[move][move-decompiler] Inline immutable aliases to remove additional lets [1/8]#26803
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cgswords merged 3 commits intoJun 8, 2026
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…ble_alias`
Rename `inline_single_use_bindings` to `inline_immutable_alias` and relax
the eligibility predicate: a `let X = Y;` binding now fires whenever `Y`
has zero writes (was: exactly one read). `Y`'s value is stable for the
function's lifetime, so substituting it for `X` preserves semantics
regardless of how many places read `Y`.
The destination guard on `X` is unchanged — one defining `LetBind`, no
re-assignment, and (when there are multiple syntactic reads) no path
that reads `X` twice. The latter still matters for non-`copy` types
even though `Y` is immutable: a double substitution would convert a
single move into two reads.
Snapshot impact: the cetus argument-staging idiom
let l40 = l13; let l39 = l12; let l38 = l11;
is_stable(freeze(l0), l38, l39, l40)
now collapses to `is_stable(freeze(l0), l11, l12, l13)`. Similar wins
across `dbke.mv` and the enum-match snapshots.
Four staging let-bindings of immutable parameters `p` and `q` — each of which is used in multiple other staging slots. Old `inline_single_use_bindings` declined (each RHS was read more than once); `inline_immutable_alias` now folds them all to `l0 + l1 + l0 + l1`.
A `let X = Y;` is only safe to inline when slot `Y` is invariant from the binding to each use of `X`. The previous `assigns(Y) == 0` check caught direct reassignment but missed mutation through a `&mut`: a site that takes `&mut Y` (or `&mut X`) hands out a reference through which a `WriteRef` — or a callee receiving the reference — can reassign the slot, invisible to `assigns`. Add a `mut_borrows` counter to `NameCounts` (bumped on `Borrow(true, Variable(n))`) and require `mut_borrows == 0` on both `X` and `Y` in `is_eligible`. Expand the module/`is_eligible` comments with the slot-invariance argument: the check is type-agnostic because pointee mutations (`*Y = e`, `f(Y)`) touch the heap, not the slot, and `X`/`Y` alias the same pointee. Regression test `mut_borrow_blocks_inline`: `let x = 0; let y = x; add_one(&mut x); y` must keep the `let y = x` binding so the function returns the binding-time snapshot (0), not the post-mutation slot (1). Snapshot impact: two staging bindings in cetus that the previous check inlined (`l44`, which is `&mut`-borrowed by `extract_..._coin_send`) are now preserved. The inline happened to be safe there — the mutation precedes the binding — but the slot-level check can't see that ordering; reaching-defs could, and is parked as a follow-up.
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Description
Rename the single-use binding inliner to
inline_immutable_aliasand generalize its logic to inline anylet x = y;whose sourceyis never mutated, not only single-use ones. This eliminates more redundantletbindings for readability, while a mut-borrow guard keeps it from inlining a slot that can be written through a&mut.Test plan
Snapshot updates with new refinements.
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