Implement jackh726's suggestions

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -689,17 +689,11 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         // Blacklist traits for which it would be nonsensical to suggest borrowing.
         // For instance, immutable references are always Copy, so suggesting to
         // borrow would always succeed, but it's probably not what the user wanted.
-        let blacklist: Vec<_> = [
+        let blacklist: Vec<_> =
-            LangItem::Copy,
+            [LangItem::Copy, LangItem::Clone, LangItem::Unpin, LangItem::Sized, LangItem::Send]
-            LangItem::Clone,
+                .iter()
-            LangItem::Pin,
+                .filter_map(|lang_item| self.tcx.lang_items().require(*lang_item).ok())
-            LangItem::Unpin,
+                .collect();
-            LangItem::Sized,
-            LangItem::Send,
-        ]
-        .iter()
-        .filter_map(|lang_item| self.tcx.lang_items().require(*lang_item).ok())
-        .collect();
 
         let span = obligation.cause.span;
         let param_env = obligation.param_env;
@@ -771,7 +765,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                         err.span_suggestion(
                             span,
                             &format!(
-                                "consider borrowing{} here",
+                                "consider{} borrowing here",
                                 if mtbl { " mutably" } else { "" }
                             ),
                             format!("&{}{}", if mtbl { "mut " } else { "" }, snippet),

src/test/ui/suggestions/imm-ref-trait-object-literal.stderr

@@ -24,7 +24,7 @@ LL |   foo(s);
    |       ^
    |       |
    |       expected an implementor of trait `Trait`
-   |       help: consider borrowing mutably here: `&mut s`
+   |       help: consider mutably borrowing here: `&mut s`
 
 error: aborting due to 2 previous errors
 

src/test/ui/suggestions/issue-84973-2.stderr

@@ -8,7 +8,7 @@ LL |     foo(a);
    |         ^
    |         |
    |         expected an implementor of trait `Tr`
-   |         help: consider borrowing mutably here: `&mut a`
+   |         help: consider mutably borrowing here: `&mut a`
 
 error: aborting due to previous error
 

src/test/ui/suggestions/issue-84973-blacklist.rs

@@ -0,0 +1,29 @@
+// Checks that certain traits for which we don't want to suggest borrowing
+// are blacklisted and don't cause the suggestion to be issued.
+
+#![feature(generators)]
+
+fn f_copy<T: Copy>(t: T) {}
+fn f_clone<T: Clone>(t: T) {}
+fn f_unpin<T: Unpin>(t: T) {}
+fn f_sized<T: Sized>(t: T) {}
+fn f_send<T: Send>(t: T) {}
+
+struct S;
+
+fn main() {
+    f_copy("".to_string()); //~ ERROR: the trait bound `String: Copy` is not satisfied [E0277]
+    f_clone(S); //~ ERROR: the trait bound `S: Clone` is not satisfied [E0277]
+    f_unpin(static || { yield; });
+    //~^ ERROR: cannot be unpinned [E0277]
+
+    let cl = || ();
+    let ref_cl: &dyn Fn() -> () = &cl;
+    f_sized(*ref_cl);
+    //~^ ERROR: the size for values of type `dyn Fn()` cannot be known at compilation time [E0277]
+    //~| ERROR: the size for values of type `dyn Fn()` cannot be known at compilation time [E0277]
+
+    use std::rc::Rc;
+    let rc = Rc::new(0);
+    f_send(rc); //~ ERROR: `Rc<{integer}>` cannot be sent between threads safely [E0277]
+}

src/test/ui/suggestions/issue-84973-blacklist.stderr

@@ -0,0 +1,64 @@
+error[E0277]: the trait bound `String: Copy` is not satisfied
+  --> $DIR/issue-84973-blacklist.rs:15:12
+   |
+LL | fn f_copy<T: Copy>(t: T) {}
+   |              ---- required by this bound in `f_copy`
+...
+LL |     f_copy("".to_string());
+   |            ^^^^^^^^^^^^^^ the trait `Copy` is not implemented for `String`
+
+error[E0277]: the trait bound `S: Clone` is not satisfied
+  --> $DIR/issue-84973-blacklist.rs:16:13
+   |
+LL | fn f_clone<T: Clone>(t: T) {}
+   |               ----- required by this bound in `f_clone`
+...
+LL |     f_clone(S);
+   |             ^ the trait `Clone` is not implemented for `S`
+
+error[E0277]: `[static generator@$DIR/issue-84973-blacklist.rs:17:13: 17:33]` cannot be unpinned
+  --> $DIR/issue-84973-blacklist.rs:17:5
+   |
+LL | fn f_unpin<T: Unpin>(t: T) {}
+   |               ----- required by this bound in `f_unpin`
+...
+LL |     f_unpin(static || { yield; });
+   |     ^^^^^^^ the trait `Unpin` is not implemented for `[static generator@$DIR/issue-84973-blacklist.rs:17:13: 17:33]`
+   |
+   = note: consider using `Box::pin`
+
+error[E0277]: the size for values of type `dyn Fn()` cannot be known at compilation time
+  --> $DIR/issue-84973-blacklist.rs:22:13
+   |
+LL | fn f_sized<T: Sized>(t: T) {}
+   |            - required by this bound in `f_sized`
+...
+LL |     f_sized(*ref_cl);
+   |             ^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `dyn Fn()`
+
+error[E0277]: `Rc<{integer}>` cannot be sent between threads safely
+  --> $DIR/issue-84973-blacklist.rs:28:12
+   |
+LL | fn f_send<T: Send>(t: T) {}
+   |              ---- required by this bound in `f_send`
+...
+LL |     f_send(rc);
+   |            ^^ `Rc<{integer}>` cannot be sent between threads safely
+   |
+   = help: the trait `Send` is not implemented for `Rc<{integer}>`
+
+error[E0277]: the size for values of type `dyn Fn()` cannot be known at compilation time
+  --> $DIR/issue-84973-blacklist.rs:22:5
+   |
+LL |     f_sized(*ref_cl);
+   |     ^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `dyn Fn()`
+   = note: all function arguments must have a statically known size
+   = help: unsized fn params are gated as an unstable feature
+
+error: aborting due to 6 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/suggestions/issue-84973-negative.rs

@@ -0,0 +1,12 @@
+// Checks that we only suggest borrowing if &T actually implements the trait.
+
+trait Tr {}
+impl Tr for &f32 {}
+fn bar<T: Tr>(t: T) {}
+
+fn main() {
+    let a = 0i32;
+    let b = 0.0f32;
+    bar(a); //~ ERROR: the trait bound `i32: Tr` is not satisfied [E0277]
+    bar(b); //~ ERROR: the trait bound `f32: Tr` is not satisfied [E0277]
+}

src/test/ui/suggestions/issue-84973-negative.stderr

@@ -0,0 +1,24 @@
+error[E0277]: the trait bound `i32: Tr` is not satisfied
+  --> $DIR/issue-84973-negative.rs:10:9
+   |
+LL | fn bar<T: Tr>(t: T) {}
+   |           -- required by this bound in `bar`
+...
+LL |     bar(a);
+   |         ^ the trait `Tr` is not implemented for `i32`
+
+error[E0277]: the trait bound `f32: Tr` is not satisfied
+  --> $DIR/issue-84973-negative.rs:11:9
+   |
+LL | fn bar<T: Tr>(t: T) {}
+   |           -- required by this bound in `bar`
+...
+LL |     bar(b);
+   |         ^
+   |         |
+   |         expected an implementor of trait `Tr`
+   |         help: consider borrowing here: `&b`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.