Do not suggest incorrect syntax on pattern type error due to borrow

src/librustc_typeck/check/_match.rs

@@ -378,12 +378,61 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                             // `fn foo(foo: &u32)`
                             if let Some(mut err) = err {
                                 if let PatKind::Binding(..) = inner.node {
-                                    if let Ok(snippet) = tcx.sess.source_map()
-                                                                    .span_to_snippet(pat.span)
-                                    {
-                                        err.help(&format!("did you mean `{}: &{}`?",
-                                                            &snippet[1..],
-                                                            expected));
+                                    let parent_id = tcx.hir().get_parent_node_by_hir_id(pat.hir_id);
+                                    let parent = tcx.hir().get_by_hir_id(parent_id);
+                                    debug!("inner {:?} pat {:?} parent {:?}", inner, pat, parent);
+                                    match parent {
+                                        hir::Node::Item(hir::Item {
+                                            node: hir::ItemKind::Fn(..), ..
+                                        }) |
+                                        hir::Node::ForeignItem(hir::ForeignItem {
+                                            node: hir::ForeignItemKind::Fn(..), ..
+                                        }) |
+                                        hir::Node::TraitItem(hir::TraitItem {
+                                            node: hir::TraitItemKind::Method(..), ..
+                                        }) |
+                                        hir::Node::ImplItem(hir::ImplItem {
+                                            node: hir::ImplItemKind::Method(..), ..
+                                        }) => { // this pat is likely an argument
+                                            if let Ok(snippet) = tcx.sess.source_map()
+                                                .span_to_snippet(inner.span)
+                                            { // FIXME: turn into structured suggestion, will need
+                                              // a span that also includes the the arg's type.
+                                                err.help(&format!(
+                                                    "did you mean `{}: &{}`?",
+                                                    snippet,
+                                                    expected,
+                                                ));
+                                            }
+                                        }
+                                        hir::Node::Expr(hir::Expr {
+                                            node: hir::ExprKind::Match(..), ..
+                                        }) => { // rely on match ergonomics
+                                            if let Ok(snippet) = tcx.sess.source_map()
+                                                .span_to_snippet(inner.span)
+                                            {
+                                                err.span_suggestion(
+                                                    pat.span,
+                                                    "you can rely on match ergonomics and remove \
+                                                     the explicit borrow",
+                                                    snippet,
+                                                    Applicability::MaybeIncorrect,
+                                                );
+                                            }
+                                        }
+                                        hir::Node::Pat(_) => {  // nested `&&pat`
+                                            if let Ok(snippet) = tcx.sess.source_map()
+                                                .span_to_snippet(inner.span)
+                                            {
+                                                err.span_suggestion(
+                                                    pat.span,
+                                                    "you can probaly remove the explicit borrow",
+                                                    snippet,
+                                                    Applicability::MaybeIncorrect,
+                                                );
+                                            }
+                                        }
+                                        _ => {} // don't provide suggestions in other cases #55175
                                     }
                                 }
                                 err.emit();

src/test/ui/destructure-trait-ref.stderr

@@ -20,21 +20,25 @@ error[E0308]: mismatched types
   --> $DIR/destructure-trait-ref.rs:31:10
    |
 LL |     let &&x = &1isize as &T;
-   |          ^^ expected trait T, found reference
+   |          ^^
+   |          |
+   |          expected trait T, found reference
+   |          help: you can probaly remove the explicit borrow: `x`
    |
    = note: expected type `dyn T`
               found type `&_`
-   = help: did you mean `x: &dyn T`?
 
 error[E0308]: mismatched types
   --> $DIR/destructure-trait-ref.rs:36:11
    |
 LL |     let &&&x = &(&1isize as &T);
-   |           ^^ expected trait T, found reference
+   |           ^^
+   |           |
+   |           expected trait T, found reference
+   |           help: you can probaly remove the explicit borrow: `x`
    |
    = note: expected type `dyn T`
               found type `&_`
-   = help: did you mean `x: &dyn T`?
 
 error[E0308]: mismatched types
   --> $DIR/destructure-trait-ref.rs:41:13

src/test/ui/mismatched_types/issue-38371.stderr

@@ -12,11 +12,13 @@ error[E0308]: mismatched types
   --> $DIR/issue-38371.rs:18:9
    |
 LL | fn agh(&&bar: &u32) {
-   |         ^^^^ expected u32, found reference
+   |         ^^^^
+   |         |
+   |         expected u32, found reference
+   |         help: you can probaly remove the explicit borrow: `bar`
    |
    = note: expected type `u32`
               found type `&_`
-   = help: did you mean `bar: &u32`?
 
 error[E0308]: mismatched types
   --> $DIR/issue-38371.rs:21:8

src/test/ui/suggestions/match-ergonomics.rs

@@ -0,0 +1,40 @@
+fn main() {
+    let x = vec![1i32];
+    match &x[..] {
+        [&v] => {}, //~ ERROR mismatched types
+        _ => {},
+    }
+    match x {
+        [&v] => {}, //~ ERROR expected an array or slice
+        _ => {},
+    }
+    match &x[..] {
+        [v] => {},
+        _ => {},
+    }
+    match &x[..] {
+        &[v] => {},
+        _ => {},
+    }
+    match x {
+        [v] => {}, //~ ERROR expected an array or slice
+        _ => {},
+    }
+    let y = 1i32;
+    match &y {
+        &v => {},
+        _ => {},
+    }
+    match y {
+        &v => {}, //~ ERROR mismatched types
+        _ => {},
+    }
+    match &y {
+        v => {},
+        _ => {},
+    }
+    match y {
+        v => {},
+        _ => {},
+    }
+}

src/test/ui/suggestions/match-ergonomics.stderr

@@ -0,0 +1,41 @@
+error[E0308]: mismatched types
+  --> $DIR/match-ergonomics.rs:4:10
+   |
+LL |         [&v] => {},
+   |          ^^
+   |          |
+   |          expected i32, found reference
+   |          help: you can probaly remove the explicit borrow: `v`
+   |
+   = note: expected type `i32`
+              found type `&_`
+
+error[E0529]: expected an array or slice, found `std::vec::Vec<i32>`
+  --> $DIR/match-ergonomics.rs:8:9
+   |
+LL |         [&v] => {},
+   |         ^^^^ pattern cannot match with input type `std::vec::Vec<i32>`
+
+error[E0529]: expected an array or slice, found `std::vec::Vec<i32>`
+  --> $DIR/match-ergonomics.rs:20:9
+   |
+LL |         [v] => {},
+   |         ^^^ pattern cannot match with input type `std::vec::Vec<i32>`
+
+error[E0308]: mismatched types
+  --> $DIR/match-ergonomics.rs:29:9
+   |
+LL |         &v => {},
+   |         ^^ expected i32, found reference
+   |
+   = note: expected type `i32`
+              found type `&_`
+help: you can rely on match ergonomics and remove the explicit borrow
+   |
+LL |         v => {},
+   |         ^
+
+error: aborting due to 4 previous errors
+
+Some errors have detailed explanations: E0308, E0529.
+For more information about an error, try `rustc --explain E0308`.