typeck: refactor default binding mode logic & improve docs

src/librustc_typeck/check/pat.rs

@@ -89,6 +89,18 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
  }
 }
 
+const INITIAL_BM: BindingMode = BindingMode::BindByValue(hir::Mutability::Not);
+
+/// Mode for adjusting the expected type and binding mode.
+enum AdjustMode {
+ /// Peel off all immediate reference types.
+ Peel,
+ /// Reset binding mode to the inital mode.
+ Reset,
+ /// Pass on the input binding mode and expected type.
+ Pass,
+}
+
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  /// Type check the given top level pattern against the `expected` type.
  ///
@@ -105,8 +117,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  span: Option<Span>,
  origin_expr: bool,
  ) {
- let def_bm = BindingMode::BindByValue(hir::Mutability::Not);
- self.check_pat(pat, expected, def_bm, TopInfo { expected, origin_expr, span });
+ self.check_pat(pat, expected, INITIAL_BM, TopInfo { expected, origin_expr, span });
  }
 
  /// Type check the given `pat` against the `expected` type
@@ -123,12 +134,12 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  ) {
  debug!("check_pat(pat={:?},expected={:?},def_bm={:?})", pat, expected, def_bm);
 
- let path_resolution = match &pat.kind {
+ let path_res = match &pat.kind {
  PatKind::Path(qpath) => Some(self.resolve_ty_and_res_ufcs(qpath, pat.hir_id, pat.span)),
  _ => None,
  };
- let is_nrp = self.is_non_ref_pat(pat, path_resolution.map(|(res, ..)| res));
- let (expected, def_bm) = self.calc_default_binding_mode(pat, expected, def_bm, is_nrp);
+ let adjust_mode = self.calc_adjust_mode(pat, path_res.map(|(res, ..)| res));
+ let (expected, def_bm) = self.calc_default_binding_mode(pat, expected, def_bm, adjust_mode);
 
  let ty = match pat.kind {
  PatKind::Wild => expected,
@@ -141,7 +152,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  self.check_pat_tuple_struct(pat, qpath, subpats, ddpos, expected, def_bm, ti)
  }
  PatKind::Path(ref qpath) => {
- self.check_pat_path(pat, path_resolution.unwrap(), qpath, expected)
+ self.check_pat_path(pat, path_res.unwrap(), qpath, expected)
  }
  PatKind::Struct(ref qpath, fields, etc) => {
  self.check_pat_struct(pat, qpath, fields, etc, expected, def_bm, ti)
@@ -223,64 +234,68 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  pat: &'tcx Pat<'tcx>,
  expected: Ty<'tcx>,
  def_bm: BindingMode,
- is_non_ref_pat: bool,
+ adjust_mode: AdjustMode,
  ) -> (Ty<'tcx>, BindingMode) {
- if is_non_ref_pat {
- debug!("pattern is non reference pattern");
- self.peel_off_references(pat, expected, def_bm)
- } else {
- // When you encounter a `&pat` pattern, reset to "by
- // value". This is so that `x` and `y` here are by value,
- // as they appear to be:
- //
- // ```
- // match &(&22, &44) {
- // (&x, &y) => ...
- // }
- // ```
- //
- // See issue #46688.
- let def_bm = match pat.kind {
- PatKind::Ref(..) => ty::BindByValue(hir::Mutability::Not),
- _ => def_bm,
- };
- (expected, def_bm)
+ match adjust_mode {
+ AdjustMode::Pass => (expected, def_bm),
+ AdjustMode::Reset => (expected, INITIAL_BM),
+ AdjustMode::Peel => self.peel_off_references(pat, expected, def_bm),
  }
  }
 
- /// Is the pattern a "non reference pattern"?
+ /// How should the binding mode and expected type be adjusted?
+ ///
  /// When the pattern is a path pattern, `opt_path_res` must be `Some(res)`.
- fn is_non_ref_pat(&self, pat: &'tcx Pat<'tcx>, opt_path_res: Option<Res>) -> bool {
- match pat.kind {
+ fn calc_adjust_mode(&self, pat: &'tcx Pat<'tcx>, opt_path_res: Option<Res>) -> AdjustMode {
+ match &pat.kind {
+ // Type checking these product-like types successfully always require
+ // that the expected type be of those types and not reference types.
  PatKind::Struct(..)
  | PatKind::TupleStruct(..)
  | PatKind::Tuple(..)
  | PatKind::Box(_)
  | PatKind::Range(..)
- | PatKind::Slice(..) => true,
- PatKind::Lit(ref lt) => {
-  let ty = self.check_expr(lt);
-  match ty.kind {
-  ty::Ref(..) => false,
-  _ => true,
- }
- }
+ | PatKind::Slice(..) => AdjustMode::Peel,
+ // String and byte-string literals result in types `&str` and `&[u8]` respectively.
+ // All other literals result in non-reference types.
+ // As a result, we allow `if let 0 = &&0 {}` but not `if let "foo" = &&"foo {}`.
+ PatKind::Lit(lt) => match self.check_expr(lt).kind {
+ ty::Ref(..) => AdjustMode::Pass,
+ _ => AdjustMode::Peel,
+ },
  PatKind::Path(_) => match opt_path_res.unwrap() {
- Res::Def(DefKind::Const, _) | Res::Def(DefKind::AssocConst, _) => false,
- _ => true,
+ // These constants can be of a reference type, e.g. `const X: &u8 = &0;`.
+ // Peeling the reference types too early will cause type checking failures.
+ // Although it would be possible to *also* peel the types of the constants too.
+ Res::Def(DefKind::Const, _) | Res::Def(DefKind::AssocConst, _) => AdjustMode::Pass,
+ // In the `ValueNS`, we have `SelfCtor(..) | Ctor(_, Const), _)` remaining which
+ // could successfully compile. The former being `Self` requires a unit struct.
+ // In either case, and unlike constants, the pattern itself cannot be
+ // a reference type wherefore peeling doesn't give up any expressivity.
+ _ => AdjustMode::Peel,
  },
- // FIXME(or_patterns; Centril | dlrobertson): To keep things compiling
- // for or-patterns at the top level, we need to make `p_0 | ... | p_n`
- // a "non reference pattern". For example the following currently compiles:
+ // When encountering a `& mut? pat` pattern, reset to "by value".
+ // This is so that `x` and `y` here are by value, as they appear to be:
+ //
  // ```
- // match &1 {
- // e @ &(1...2) | e @ &(3...4) => {}
- // _ => {}
+ // match &(&22, &44) {
+ // (&x, &y) => ...
  // }
  // ```
  //
- // We should consider whether we should do something special in nested or-patterns.
- PatKind::Or(_) | PatKind::Wild | PatKind::Binding(..) | PatKind::Ref(..) => false,
+ // See issue #46688.
+ PatKind::Ref(..) => AdjustMode::Reset,
+ // A `_` pattern works with any expected type, so there's no need to do anything.
+ PatKind::Wild
+ // Bindings also work with whatever the expected type is,
+ // and moreover if we peel references off, that will give us the wrong binding type.
+ // Also, we can have a subpattern `binding @ pat`.
+ // Each side of the `@` should be treated independently (like with OR-patterns).
+ | PatKind::Binding(..)
+ // An OR-pattern just propagates to each individual alternative.
+ // This is maximally flexible, allowing e.g., `Some(mut x) | &Some(mut x)`.
+ // In that example, `Some(mut x)` results in `Peel` whereas `&Some(mut x)` in `Reset`.
+ | PatKind::Or(_) => AdjustMode::Pass,
  }
  }
 
@@ -508,7 +523,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  let local_ty = self.local_ty(pat.span, pat.hir_id).decl_ty;
  let eq_ty = match bm {
  ty::BindByReference(mutbl) => {
- // If the binding is like `ref x | ref const x | ref mut x`
+ // If the binding is like `ref x | ref mut x`,
  // then `x` is assigned a value of type `&M T` where M is the
  // mutability and T is the expected type.
  //