Auto merge of rust-lang#53147 - ashtneoi:dont-suggest-ref, r=estebank

For move errors, suggest match ergonomics instead of `ref`

Partially fixes issue rust-lang#52423. Also makes errors and suggestions more consistent between move-from-place and move-from-value errors.

Limitations:
- Only the first pattern in a match arm can have a "consider removing this borrow operator" suggestion.
- Suggestions don't always compile as-is (see the TODOs in the test for details).

Sorry for the really long test. I wanted to make sure I handled every case I could think of, and it turned out there were a lot of them.

Questions:
- Is there any particular applicability I should set on those suggestions?
- Are the notes about the `Copy` trait excessive?

Author: bors

src/librustc/mir/mod.rs

@@ -429,8 +429,8 @@ pub enum BorrowKind {
 
     /// Data must be immutable but not aliasable.  This kind of borrow
     /// cannot currently be expressed by the user and is used only in
-    /// implicit closure bindings. It is needed when you the closure
-    /// is borrowing or mutating a mutable referent, e.g.:
+    /// implicit closure bindings. It is needed when the closure is
+    /// borrowing or mutating a mutable referent, e.g.:
     ///
     ///    let x: &mut isize = ...;
     ///    let y = || *x += 5;
@@ -443,7 +443,7 @@ pub enum BorrowKind {
     ///    let y = (&mut Env { &x }, fn_ptr);  // Closure is pair of env and fn
     ///    fn fn_ptr(env: &mut Env) { **env.x += 5; }
     ///
-    /// This is then illegal because you cannot mutate a `&mut` found
+    /// This is then illegal because you cannot mutate an `&mut` found
     /// in an aliasable location. To solve, you'd have to translate with
     /// an `&mut` borrow:
     ///
@@ -523,6 +523,8 @@ pub struct VarBindingForm<'tcx> {
     /// (b) it gives a way to separate this case from the remaining cases
     ///     for diagnostics.
     pub opt_match_place: Option<(Option<Place<'tcx>>, Span)>,
+    /// Span of the pattern in which this variable was bound.
+    pub pat_span: Span,
 }
 
 #[derive(Clone, PartialEq, Eq, Hash, Debug, RustcEncodable, RustcDecodable)]
@@ -540,7 +542,8 @@ CloneTypeFoldableAndLiftImpls! { BindingForm<'tcx>, }
 impl_stable_hash_for!(struct self::VarBindingForm<'tcx> {
     binding_mode,
     opt_ty_info,
-    opt_match_place
+    opt_match_place,
+    pat_span
 });
 
 mod binding_form_impl {
@@ -673,7 +676,7 @@ pub struct LocalDecl<'tcx> {
     /// ROOT SCOPE
     ///  │{ argument x: &str }
     ///  │
-    ///  │ │{ #[allow(unused_mut] } // this is actually split into 2 scopes
+    ///  │ │{ #[allow(unused_mut)] } // this is actually split into 2 scopes
     ///  │ │                        // in practice because I'm lazy.
     ///  │ │
     ///  │ │← x.source_info.scope
@@ -710,6 +713,7 @@ impl<'tcx> LocalDecl<'tcx> {
                 binding_mode: ty::BindingMode::BindByValue(_),
                 opt_ty_info: _,
                 opt_match_place: _,
+                pat_span: _,
             }))) => true,
 
             // FIXME: might be able to thread the distinction between
@@ -729,6 +733,7 @@ impl<'tcx> LocalDecl<'tcx> {
                 binding_mode: ty::BindingMode::BindByValue(_),
                 opt_ty_info: _,
                 opt_match_place: _,
+                pat_span: _,
             }))) => true,
 
             Some(ClearCrossCrate::Set(BindingForm::ImplicitSelf)) => true,
@@ -906,7 +911,7 @@ pub enum TerminatorKind<'tcx> {
 
     /// Drop the Place and assign the new value over it. This ensures
     /// that the assignment to `P` occurs *even if* the destructor for
-    /// place unwinds. Its semantics are best explained by by the
+    /// place unwinds. Its semantics are best explained by the
     /// elaboration:
     ///
     /// ```

src/librustc_borrowck/borrowck/mod.rs

@@ -1235,7 +1235,7 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                     ty::BindByReference(..) => {
                         let let_span = self.tcx.hir.span(node_id);
                         let suggestion = suggest_ref_mut(self.tcx, let_span);
-                        if let Some((let_span, replace_str)) = suggestion {
+                        if let Some(replace_str) = suggestion {
                             db.span_suggestion(
                                 let_span,
                                 "use a mutable reference instead",