Auto merge of #51686 - nikomatsakis:issue-51415-borrowck-match-default-bindings-bug, r=eddyb

yet another "old borrowck" bug around match default bindings

We were getting the type of the parameter from its pattern, but that didn't include adjustments. I did a `ripgrep` around and this seemed to be the only affected case.

The reason this didn't show up as an ICE earlier is that mem-categorization is lenient with respect to weird discrepancies. I am going to add more delay-span-bug calls shortly around that (I'll push onto the PR).

This example is an ICE, but I presume that there is a way to make a soundness example out of this -- it basically ignores borrows occuring inside match-default-bindings in a closure, though only if the implicit deref is at the top-level. It happens though that this occurs frequently in iterators, which often give a `&T` parameter.

Fixes #51415
Fixes #49534

r? @eddyb

src/librustc/middle/expr_use_visitor.rs

@@ -313,7 +313,8 @@ impl<'a, 'gcx, 'tcx> ExprUseVisitor<'a, 'gcx, 'tcx> {
         debug!("consume_body(body={:?})", body);
 
         for arg in &body.arguments {
-            let arg_ty = return_if_err!(self.mc.node_ty(arg.pat.hir_id));
+            let arg_ty = return_if_err!(self.mc.pat_ty_adjusted(&arg.pat));
+            debug!("consume_body: arg_ty = {:?}", arg_ty);
 
             let fn_body_scope_r =
                 self.tcx().mk_region(ty::ReScope(region::Scope::Node(body.value.hir_id.local_id)));

src/librustc/middle/mem_categorization.rs

@@ -517,7 +517,7 @@ impl<'a, 'gcx, 'tcx> MemCategorizationContext<'a, 'gcx, 'tcx> {
     ///   implicit deref patterns attached (e.g., it is really
     ///   `&Some(x)`). In that case, we return the "outermost" type
     ///   (e.g., `&Option<T>).
-    fn pat_ty(&self, pat: &hir::Pat) -> McResult<Ty<'tcx>> {
+    pub fn pat_ty_adjusted(&self, pat: &hir::Pat) -> McResult<Ty<'tcx>> {
         // Check for implicit `&` types wrapping the pattern; note
         // that these are never attached to binding patterns, so
         // actually this is somewhat "disjoint" from the code below
@@ -1300,7 +1300,7 @@ impl<'a, 'gcx, 'tcx> MemCategorizationContext<'a, 'gcx, 'tcx> {
             };
 
             for (i, subpat) in subpats.iter().enumerate_and_adjust(expected_len, ddpos) {
-                let subpat_ty = self.pat_ty(&subpat)?; // see (*2)
+                let subpat_ty = self.pat_ty_adjusted(&subpat)?; // see (*2)
                 let interior = InteriorField(FieldIndex(i, Name::intern(&i.to_string())));
                 let subcmt = Rc::new(self.cat_imm_interior(pat, cmt.clone(), subpat_ty, interior));
                 self.cat_pattern_(subcmt, &subpat, op)?;
@@ -1323,7 +1323,7 @@ impl<'a, 'gcx, 'tcx> MemCategorizationContext<'a, 'gcx, 'tcx> {
             };
 
             for fp in field_pats {
-                let field_ty = self.pat_ty(&fp.node.pat)?; // see (*2)
+                let field_ty = self.pat_ty_adjusted(&fp.node.pat)?; // see (*2)
                 let f_index = self.tcx.field_index(fp.node.id, self.tables);
                 let cmt_field = Rc::new(self.cat_field(pat, cmt.clone(), f_index,
                                                        fp.node.ident, field_ty));
@@ -1342,7 +1342,7 @@ impl<'a, 'gcx, 'tcx> MemCategorizationContext<'a, 'gcx, 'tcx> {
                 ref ty => span_bug!(pat.span, "tuple pattern unexpected type {:?}", ty),
             };
             for (i, subpat) in subpats.iter().enumerate_and_adjust(expected_len, ddpos) {
-                let subpat_ty = self.pat_ty(&subpat)?; // see (*2)
+                let subpat_ty = self.pat_ty_unadjusted(&subpat)?; // see (*2)
                 let interior = InteriorField(FieldIndex(i, Name::intern(&i.to_string())));
                 let subcmt = Rc::new(self.cat_imm_interior(pat, cmt.clone(), subpat_ty, interior));
                 self.cat_pattern_(subcmt, &subpat, op)?;

src/test/ui/borrowck/issue-51415.nll.stderr

@@ -0,0 +1,9 @@
+error[E0507]: cannot move out of borrowed content
+  --> $DIR/issue-51415.rs:16:47
+   |
+LL |     let opt = a.iter().enumerate().find(|(_, &s)| {
+   |                                               ^ cannot move out of borrowed content
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0507`.

src/test/ui/borrowck/issue-51415.rs

@@ -0,0 +1,21 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Regression test for #51415: match default bindings were failing to
+// see the "move out" implied by `&s` below.
+
+fn main() {
+    let a = vec![String::from("a")];
+    let opt = a.iter().enumerate().find(|(_, &s)| {
+        //~^ ERROR cannot move out
+        *s == String::from("d")
+    }).map(|(i, _)| i);
+    println!("{:?}", opt);
+}

src/test/ui/borrowck/issue-51415.stderr

@@ -0,0 +1,12 @@
+error[E0507]: cannot move out of borrowed content
+  --> $DIR/issue-51415.rs:16:46
+   |
+LL |     let opt = a.iter().enumerate().find(|(_, &s)| {
+   |                                              ^-
+   |                                              ||
+   |                                              |hint: to prevent move, use `ref s` or `ref mut s`
+   |                                              cannot move out of borrowed content
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0507`.