize!(trait_count),);
             if let Some(spans) = trait_spans {
                 let mut spans = spans.iter();
                 if let Some(span) = spans.next() {
-                    err.span_label(
-                        *span,
-                        format!(
-                            "expected {} {} parameter{}",
-                            trait_count,
-                            kind,
-                            pluralize!(trait_count),
-                        ),
-                    );
+                    err.span_label(*span, msg);
                 }
                 for span in spans {
                     err.span_label(*span, "");
                 }
             } else {
-                suffix = Some(format!(", expected {trait_count}"));
+                err.span_label(tcx.def_span(trait_.def_id), msg);
             }
 
             if let Some(span) = span {
                 err.span_label(
                     span,
-                    format!(
-                        "found {} {} parameter{}{}",
-                        impl_count,
-                        kind,
-                        pluralize!(impl_count),
-                        suffix.unwrap_or_default(),
-                    ),
+                    format!("found {} {} parameter{}", impl_count, kind, pluralize!(impl_count),),
                 );
             }
 
diff --git a/tests/ui/typeck/issue-36708.stderr b/tests/ui/typeck/issue-36708.stderr
index 140f19f1ff774..f1e0f4719281c 100644
--- a/tests/ui/typeck/issue-36708.stderr
+++ b/tests/ui/typeck/issue-36708.stderr
@@ -2,7 +2,12 @@ error[E0049]: method `foo` has 1 type parameter but its trait declaration has 0
   --> $DIR/issue-36708.rs:8:12
    |
 LL |     fn foo<T>() {}
-   |            ^ found 1 type parameter, expected 0
+   |            ^ found 1 type parameter
+   |
+  ::: $DIR/auxiliary/issue-36708.rs:4:5
+   |
+LL |     fn foo();
+   |     --------- expected 0 type parameters
 
 error: aborting due to previous error