rustc_hir_analysis/hir_wf_check.rs
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use rustc_hir as hir;
use rustc_hir::intravisit::{self, Visitor};
use rustc_hir::{ForeignItem, ForeignItemKind};
use rustc_infer::infer::TyCtxtInferExt;
use rustc_infer::traits::{ObligationCause, WellFormedLoc};
use rustc_middle::bug;
use rustc_middle::query::Providers;
use rustc_middle::ty::{self, TyCtxt, TypingMode};
use rustc_span::def_id::LocalDefId;
use rustc_trait_selection::traits::{self, ObligationCtxt};
use tracing::debug;
use crate::collect::ItemCtxt;
pub(crate) fn provide(providers: &mut Providers) {
*providers = Providers { diagnostic_hir_wf_check, ..*providers };
}
// Ideally, this would be in `rustc_trait_selection`, but we
// need access to `ItemCtxt`
fn diagnostic_hir_wf_check<'tcx>(
tcx: TyCtxt<'tcx>,
(predicate, loc): (ty::Predicate<'tcx>, WellFormedLoc),
) -> Option<ObligationCause<'tcx>> {
let hir = tcx.hir();
let def_id = match loc {
WellFormedLoc::Ty(def_id) => def_id,
WellFormedLoc::Param { function, param_idx: _ } => function,
};
let hir_id = tcx.local_def_id_to_hir_id(def_id);
// HIR wfcheck should only ever happen as part of improving an existing error
tcx.dcx()
.span_delayed_bug(tcx.def_span(def_id), "Performed HIR wfcheck without an existing error!");
let icx = ItemCtxt::new(tcx, def_id);
// To perform HIR-based WF checking, we iterate over all HIR types
// that occur 'inside' the item we're checking. For example,
// given the type `Option<MyStruct<u8>>`, we will check
// `Option<MyStruct<u8>>`, `MyStruct<u8>`, and `u8`.
// For each type, we perform a well-formed check, and see if we get
// an error that matches our expected predicate. We save
// the `ObligationCause` corresponding to the *innermost* type,
// which is the most specific type that we can point to.
// In general, the different components of an `hir::Ty` may have
// completely different spans due to macro invocations. Pointing
// to the most accurate part of the type can be the difference
// between a useless span (e.g. the macro invocation site)
// and a useful span (e.g. a user-provided type passed into the macro).
//
// This approach is quite inefficient - we redo a lot of work done
// by the normal WF checker. However, this code is run at most once
// per reported error - it will have no impact when compilation succeeds,
// and should only have an impact if a very large number of errors is
// displayed to the user.
struct HirWfCheck<'tcx> {
tcx: TyCtxt<'tcx>,
predicate: ty::Predicate<'tcx>,
cause: Option<ObligationCause<'tcx>>,
cause_depth: usize,
icx: ItemCtxt<'tcx>,
def_id: LocalDefId,
param_env: ty::ParamEnv<'tcx>,
depth: usize,
}
impl<'tcx> Visitor<'tcx> for HirWfCheck<'tcx> {
fn visit_ty(&mut self, ty: &'tcx hir::Ty<'tcx>) {
let infcx = self.tcx.infer_ctxt().build(TypingMode::non_body_analysis());
let ocx = ObligationCtxt::new_with_diagnostics(&infcx);
let tcx_ty = self.icx.lower_ty(ty);
// This visitor can walk into binders, resulting in the `tcx_ty` to
// potentially reference escaping bound variables. We simply erase
// those here.
let tcx_ty = self.tcx.fold_regions(tcx_ty, |r, _| {
if r.is_bound() { self.tcx.lifetimes.re_erased } else { r }
});
let cause = traits::ObligationCause::new(
ty.span,
self.def_id,
traits::ObligationCauseCode::WellFormed(None),
);
ocx.register_obligation(traits::Obligation::new(
self.tcx,
cause,
self.param_env,
ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(tcx_ty.into())),
));
for error in ocx.select_all_or_error() {
debug!("Wf-check got error for {:?}: {:?}", ty, error);
if error.obligation.predicate == self.predicate {
// Save the cause from the greatest depth - this corresponds
// to picking more-specific types (e.g. `MyStruct<u8>`)
// over less-specific types (e.g. `Option<MyStruct<u8>>`)
if self.depth >= self.cause_depth {
self.cause = Some(error.obligation.cause);
self.cause_depth = self.depth
}
}
}
self.depth += 1;
intravisit::walk_ty(self, ty);
self.depth -= 1;
}
}
let mut visitor = HirWfCheck {
tcx,
predicate,
cause: None,
cause_depth: 0,
icx,
def_id,
param_env: tcx.param_env(def_id.to_def_id()),
depth: 0,
};
// Get the starting `hir::Ty` using our `WellFormedLoc`.
// We will walk 'into' this type to try to find
// a more precise span for our predicate.
let tys = match loc {
WellFormedLoc::Ty(_) => match tcx.hir_node(hir_id) {
hir::Node::ImplItem(item) => match item.kind {
hir::ImplItemKind::Type(ty) => vec![ty],
hir::ImplItemKind::Const(ty, _) => vec![ty],
ref item => bug!("Unexpected ImplItem {:?}", item),
},
hir::Node::TraitItem(item) => match item.kind {
hir::TraitItemKind::Type(_, ty) => ty.into_iter().collect(),
hir::TraitItemKind::Const(ty, _) => vec![ty],
ref item => bug!("Unexpected TraitItem {:?}", item),
},
hir::Node::Item(item) => match item.kind {
hir::ItemKind::TyAlias(ty, _)
| hir::ItemKind::Static(ty, _, _)
| hir::ItemKind::Const(ty, _, _) => vec![ty],
hir::ItemKind::Impl(impl_) => match &impl_.of_trait {
Some(t) => t
.path
.segments
.last()
.iter()
.flat_map(|seg| seg.args().args)
.filter_map(|arg| {
if let hir::GenericArg::Type(ty) = arg { Some(*ty) } else { None }
})
.chain([impl_.self_ty])
.collect(),
None => {
vec![impl_.self_ty]
}
},
ref item => bug!("Unexpected item {:?}", item),
},
hir::Node::Field(field) => vec![field.ty],
hir::Node::ForeignItem(ForeignItem {
kind: ForeignItemKind::Static(ty, _, _), ..
}) => vec![*ty],
hir::Node::GenericParam(hir::GenericParam {
kind: hir::GenericParamKind::Type { default: Some(ty), .. },
..
}) => vec![*ty],
hir::Node::AnonConst(_) => {
if let Some(const_param_id) = tcx.hir().opt_const_param_default_param_def_id(hir_id)
&& let hir::Node::GenericParam(hir::GenericParam {
kind: hir::GenericParamKind::Const { ty, .. },
..
}) = tcx.hir_node_by_def_id(const_param_id)
{
vec![*ty]
} else {
vec![]
}
}
ref node => bug!("Unexpected node {:?}", node),
},
WellFormedLoc::Param { function: _, param_idx } => {
let fn_decl = hir.fn_decl_by_hir_id(hir_id).unwrap();
// Get return type
if param_idx as usize == fn_decl.inputs.len() {
match fn_decl.output {
hir::FnRetTy::Return(ty) => vec![ty],
// The unit type `()` is always well-formed
hir::FnRetTy::DefaultReturn(_span) => vec![],
}
} else {
vec![&fn_decl.inputs[param_idx as usize]]
}
}
};
for ty in tys {
visitor.visit_ty(ty);
}
visitor.cause
}