refactoring: move const_to_pat code into its own submodule.

src/librustc_mir/hair/pattern/const_to_pat.rs

@@ -0,0 +1,216 @@
+use crate::const_eval::const_variant_index;
+
+use rustc::hir;
+use rustc::lint;
+use rustc::mir::Field;
+use rustc::traits::{ObligationCause, PredicateObligation};
+use rustc::ty;
+
+use rustc_index::vec::Idx;
+
+use syntax::symbol::sym;
+use syntax_pos::Span;
+
+use super::{FieldPat, Pat, PatCtxt, PatKind};
+
+impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
+    /// Converts an evaluated constant to a pattern (if possible).
+    /// This means aggregate values (like structs and enums) are converted
+    /// to a pattern that matches the value (as if you'd compared via structural equality).
+    pub(super) fn const_to_pat(
+        &self,
+        instance: ty::Instance<'tcx>,
+        cv: &'tcx ty::Const<'tcx>,
+        id: hir::HirId,
+        span: Span,
+    ) -> Pat<'tcx> {
+        // This method is just a warpper handling a validity check; the heavy lifting is
+        // performed by the recursive const_to_pat_inner method, which is not meant to be
+        // invoked except by this method.
+        //
+        // once indirect_structural_match is a full fledged error, this
+        // level of indirection can be eliminated
+
+        debug!("const_to_pat: cv={:#?} id={:?}", cv, id);
+        debug!("const_to_pat: cv.ty={:?} span={:?}", cv.ty, span);
+
+        let mut saw_error = false;
+        let inlined_const_as_pat = self.const_to_pat_inner(instance, cv, id, span, &mut saw_error);
+
+        if self.include_lint_checks && !saw_error {
+            // If we were able to successfully convert the const to some pat, double-check
+            // that the type of the const obeys `#[structural_match]` constraint.
+            if let Some(non_sm_ty) = ty::search_for_structural_match_violation(self.tcx, cv.ty) {
+                let msg = match non_sm_ty {
+                    ty::NonStructuralMatchTy::Adt(adt_def) => {
+                        let path = self.tcx.def_path_str(adt_def.did);
+                        format!(
+                            "to use a constant of type `{}` in a pattern, \
+                             `{}` must be annotated with `#[derive(PartialEq, Eq)]`",
+                            path,
+                            path,
+                        )
+                    }
+                    ty::NonStructuralMatchTy::Param => {
+                        bug!("use of constant whose type is a parameter inside a pattern");
+                    }
+                };
+
+                // before issuing lint, double-check there even *is* a
+                // semantic PartialEq for us to dispatch to.
+                //
+                // (If there isn't, then we can safely issue a hard
+                // error, because that's never worked, due to compiler
+                // using PartialEq::eq in this scenario in the past.)
+
+                let ty_is_partial_eq: bool = {
+                    let partial_eq_trait_id = self.tcx.lang_items().eq_trait().unwrap();
+                    let obligation: PredicateObligation<'_> =
+                        self.tcx.predicate_for_trait_def(self.param_env,
+                                                         ObligationCause::misc(span, id),
+                                                         partial_eq_trait_id,
+                                                         0,
+                                                         cv.ty,
+                                                         &[]);
+                    self.tcx
+                        .infer_ctxt()
+                        .enter(|infcx| infcx.predicate_may_hold(&obligation))
+                };
+
+                if !ty_is_partial_eq {
+                    // span_fatal avoids ICE from resolution of non-existent method (rare case).
+                    self.tcx.sess.span_fatal(span, &msg);
+                } else {
+                    self.tcx.lint_hir(lint::builtin::INDIRECT_STRUCTURAL_MATCH, id, span, &msg);
+                }
+            }
+        }
+
+        inlined_const_as_pat
+    }
+
+    /// Recursive helper for `const_to_pat`; invoke that (instead of calling this directly).
+    fn const_to_pat_inner(
+        &self,
+        instance: ty::Instance<'tcx>,
+        cv: &'tcx ty::Const<'tcx>,
+        id: hir::HirId,
+        span: Span,
+        // This tracks if we signal some hard error for a given const
+        // value, so that we will not subsequently issue an irrelevant
+        // lint for the same const value.
+        saw_const_match_error: &mut bool,
+    ) -> Pat<'tcx> {
+
+        let mut adt_subpattern = |i, variant_opt| {
+            let field = Field::new(i);
+            let val = crate::const_eval::const_field(
+                self.tcx, self.param_env, variant_opt, field, cv
+            );
+            self.const_to_pat_inner(instance, val, id, span, saw_const_match_error)
+        };
+        let mut adt_subpatterns = |n, variant_opt| {
+            (0..n).map(|i| {
+                let field = Field::new(i);
+                FieldPat {
+                    field,
+                    pattern: adt_subpattern(i, variant_opt),
+                }
+            }).collect::<Vec<_>>()
+        };
+
+
+        let kind = match cv.ty.kind {
+            ty::Float(_) => {
+                self.tcx.lint_hir(
+                    ::rustc::lint::builtin::ILLEGAL_FLOATING_POINT_LITERAL_PATTERN,
+                    id,
+                    span,
+                    "floating-point types cannot be used in patterns",
+                );
+                PatKind::Constant {
+                    value: cv,
+                }
+            }
+            ty::Adt(adt_def, _) if adt_def.is_union() => {
+                // Matching on union fields is unsafe, we can't hide it in constants
+                *saw_const_match_error = true;
+                self.tcx.sess.span_err(span, "cannot use unions in constant patterns");
+                PatKind::Wild
+            }
+            // keep old code until future-compat upgraded to errors.
+            ty::Adt(adt_def, _) if !self.tcx.has_attr(adt_def.did, sym::structural_match) => {
+                let path = self.tcx.def_path_str(adt_def.did);
+                let msg = format!(
+                    "to use a constant of type `{}` in a pattern, \
+                     `{}` must be annotated with `#[derive(PartialEq, Eq)]`",
+                    path,
+                    path,
+                );
+                *saw_const_match_error = true;
+                self.tcx.sess.span_err(span, &msg);
+                PatKind::Wild
+            }
+            // keep old code until future-compat upgraded to errors.
+            ty::Ref(_, ty::TyS { kind: ty::Adt(adt_def, _), .. }, _)
+            if !self.tcx.has_attr(adt_def.did, sym::structural_match) => {
+                // HACK(estebank): Side-step ICE #53708, but anything other than erroring here
+                // would be wrong. Returnging `PatKind::Wild` is not technically correct.
+                let path = self.tcx.def_path_str(adt_def.did);
+                let msg = format!(
+                    "to use a constant of type `{}` in a pattern, \
+                     `{}` must be annotated with `#[derive(PartialEq, Eq)]`",
+                    path,
+                    path,
+                );
+                *saw_const_match_error = true;
+                self.tcx.sess.span_err(span, &msg);
+                PatKind::Wild
+            }
+            ty::Adt(adt_def, substs) if adt_def.is_enum() => {
+                let variant_index = const_variant_index(self.tcx, self.param_env, cv);
+                let subpatterns = adt_subpatterns(
+                    adt_def.variants[variant_index].fields.len(),
+                    Some(variant_index),
+                );
+                PatKind::Variant {
+                    adt_def,
+                    substs,
+                    variant_index,
+                    subpatterns,
+                }
+            }
+            ty::Adt(adt_def, _) => {
+                let struct_var = adt_def.non_enum_variant();
+                PatKind::Leaf {
+                    subpatterns: adt_subpatterns(struct_var.fields.len(), None),
+                }
+            }
+            ty::Tuple(fields) => {
+                PatKind::Leaf {
+                    subpatterns: adt_subpatterns(fields.len(), None),
+                }
+            }
+            ty::Array(_, n) => {
+                PatKind::Array {
+                    prefix: (0..n.eval_usize(self.tcx, self.param_env))
+                        .map(|i| adt_subpattern(i as usize, None))
+                        .collect(),
+                    slice: None,
+                    suffix: Vec::new(),
+                }
+            }
+            _ => {
+                PatKind::Constant {
+                    value: cv,
+                }
+            }
+        };
+
+        Pat {
+            span,
+            ty: cv.ty,
+            kind: Box::new(kind),
+        }
+    }
+}