Auto merge of #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.

1 files changed, 8 insertions, 8 deletions

diff --git a/tests/mir-opt/coverage/branch_match_arms.main.InstrumentCoverage.diff b/tests/mir-opt/coverage/branch_match_arms.main.InstrumentCoverage.diff
index 3d791734f46..eeeac70b5b8 100644
--- a/tests/mir-opt/coverage/branch_match_arms.main.InstrumentCoverage.diff
+++ b/tests/mir-opt/coverage/branch_match_arms.main.InstrumentCoverage.diff
@@ -71,33 +71,33 @@
       bb5: {
 +         Coverage::ExpressionUsed(2);
           StorageLive(_9);
-          _9 = ((_1 as A).0: u32);
+          _9 = copy ((_1 as A).0: u32);
           StorageLive(_10);
-          _10 = _9;
+          _10 = copy _9;
           _0 = consume(move _10) -> [return: bb12, unwind: bb14];
       }
   
       bb6: {
           StorageLive(_7);
-          _7 = ((_1 as B).0: u32);
+          _7 = copy ((_1 as B).0: u32);
           StorageLive(_8);
-          _8 = _7;
+          _8 = copy _7;
           _0 = consume(move _8) -> [return: bb11, unwind: bb14];
       }
   
       bb7: {
           StorageLive(_5);
-          _5 = ((_1 as C).0: u32);
+          _5 = copy ((_1 as C).0: u32);
           StorageLive(_6);
-          _6 = _5;
+          _6 = copy _5;
           _0 = consume(move _6) -> [return: bb10, unwind: bb14];
       }
   
       bb8: {
           StorageLive(_3);
-          _3 = ((_1 as D).0: u32);
+          _3 = copy ((_1 as D).0: u32);
           StorageLive(_4);
-          _4 = _3;
+          _4 = copy _3;
           _0 = consume(move _4) -> [return: bb9, unwind: bb14];
       }