typeck/pat.rs: check_pat_top is the entry point.

This clarifies the fact that type checking patterns unconditionally
starts with `BindByValue` as the default binding mode making the
notion of a default binding mode internal to type checking patterns.

Author: Centril

src/librustc_typeck/check/_match.rs

@@ -3,7 +3,7 @@ use crate::check::coercion::CoerceMany;
 use rustc::hir::{self, ExprKind};
 use rustc::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc::traits::{ObligationCause, ObligationCauseCode};
-use rustc::ty::{self, Ty};
+use rustc::ty::Ty;
 use syntax_pos::Span;
 
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
@@ -59,8 +59,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             let mut all_pats_diverge = Diverges::WarnedAlways;
             for p in &arm.pats {
                 self.diverges.set(Diverges::Maybe);
-                let binding_mode = ty::BindingMode::BindByValue(hir::Mutability::MutImmutable);
-                self.check_pat_walk(&p, discrim_ty, binding_mode, Some(discrim.span));
+                self.check_pat_top(&p, discrim_ty, Some(discrim.span));
                 all_pats_diverge &= self.diverges.get();
             }
 

src/librustc_typeck/check/mod.rs

@@ -1104,8 +1104,7 @@ fn check_fn<'a, 'tcx>(
     // Add formal parameters.
     for (arg_ty, arg) in fn_sig.inputs().iter().zip(&body.arguments) {
         // Check the pattern.
-        let binding_mode = ty::BindingMode::BindByValue(hir::Mutability::MutImmutable);
-        fcx.check_pat_walk(&arg.pat, arg_ty, binding_mode, None);
+        fcx.check_pat_top(&arg.pat, arg_ty, None);
 
         // Check that argument is Sized.
         // The check for a non-trivial pattern is a hack to avoid duplicate warnings
@@ -3631,12 +3630,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }
         }
 
-        self.check_pat_walk(
-            &local.pat,
-            t,
-            ty::BindingMode::BindByValue(hir::Mutability::MutImmutable),
-            None,
-        );
+        self.check_pat_top(&local.pat, t, None);
         let pat_ty = self.node_ty(local.pat.hir_id);
         if pat_ty.references_error() {
             self.write_ty(local.hir_id, pat_ty);

src/librustc_typeck/check/pat.rs

@@ -29,6 +29,16 @@ You can read more about trait objects in the Trait Objects section of the Refere
 https://doc.rust-lang.org/reference/types.html#trait-objects";
 
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
+    pub fn check_pat_top(
+        &self,
+        pat: &'tcx hir::Pat,
+        expected: Ty<'tcx>,
+        discrim_span: Option<Span>,
+    ) {
+        let def_bm = ty::BindingMode::BindByValue(hir::Mutability::MutImmutable);
+        self.check_pat_walk(pat, expected, def_bm, discrim_span);
+    }
+
     /// `discrim_span` argument having a `Span` indicates that this pattern is part of a match
     /// expression arm guard, and it points to the match discriminant to add context in type errors.
     /// In the following example, `discrim_span` corresponds to the `a + b` expression:
@@ -45,7 +55,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     ///   = note: expected type `usize`
     ///              found type `std::result::Result<_, _>`
     /// ```
-    pub fn check_pat_walk(
+    fn check_pat_walk(
         &self,
         pat: &'tcx hir::Pat,
         expected: Ty<'tcx>,