On unsized locals with explicit types suggest &

Fix #72742.

src/test/ui/unsized-locals/suggest-borrow.rs

@@ -0,0 +1,7 @@
+fn main() {
+    let x: [u8] = vec!(1, 2, 3)[..]; //~ ERROR E0277
+    let x: &[u8] = vec!(1, 2, 3)[..]; //~ ERROR E0308
+    let x: [u8] = &vec!(1, 2, 3)[..]; //~ ERROR E0308
+    //~^ ERROR E0277
+    let x: &[u8] = &vec!(1, 2, 3)[..];
+}

src/test/ui/unsized-locals/suggest-borrow.stderr

@@ -0,0 +1,60 @@
+error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
+  --> $DIR/suggest-borrow.rs:2:9
+   |
+LL |     let x: [u8] = vec!(1, 2, 3)[..];
+   |         ^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `[u8]`
+   = note: all local variables must have a statically known size
+   = help: unsized locals are gated as an unstable feature
+help: consider borrowing here
+   |
+LL |     let x: &[u8] = vec!(1, 2, 3)[..];
+   |            +
+
+error[E0308]: mismatched types
+  --> $DIR/suggest-borrow.rs:3:20
+   |
+LL |     let x: &[u8] = vec!(1, 2, 3)[..];
+   |            -----   ^^^^^^^^^^^^^^^^^
+   |            |       |
+   |            |       expected `&[u8]`, found slice `[{integer}]`
+   |            |       help: consider borrowing here: `&vec!(1, 2, 3)[..]`
+   |            expected due to this
+
+error[E0308]: mismatched types
+  --> $DIR/suggest-borrow.rs:4:19
+   |
+LL |     let x: [u8] = &vec!(1, 2, 3)[..];
+   |            ----   ^^^^^^^^^^^^^^^^^^ expected slice `[u8]`, found `&[{integer}]`
+   |            |
+   |            expected due to this
+   |
+help: consider removing the borrow
+   |
+LL -     let x: [u8] = &vec!(1, 2, 3)[..];
+LL +     let x: [u8] = vec!(1, 2, 3)[..];
+   |
+help: alternatively, consider changing the type annotation
+   |
+LL |     let x: &[u8] = &vec!(1, 2, 3)[..];
+   |            +
+
+error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
+  --> $DIR/suggest-borrow.rs:4:9
+   |
+LL |     let x: [u8] = &vec!(1, 2, 3)[..];
+   |         ^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `[u8]`
+   = note: all local variables must have a statically known size
+   = help: unsized locals are gated as an unstable feature
+help: consider borrowing here
+   |
+LL |     let x: &[u8] = &vec!(1, 2, 3)[..];
+   |            +
+
+error: aborting due to 4 previous errors
+
+Some errors have detailed explanations: E0277, E0308.
+For more information about an error, try `rustc --explain E0277`.

src/test/ui/unsized-locals/unsized-locals-using-unsized-fn-params.stderr

@@ -27,6 +27,10 @@ LL |     let _foo: [u8] = *foo;
    = help: the trait `Sized` is not implemented for `[u8]`
    = note: all local variables must have a statically known size
    = help: unsized locals are gated as an unstable feature
+help: consider borrowing here
+   |
+LL |     let _foo: &[u8] = *foo;
+   |               +
 
 error: aborting due to 3 previous errors
 

src/test/ui/unsized/unsized6.stderr

@@ -14,6 +14,10 @@ help: consider removing the `?Sized` bound to make the type parameter `Sized`
 LL - fn f1<W: ?Sized, X: ?Sized, Y: ?Sized, Z: ?Sized>(x: &X) {
 LL + fn f1<W: ?Sized, X: ?Sized, Y, Z: ?Sized>(x: &X) {
    |
+help: consider borrowing here
+   |
+LL |     let y: &Y;
+   |            +
 
 error[E0277]: the size for values of type `X` cannot be known at compilation time
   --> $DIR/unsized6.rs:7:12
@@ -62,6 +66,10 @@ help: consider removing the `?Sized` bound to make the type parameter `Sized`
 LL - fn f2<X: ?Sized, Y: ?Sized>(x: &X) {
 LL + fn f2<X, Y: ?Sized>(x: &X) {
    |
+help: consider borrowing here
+   |
+LL |     let y: &X;
+   |            +
 
 error[E0277]: the size for values of type `Y` cannot be known at compilation time
   --> $DIR/unsized6.rs:17:12
@@ -94,6 +102,10 @@ help: consider removing the `?Sized` bound to make the type parameter `Sized`
 LL - fn f3<X: ?Sized>(x1: Box<X>, x2: Box<X>, x3: Box<X>) {
 LL + fn f3<X>(x1: Box<X>, x2: Box<X>, x3: Box<X>) {
    |
+help: consider borrowing here
+   |
+LL |     let y: &X = *x1;
+   |            +
 
 error[E0277]: the size for values of type `X` cannot be known at compilation time
   --> $DIR/unsized6.rs:24:9
@@ -144,6 +156,10 @@ help: consider removing the `?Sized` bound to make the type parameter `Sized`
 LL - fn f4<X: ?Sized + T>(x1: Box<X>, x2: Box<X>, x3: Box<X>) {
 LL + fn f4<X: T>(x1: Box<X>, x2: Box<X>, x3: Box<X>) {
    |
+help: consider borrowing here
+   |
+LL |     let y: &X = *x1;
+   |            +
 
 error[E0277]: the size for values of type `X` cannot be known at compilation time
   --> $DIR/unsized6.rs:32:9