13c38730d9
GVN: Only propagate borrows from SSA locals Fixes https://github.com/rust-lang/rust/issues/141313. This is a more principled fix than https://github.com/rust-lang/rust/pull/147886. Using a reference that is not a borrowing of an SSA local at a new location may be UB. The PR has two major changes. The first one, when introducing a new dereference at a new location, is that the reference must point to an SSA local or be an immutable argument. `dereference_address` has handled SSA locals. The second one, if we cannot guard to the reference point to an SSA local in `visit_assign`, we have to rewrite the value to opaque. This avoids unifying the following dereferences that also are references: ```rust let b: &T = *a; // ... `a` is allowed to be modified. `c` and `b` have different borrowing lifetime. // Unifying them will extend the lifetime of `b`. let c: &T = *a; ``` See also https://github.com/rust-lang/rust/issues/130853. This still allows unifying non-reference dereferences: ```rust let a: &T = ...; let b: T = *a; // ... a is NOT allowed to be modified. let c: T = *a; ``` r? @cjgillot |
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