Rollup merge of rust-lang#101111 - saethlin:better-fnentry-spans, r=RalfJung

Use the declaration's SourceInfo for FnEntry retags, not the outermost

This addresses a long-standing `// FIXME` in the pass that adds retags.

The changes to Miri's UI tests will look like this:
```
   --> $DIR/aliasing_mut1.rs:LL:CC
    |
 LL | pub fn safe(_x: &mut i32, _y: &mut i32) {}
<   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not granting access to tag <TAG> because incompatible item [Unique for <TAG>] is protected by call ID
>   |                           ^^ not granting access to tag <TAG> because incompatible item [Unique for <TAG>] is protected by call ID
    |
```

r? ````@RalfJung````

Author: matthiaskrgr

compiler/rustc_mir_transform/src/add_retag.rs

@@ -66,7 +66,6 @@ impl<'tcx> MirPass<'tcx> for AddRetag {
         // We need an `AllCallEdges` pass before we can do any work.
         super::add_call_guards::AllCallEdges.run_pass(tcx, body);
 
-        let (span, arg_count) = (body.span, body.arg_count);
         let basic_blocks = body.basic_blocks.as_mut();
         let local_decls = &body.local_decls;
         let needs_retag = |place: &Place<'tcx>| {
@@ -90,20 +89,18 @@ impl<'tcx> MirPass<'tcx> for AddRetag {
         // PART 1
         // Retag arguments at the beginning of the start block.
         {
-            // FIXME: Consider using just the span covering the function
-            // argument declaration.
-            let source_info = SourceInfo::outermost(span);
             // Gather all arguments, skip return value.
-            let places = local_decls
-                .iter_enumerated()
-                .skip(1)
-                .take(arg_count)
-                .map(|(local, _)| Place::from(local))
-                .filter(needs_retag);
+            let places = local_decls.iter_enumerated().skip(1).take(body.arg_count).filter_map(
+                |(local, decl)| {
+                    let place = Place::from(local);
+                    needs_retag(&place).then_some((place, decl.source_info))
+                },
+            );
+
             // Emit their retags.
             basic_blocks[START_BLOCK].statements.splice(
                 0..0,
-                places.map(|place| Statement {
+                places.map(|(place, source_info)| Statement {
                     source_info,
                     kind: StatementKind::Retag(RetagKind::FnEntry, Box::new(place)),
                 }),

src/test/mir-opt/dead-store-elimination/provenance_soundness.retags.DeadStoreElimination.diff

@@ -6,7 +6,7 @@
       let mut _0: ();                      // return place in scope 0 at $DIR/provenance_soundness.rs:+0:25: +0:25
 
       bb0: {
-          Retag([fn entry] _1);            // scope 0 at $DIR/provenance_soundness.rs:+0:1: +0:27
+          Retag([fn entry] _1);            // scope 0 at $DIR/provenance_soundness.rs:+0:11: +0:13
           _0 = const ();                   // scope 0 at $DIR/provenance_soundness.rs:+0:25: +0:27
           return;                          // scope 0 at $DIR/provenance_soundness.rs:+0:27: +0:27
       }

src/test/mir-opt/inline/inline_retag.bar.Inline.after.mir

@@ -52,8 +52,8 @@ fn bar() -> bool {
         Retag(_7);                       // scope 1 at $DIR/inline-retag.rs:+2:11: +2:14
         _6 = &(*_7);                     // scope 1 at $DIR/inline-retag.rs:+2:11: +2:14
         Retag(_6);                       // scope 1 at $DIR/inline-retag.rs:+2:11: +2:14
-        Retag(_3);                       // scope 2 at $DIR/inline-retag.rs:16:1: 18:2
-        Retag(_6);                       // scope 2 at $DIR/inline-retag.rs:16:1: 18:2
+        Retag(_3);                       // scope 2 at $DIR/inline-retag.rs:16:8: 16:9
+        Retag(_6);                       // scope 2 at $DIR/inline-retag.rs:16:17: 16:18
         StorageLive(_11);                // scope 2 at $DIR/inline-retag.rs:17:5: 17:7
         _11 = (*_3);                     // scope 2 at $DIR/inline-retag.rs:17:5: 17:7
         StorageLive(_12);                // scope 2 at $DIR/inline-retag.rs:17:11: 17:13

src/test/mir-opt/retag.main-{closure#0}.SimplifyCfg-elaborate-drops.after.mir

@@ -10,7 +10,7 @@ fn main::{closure#0}(_1: &[closure@main::{closure#0}], _2: &i32) -> &i32 {
 
     bb0: {
         Retag([fn entry] _1);            // scope 0 at $DIR/retag.rs:+0:31: +0:48
-        Retag([fn entry] _2);            // scope 0 at $DIR/retag.rs:+0:31: +0:48
+        Retag([fn entry] _2);            // scope 0 at $DIR/retag.rs:+0:32: +0:33
         StorageLive(_3);                 // scope 0 at $DIR/retag.rs:42:13: 42:15
         _3 = _2;                         // scope 0 at $DIR/retag.rs:42:18: 42:19
         Retag(_3);                       // scope 0 at $DIR/retag.rs:42:18: 42:19

src/test/mir-opt/retag.{impl#0}-foo.SimplifyCfg-elaborate-drops.after.mir

@@ -7,8 +7,8 @@ fn <impl at $DIR/retag.rs:12:1: 12:10>::foo(_1: &Test, _2: &mut i32) -> &mut i32
     let mut _3: &mut i32;                // in scope 0 at $DIR/retag.rs:+1:9: +1:10
 
     bb0: {
-        Retag([fn entry] _1);            // scope 0 at $DIR/retag.rs:+0:5: +2:6
-        Retag([fn entry] _2);            // scope 0 at $DIR/retag.rs:+0:5: +2:6
+        Retag([fn entry] _1);            // scope 0 at $DIR/retag.rs:+0:16: +0:21
+        Retag([fn entry] _2);            // scope 0 at $DIR/retag.rs:+0:23: +0:24
         StorageLive(_3);                 // scope 0 at $DIR/retag.rs:+1:9: +1:10
         _3 = &mut (*_2);                 // scope 0 at $DIR/retag.rs:+1:9: +1:10
         Retag(_3);                       // scope 0 at $DIR/retag.rs:+1:9: +1:10

src/test/mir-opt/retag.{impl#0}-foo_shr.SimplifyCfg-elaborate-drops.after.mir

@@ -6,8 +6,8 @@ fn <impl at $DIR/retag.rs:12:1: 12:10>::foo_shr(_1: &Test, _2: &i32) -> &i32 {
     let mut _0: &i32;                    // return place in scope 0 at $DIR/retag.rs:+0:42: +0:49
 
     bb0: {
-        Retag([fn entry] _1);            // scope 0 at $DIR/retag.rs:+0:5: +2:6
-        Retag([fn entry] _2);            // scope 0 at $DIR/retag.rs:+0:5: +2:6
+        Retag([fn entry] _1);            // scope 0 at $DIR/retag.rs:+0:20: +0:25
+        Retag([fn entry] _2);            // scope 0 at $DIR/retag.rs:+0:27: +0:28
         _0 = _2;                         // scope 0 at $DIR/retag.rs:+1:9: +1:10
         Retag(_0);                       // scope 0 at $DIR/retag.rs:+1:9: +1:10
         return;                          // scope 0 at $DIR/retag.rs:+2:6: +2:6