refactor: emit "unused assoc fn" in lexical order

with repect to other dead code lints

compiler/rustc_passes/src/dead.rs

@@ -836,6 +836,13 @@ fn check_mod_deathness(tcx: TyCtxt<'_>, module: LocalDefId) {
     let module_items = tcx.hir_module_items(module);
 
     for item in module_items.items() {
+        if let hir::ItemKind::Impl(impl_item) = tcx.hir().item(item).kind {
+            for item in impl_item.items {
+                visitor.check_definition(item.id.owner_id.def_id);
+            }
+            continue;
+        }
+
         if !live_symbols.contains(&item.owner_id.def_id) {
             let parent = tcx.local_parent(item.owner_id.def_id);
             if parent != module && !live_symbols.contains(&parent) {
@@ -900,10 +907,6 @@ fn check_mod_deathness(tcx: TyCtxt<'_>, module: LocalDefId) {
         }
     }
 
-    for impl_item in module_items.impl_items() {
-        visitor.check_definition(impl_item.owner_id.def_id);
-    }
-
     for foreign_item in module_items.foreign_items() {
         visitor.check_definition(foreign_item.owner_id.def_id);
     }

tests/ui/lint/dead-code/issue-85255.stderr

@@ -14,26 +14,6 @@ note: the lint level is defined here
 LL | #![warn(dead_code)]
    |         ^^^^^^^^^
 
-warning: fields `a` and `b` are never read
-  --> $DIR/issue-85255.rs:19:5
-   |
-LL | pub(crate) struct Foo1 {
-   |                   ---- fields in this struct
-LL |     a: i32,
-   |     ^
-LL |     pub b: i32,
-   |         ^
-
-warning: fields `a` and `b` are never read
-  --> $DIR/issue-85255.rs:31:5
-   |
-LL | pub(crate) struct Foo2 {
-   |                   ---- fields in this struct
-LL |     a: i32,
-   |     ^
-LL |     pub b: i32,
-   |         ^
-
 warning: method `a` is never used
   --> $DIR/issue-85255.rs:14:8
    |
@@ -46,6 +26,16 @@ warning: method `b` is never used
 LL |     pub fn b(&self) -> i32 { 6 }
    |            ^
 
+warning: fields `a` and `b` are never read
+  --> $DIR/issue-85255.rs:19:5
+   |
+LL | pub(crate) struct Foo1 {
+   |                   ---- fields in this struct
+LL |     a: i32,
+   |     ^
+LL |     pub b: i32,
+   |         ^
+
 warning: method `a` is never used
   --> $DIR/issue-85255.rs:26:8
    |
@@ -58,6 +48,16 @@ warning: method `b` is never used
 LL |     pub fn b(&self) -> i32 { 6 }
    |            ^
 
+warning: fields `a` and `b` are never read
+  --> $DIR/issue-85255.rs:31:5
+   |
+LL | pub(crate) struct Foo2 {
+   |                   ---- fields in this struct
+LL |     a: i32,
+   |     ^
+LL |     pub b: i32,
+   |         ^
+
 warning: method `a` is never used
   --> $DIR/issue-85255.rs:38:8
    |

tests/ui/lint/dead-code/lint-dead-code-3.stderr

@@ -10,6 +10,12 @@ note: the lint level is defined here
 LL | #![deny(dead_code)]
    |         ^^^^^^^^^
 
+error: method `foo` is never used
+  --> $DIR/lint-dead-code-3.rs:16:8
+   |
+LL |     fn foo(&self) {
+   |        ^^^
+
 error: function `bar` is never used
   --> $DIR/lint-dead-code-3.rs:21:4
    |
@@ -34,12 +40,6 @@ error: function `blah` is never used
 LL |         fn blah() {}
    |            ^^^^
 
-error: method `foo` is never used
-  --> $DIR/lint-dead-code-3.rs:16:8
-   |
-LL |     fn foo(&self) {
-   |        ^^^
-
 error: function `free` is never used
   --> $DIR/lint-dead-code-3.rs:62:8
    |

tests/ui/lint/dead-code/unused-assoc-fns.rs

@@ -0,0 +1,27 @@
+#![deny(unused)]
+
+struct Foo;
+
+impl Foo {
+    fn one() {}
+    //~^ ERROR associated function `one` is never used [dead_code]
+
+    fn two(&self) {}
+    //~^ ERROR method `two` is never used [dead_code]
+
+    // seperation between functions
+    // ...
+    // ...
+
+    fn used() {}
+
+    fn three(&self) {
+    //~^ ERROR method `three` is never used [dead_code]
+        Foo::one();
+        // ...
+    }
+}
+
+fn main() {
+    Foo::used();
+}

tests/ui/lint/dead-code/unused-assoc-fns.stderr

@@ -0,0 +1,27 @@
+error: associated function `one` is never used
+  --> $DIR/unused-assoc-fns.rs:6:8
+   |
+LL |     fn one() {}
+   |        ^^^
+   |
+note: the lint level is defined here
+  --> $DIR/unused-assoc-fns.rs:1:9
+   |
+LL | #![deny(unused)]
+   |         ^^^^^^
+   = note: `#[deny(dead_code)]` implied by `#[deny(unused)]`
+
+error: method `two` is never used
+  --> $DIR/unused-assoc-fns.rs:9:8
+   |
+LL |     fn two(&self) {}
+   |        ^^^
+
+error: method `three` is never used
+  --> $DIR/unused-assoc-fns.rs:18:8
+   |
+LL |     fn three(&self) {
+   |        ^^^^^
+
+error: aborting due to 3 previous errors
+