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rustc: fix ty::Const::eval's handling of inference variables.

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eddyb committed Nov 29, 2019
1 parent 25d8a94 commit de30d319761c6fed04c87a41e62828d0b275e01e
Showing with 35 additions and 14 deletions.
  1. +35 −14 src/librustc/ty/sty.rs
@@ -2330,22 +2330,43 @@ impl<'tcx> Const<'tcx> {
tcx: TyCtxt<'tcx>,
param_env: ParamEnv<'tcx>,
) -> &Const<'tcx> {
// FIXME(const_generics): this doesn't work right now,
// because it tries to relate an `Infer` to a `Param`.
let try_const_eval = |did, param_env: ParamEnv<'tcx>, substs| {
let param_env_and_substs = param_env.with_reveal_all().and(substs);

// Avoid querying `tcx.const_eval(...)` with any e.g. inference vars.
if param_env_and_substs.has_local_value() {
return None;
}

let (param_env, substs) = param_env_and_substs.into_parts();

// try to resolve e.g. associated constants to their definition on an impl
let instance = ty::Instance::resolve(tcx, param_env, did, substs)?;
let gid = GlobalId {
instance,
promoted: None,
};
tcx.const_eval(param_env.and(gid)).ok()
};

match self.val {
ConstKind::Unevaluated(did, substs) => {
// if `substs` has no unresolved components, use and empty param_env
let (param_env, substs) = param_env.with_reveal_all().and(substs).into_parts();
// try to resolve e.g. associated constants to their definition on an impl
let instance = match ty::Instance::resolve(tcx, param_env, did, substs) {
Some(instance) => instance,
None => return self,
};
let gid = GlobalId {
instance,
promoted: None,
};
tcx.const_eval(param_env.and(gid)).unwrap_or(self)
// HACK(eddyb) when substs contain e.g. inference variables,
// attempt using identity substs instead, that will succeed
// when the expression doesn't depend on any parameters.
// FIXME(eddyb) make `const_eval` a canonical query instead,
// that would properly handle inference variables in `substs`.
if substs.has_local_value() {
let identity_substs = InternalSubsts::identity_for_item(tcx, did);
// The `ParamEnv` needs to match the `identity_substs`.
let identity_param_env = tcx.param_env(did);
match try_const_eval(did, identity_param_env, identity_substs) {
Some(ct) => ct.subst(tcx, substs),
None => self,
}
} else {
try_const_eval(did, param_env, substs).unwrap_or(self)
}
},
_ => self,
}

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