From 84dd2a77769be235c19302d69b3bd1916718819e Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Tomasz=20Mi=C4=85sko?= Date: Tue, 4 Mar 2025 12:24:09 +0100 Subject: [PATCH] Remove obsolete comment from DeduceReadOnly The situation described in the comment does arise in practice now and described panic is long gone. --- .../src/deduce_param_attrs.rs | 29 ------------------- 1 file changed, 29 deletions(-) diff --git a/compiler/rustc_mir_transform/src/deduce_param_attrs.rs b/compiler/rustc_mir_transform/src/deduce_param_attrs.rs index 049f13ce96d74..a0db8bdb7ed88 100644 --- a/compiler/rustc_mir_transform/src/deduce_param_attrs.rs +++ b/compiler/rustc_mir_transform/src/deduce_param_attrs.rs @@ -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 {