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Rollup merge of #137993 - tmiasko:deduce-comment, r=compiler-errors

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Remove obsolete comment from DeduceReadOnly

The situation described in the comment does arise in practice now and described panic is long gone.
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compiler/rustc_mir_transform/src/deduce_param_attrs.rs
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@ -80,35 +80,6 @@ impl<'tcx> Visitor<'tcx> for DeduceReadOnly {
// `f` passes. Note that function arguments are the only situation in which this problem can
// arise: every other use of `move` in MIR doesn't actually write to the value it moves
// from.
//
// Anyway, right now this situation doesn't actually arise in practice. Instead, the MIR for
// that function looks like this:
//
// fn f(_1: BigStruct) -> () {
// let mut _0: ();
// let mut _2: BigStruct;
// bb0: {
// _2 = move _1;
// _0 = g(move _2) -> bb1;
// }
// ...
// }
//
// Because of that extra move that MIR construction inserts, `x` (i.e. `_1`) can *in
// practice* safely be marked `readonly`.
//
// To handle the possibility that other optimizations (for example, destination propagation)
// might someday generate MIR like the first example above, we panic upon seeing an argument
// to *our* function that is directly moved into *another* function as an argument. Having
// eliminated that problematic case, we can safely treat moves as copies in this analysis.
//
// In the future, if MIR optimizations cause arguments of a caller to be directly moved into
// the argument of a callee, we can just add that argument to `mutated_args` instead of
// panicking.
//
// Note that, because the problematic MIR is never actually generated, we can't add a test
// case for this.
if let TerminatorKind::Call { ref args, .. } = terminator.kind {
for arg in args {
if let Operand::Move(place) = arg.node {