Auto merge of rust-lang#122551 - RayMuir:copy_fmt, r=saethlin

Added "copy" to Debug fmt for copy operands

In MIR's debug mode (--emit mir) the printing for Operands is slightly inconsistent.

The RValues - values on the right side of an Assign - are usually printed with their Operand when they are Places.

Example:
_2 = move _3

But for arguments, the operand is omitted.

_2 = _1

I propose a change be made, to display the place with the operand.

_2 = copy _1

Move and copy have different semantics, meaning this difference is important and helpful to the user. It also adds consistency to the pretty printing.

-- EDIT --

 Consider this example Rust program and its MIR output with the **updated pretty printer.**

This was generated with the arguments --emit mir --crate-type lib -Zmir-opt-level=0 (Otherwise, it's optimised away since it's a junk program).

```rust
fn main(foo: i32) {
    let v = 10;

    if v == 20 {
        foo;
    }
    else {
        v;
    }
}
```

```MIR
// WARNING: This output format is intended for human consumers only
// and is subject to change without notice. Knock yourself out.
fn main(_1: i32) -> () {
    debug foo => _1;
    let mut _0: ();
    let _2: i32;
    let mut _3: bool;
    let mut _4: i32;
    let _5: i32;
    let _6: i32;
    scope 1 {
        debug v => _2;
    }

    bb0: {
        StorageLive(_2);
        _2 = const 10_i32;
        StorageLive(_3);
        StorageLive(_4);
        _4 = copy _2;
        _3 = Eq(move _4, const 20_i32);
        switchInt(move _3) -> [0: bb2, otherwise: bb1];
    }

    bb1: {
        StorageDead(_4);
        StorageLive(_5);
        _5 = copy _1;
        StorageDead(_5);
        _0 = const ();
        goto -> bb3;
    }

    bb2: {
        StorageDead(_4);
        StorageLive(_6);
        _6 = copy _2;
        StorageDead(_6);
        _0 = const ();
        goto -> bb3;
    }

    bb3: {
        StorageDead(_3);
        StorageDead(_2);
        return;
    }
}
```

In this example program, we can see that when we move a place, it is preceded by "move". e.g. ``` _3 = Eq(move _4, const 20_i32);```. However, when we copy a place such as ```_5 = _1;```, it is not preceded by the operand in the original printout. I propose to change the print to include the copy ```_5 = copy _1``` as in this example.

Regarding the arguments part. When I originally submitted this PR, I was under the impression this only affected the print for arguments to a function, but actually, it affects anything that uses a copy. This is preferable anyway with regard to consistency. The PR is about making ```copy``` explicit.

Author: bors

.git-blame-ignore-revs

@@ -25,3 +25,5 @@ b2d2184edea578109a48ec3d8decbee5948e8f35
 ec2cc761bc7067712ecc7734502f703fe3b024c8
 # format use declarations
 84ac80f1921afc243d71fd0caaa4f2838c294102
+# bless mir-opt tests to add `copy`
+99cb0c6bc399fb94a0ddde7e9b38e9c00d523bad

compiler/rustc_middle/src/mir/pretty.rs

@@ -1159,7 +1159,7 @@ impl<'tcx> Debug for Operand<'tcx> {
         use self::Operand::*;
         match *self {
             Constant(ref a) => write!(fmt, "{a:?}"),
-            Copy(ref place) => write!(fmt, "{place:?}"),
+            Copy(ref place) => write!(fmt, "copy {place:?}"),
             Move(ref place) => write!(fmt, "move {place:?}"),
         }
     }

tests/mir-opt/address_of.address_of_reborrow.SimplifyCfg-initial.after.mir

@@ -140,7 +140,7 @@ fn address_of_reborrow() -> () {
         StorageLive(_6);
         _6 = &raw const (*_1);
         AscribeUserType(_6, o, UserTypeProjection { base: UserType(0), projs: [] });
-        _5 = _6;
+        _5 = copy _6;
         StorageDead(_6);
         StorageDead(_5);
         StorageLive(_7);
@@ -153,7 +153,7 @@ fn address_of_reborrow() -> () {
         _9 = move _10 as *const dyn std::marker::Send (PointerCoercion(Unsize));
         StorageDead(_10);
         AscribeUserType(_9, o, UserTypeProjection { base: UserType(1), projs: [] });
-        _8 = _9;
+        _8 = copy _9;
         StorageDead(_9);
         StorageDead(_8);
         StorageLive(_11);
@@ -194,7 +194,7 @@ fn address_of_reborrow() -> () {
         StorageLive(_22);
         _22 = &raw const (*_3);
         AscribeUserType(_22, o, UserTypeProjection { base: UserType(10), projs: [] });
-        _21 = _22;
+        _21 = copy _22;
         StorageDead(_22);
         StorageDead(_21);
         StorageLive(_23);
@@ -207,7 +207,7 @@ fn address_of_reborrow() -> () {
         _25 = move _26 as *const dyn std::marker::Send (PointerCoercion(Unsize));
         StorageDead(_26);
         AscribeUserType(_25, o, UserTypeProjection { base: UserType(11), projs: [] });
-        _24 = _25;
+        _24 = copy _25;
         StorageDead(_25);
         StorageDead(_24);
         StorageLive(_27);
@@ -242,7 +242,7 @@ fn address_of_reborrow() -> () {
         StorageLive(_36);
         _36 = &raw mut (*_3);
         AscribeUserType(_36, o, UserTypeProjection { base: UserType(20), projs: [] });
-        _35 = _36;
+        _35 = copy _36;
         StorageDead(_36);
         StorageDead(_35);
         StorageLive(_37);
@@ -255,7 +255,7 @@ fn address_of_reborrow() -> () {
         _39 = move _40 as *mut dyn std::marker::Send (PointerCoercion(Unsize));
         StorageDead(_40);
         AscribeUserType(_39, o, UserTypeProjection { base: UserType(21), projs: [] });
-        _38 = _39;
+        _38 = copy _39;
         StorageDead(_39);
         StorageDead(_38);
         StorageLive(_41);

tests/mir-opt/array_index_is_temporary.main.SimplifyCfg-pre-optimizations.after.panic-abort.mir

@@ -33,17 +33,17 @@ fn main() -> () {
         StorageDead(_4);
         StorageLive(_5);
         StorageLive(_6);
-        _6 = _3;
+        _6 = copy _3;
         _5 = foo(move _6) -> [return: bb1, unwind unreachable];
     }
 
     bb1: {
         StorageDead(_6);
         StorageLive(_7);
-        _7 = _2;
+        _7 = copy _2;
         _8 = Len(_1);
-        _9 = Lt(_7, _8);
-        assert(move _9, "index out of bounds: the length is {} but the index is {}", move _8, _7) -> [success: bb2, unwind unreachable];
+        _9 = Lt(copy _7, copy _8);
+        assert(move _9, "index out of bounds: the length is {} but the index is {}", move _8, copy _7) -> [success: bb2, unwind unreachable];
     }
 
     bb2: {

tests/mir-opt/array_index_is_temporary.main.SimplifyCfg-pre-optimizations.after.panic-unwind.mir

@@ -33,17 +33,17 @@ fn main() -> () {
         StorageDead(_4);
         StorageLive(_5);
         StorageLive(_6);
-        _6 = _3;
+        _6 = copy _3;
         _5 = foo(move _6) -> [return: bb1, unwind continue];
     }
 
     bb1: {
         StorageDead(_6);
         StorageLive(_7);
-        _7 = _2;
+        _7 = copy _2;
         _8 = Len(_1);
-        _9 = Lt(_7, _8);
-        assert(move _9, "index out of bounds: the length is {} but the index is {}", move _8, _7) -> [success: bb2, unwind continue];
+        _9 = Lt(copy _7, copy _8);
+        assert(move _9, "index out of bounds: the length is {} but the index is {}", move _8, copy _7) -> [success: bb2, unwind continue];
     }
 
     bb2: {

tests/mir-opt/array_index_is_temporary.rs

@@ -15,7 +15,7 @@ fn main() {
     // CHECK: debug x => [[x:_.*]];
     // CHECK: debug y => [[y:_.*]];
     // CHECK: [[y]] = const 1_usize;
-    // CHECK: [[tmp:_.*]] = [[y]];
+    // CHECK: [[tmp:_.*]] = copy [[y]];
     // CHECK: [[x]][[[tmp]]] =
     let mut x = [42, 43, 44];
     let mut y = 1;

tests/mir-opt/async_closure_shims.main-{closure#0}-{closure#0}-{closure#0}.coroutine_by_move.0.panic-abort.mir

@@ -22,7 +22,7 @@ yields ()
 
     bb0: {
         StorageLive(_3);
-        _3 = (_1.0: i32);
+        _3 = copy (_1.0: i32);
         FakeRead(ForLet(None), _3);
         StorageLive(_4);
         _4 = &_3;

tests/mir-opt/async_closure_shims.main-{closure#0}-{closure#0}-{closure#0}.coroutine_by_move.0.panic-unwind.mir

@@ -22,7 +22,7 @@ yields ()
 
     bb0: {
         StorageLive(_3);
-        _3 = (_1.0: i32);
+        _3 = copy (_1.0: i32);
         FakeRead(ForLet(None), _3);
         StorageLive(_4);
         _4 = &_3;

tests/mir-opt/async_closure_shims.main-{closure#0}-{closure#1}-{closure#0}.coroutine_by_move.0.panic-abort.mir

@@ -22,7 +22,7 @@ yields ()
 
     bb0: {
         StorageLive(_3);
-        _3 = (_1.0: i32);
+        _3 = copy (_1.0: i32);
         FakeRead(ForLet(None), _3);
         StorageLive(_4);
         _4 = &_3;

tests/mir-opt/async_closure_shims.main-{closure#0}-{closure#1}-{closure#0}.coroutine_by_move.0.panic-unwind.mir

@@ -22,7 +22,7 @@ yields ()
 
     bb0: {
         StorageLive(_3);
-        _3 = (_1.0: i32);
+        _3 = copy (_1.0: i32);
         FakeRead(ForLet(None), _3);
         StorageLive(_4);
         _4 = &_3;