Rollup merge of #71947 - mibac138:dead-code, r=cramertj

Dead-code pass highlights too much of impl functions

Fixes #66627.
Previous diagnostic:
```
error: method is never used: `unused_impl_fn_3`
  --> src/main.rs:28:5
   |
28 | /     fn unused_impl_fn_3(
29 | |         var: i32,
30 | |     ) {
31 | |         println!("bar {}", var);
32 | |     }
   | |_____^
```
New diagnostic:
```
error: associated function is never used: `unused_impl_fn_3`
  --> $DIR/lint-dead-code-6.rs:13:8
   |
LL |     fn unused_impl_fn_3(
   |        ^^^^^^^^^^^^^^^^
```

This makes associated functions in line with free-standing functions.

Author: Dylan-DPC

src/librustc_passes/dead.rs

@@ -589,7 +589,7 @@ impl Visitor<'tcx> for DeadVisitor<'tcx> {
                     // FIXME(66095): Because item.span is annotated with things
                     // like expansion data, and ident.span isn't, we use the
                     // def_span method if it's part of a macro invocation
-                    // (and thus has asource_callee set).
+                    // (and thus has a source_callee set).
                     // We should probably annotate ident.span with the macro
                     // context, but that's a larger change.
                     if item.span.source_callee().is_some() {
@@ -653,7 +653,17 @@ impl Visitor<'tcx> for DeadVisitor<'tcx> {
             }
             hir::ImplItemKind::Fn(_, body_id) => {
                 if !self.symbol_is_live(impl_item.hir_id) {
-                    let span = self.tcx.sess.source_map().guess_head_span(impl_item.span);
+                    // FIXME(66095): Because impl_item.span is annotated with things
+                    // like expansion data, and ident.span isn't, we use the
+                    // def_span method if it's part of a macro invocation
+                    // (and thus has a source_callee set).
+                    // We should probably annotate ident.span with the macro
+                    // context, but that's a larger change.
+                    let span = if impl_item.span.source_callee().is_some() {
+                        self.tcx.sess.source_map().guess_head_span(impl_item.span)
+                    } else {
+                        impl_item.ident.span
+                    };
                     self.warn_dead_code(impl_item.hir_id, span, impl_item.ident.name, "used");
                 }
                 self.visit_nested_body(body_id)

src/test/ui/lint/dead-code/lint-dead-code-3.stderr

@@ -11,10 +11,10 @@ LL | #![deny(dead_code)]
    |         ^^^^^^^^^
 
 error: associated function is never used: `foo`
-  --> $DIR/lint-dead-code-3.rs:15:5
+  --> $DIR/lint-dead-code-3.rs:15:8
    |
 LL |     fn foo(&self) {
-   |     ^^^^^^^^^^^^^
+   |        ^^^
 
 error: function is never used: `bar`
   --> $DIR/lint-dead-code-3.rs:20:4

src/test/ui/lint/dead-code/lint-dead-code-6.rs

@@ -0,0 +1,20 @@
+#![deny(dead_code)]
+
+struct UnusedStruct; //~ ERROR struct is never constructed: `UnusedStruct`
+impl UnusedStruct {
+    fn unused_impl_fn_1() { //~ ERROR associated function is never used: `unused_impl_fn_1`
+        println!("blah");
+    }
+
+    fn unused_impl_fn_2(var: i32) { //~ ERROR associated function is never used: `unused_impl_fn_2`
+        println!("foo {}", var);
+    }
+
+    fn unused_impl_fn_3( //~ ERROR associated function is never used: `unused_impl_fn_3`
+        var: i32,
+    ) {
+        println!("bar {}", var);
+    }
+}
+
+fn main() {}

src/test/ui/lint/dead-code/lint-dead-code-6.stderr

@@ -0,0 +1,32 @@
+error: struct is never constructed: `UnusedStruct`
+  --> $DIR/lint-dead-code-6.rs:3:8
+   |
+LL | struct UnusedStruct;
+   |        ^^^^^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/lint-dead-code-6.rs:1:9
+   |
+LL | #![deny(dead_code)]
+   |         ^^^^^^^^^
+
+error: associated function is never used: `unused_impl_fn_1`
+  --> $DIR/lint-dead-code-6.rs:5:8
+   |
+LL |     fn unused_impl_fn_1() {
+   |        ^^^^^^^^^^^^^^^^
+
+error: associated function is never used: `unused_impl_fn_2`
+  --> $DIR/lint-dead-code-6.rs:9:8
+   |
+LL |     fn unused_impl_fn_2(var: i32) {
+   |        ^^^^^^^^^^^^^^^^
+
+error: associated function is never used: `unused_impl_fn_3`
+  --> $DIR/lint-dead-code-6.rs:13:8
+   |
+LL |     fn unused_impl_fn_3(
+   |        ^^^^^^^^^^^^^^^^
+
+error: aborting due to 4 previous errors
+