Correct suggestions for closure arguments that need a borrow

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

@@ -3758,13 +3758,13 @@ fn hint_missing_borrow<'tcx>(
     err: &mut Diagnostic,
 ) {
     let found_args = match found.kind() {
-        ty::FnPtr(f) => f.inputs().skip_binder().iter(),
+        ty::FnPtr(f) => infcx.replace_bound_vars_with_placeholders(*f).inputs().iter(),
         kind => {
             span_bug!(span, "found was converted to a FnPtr above but is now {:?}", kind)
         }
     };
     let expected_args = match expected.kind() {
-        ty::FnPtr(f) => f.inputs().skip_binder().iter(),
+        ty::FnPtr(f) => infcx.replace_bound_vars_with_placeholders(*f).inputs().iter(),
         kind => {
             span_bug!(span, "expected was converted to a FnPtr above but is now {:?}", kind)
         }
@@ -3775,12 +3775,12 @@ fn hint_missing_borrow<'tcx>(
 
     let args = fn_decl.inputs.iter().map(|ty| ty);
 
-    fn get_deref_type_and_refs(mut ty: Ty<'_>) -> (Ty<'_>, usize) {
-        let mut refs = 0;
+    fn get_deref_type_and_refs(mut ty: Ty<'_>) -> (Ty<'_>, Vec<hir::Mutability>) {
+        let mut refs = vec![];
 
-        while let ty::Ref(_, new_ty, _) = ty.kind() {
+        while let ty::Ref(_, new_ty, mutbl) = ty.kind() {
             ty = *new_ty;
-            refs += 1;
+            refs.push(*mutbl);
         }
 
         (ty, refs)
@@ -3794,11 +3794,21 @@ fn hint_missing_borrow<'tcx>(
         let (expected_ty, expected_refs) = get_deref_type_and_refs(*expected_arg);
 
         if infcx.can_eq(param_env, found_ty, expected_ty).is_ok() {
-            if found_refs < expected_refs {
-                to_borrow.push((arg.span.shrink_to_lo(), "&".repeat(expected_refs - found_refs)));
-            } else if found_refs > expected_refs {
+            // FIXME: This could handle more exotic cases like mutability mismatches too!
+            if found_refs.len() < expected_refs.len()
+                && found_refs[..] == expected_refs[expected_refs.len() - found_refs.len()..]
+            {
+                to_borrow.push((
+                    arg.span.shrink_to_lo(),
+                    expected_refs[..expected_refs.len() - found_refs.len()]
+                        .iter()
+                        .map(|mutbl| format!("&{}", mutbl.prefix_str()))
+                        .collect::<Vec<_>>()
+                        .join(""),
+                ));
+            } else if found_refs.len() > expected_refs.len() {
                 let mut span = arg.span.shrink_to_lo();
-                let mut left = found_refs - expected_refs;
+                let mut left = found_refs.len() - expected_refs.len();
                 let mut ty = arg;
                 while let hir::TyKind::Ref(_, mut_ty) = &ty.kind && left > 0 {
                     span = span.with_hi(mut_ty.ty.span.lo());

tests/ui/suggestions/late-bound-in-borrow-closure-sugg.rs

@@ -0,0 +1,28 @@
+use std::cell::RefCell;
+use std::collections::HashMap;
+use std::rc::Rc;
+
+pub struct Trader<'a> {
+    closure: Box<dyn Fn(&mut Trader) + 'a>,
+}
+
+impl<'a> Trader<'a> {
+    pub fn new() -> Self {
+        Trader {
+            closure: Box::new(|_| {}),
+        }
+    }
+    pub fn set_closure(&mut self, function: impl Fn(&mut Trader) + 'a) {
+        //foo
+    }
+}
+
+fn main() {
+    let closure = |trader : Trader| {
+        println!("Woooosh!");
+    };
+
+    let mut trader = Trader::new();
+    trader.set_closure(closure);
+    //~^ ERROR type mismatch in closure arguments
+}

tests/ui/suggestions/late-bound-in-borrow-closure-sugg.stderr

@@ -0,0 +1,26 @@
+error[E0631]: type mismatch in closure arguments
+  --> $DIR/late-bound-in-borrow-closure-sugg.rs:26:24
+   |
+LL |     let closure = |trader : Trader| {
+   |                   ----------------- found signature defined here
+...
+LL |     trader.set_closure(closure);
+   |            ----------- ^^^^^^^ expected due to this
+   |            |
+   |            required by a bound introduced by this call
+   |
+   = note: expected closure signature `for<'a, 'b> fn(&'a mut Trader<'b>) -> _`
+              found closure signature `for<'a> fn(Trader<'a>) -> _`
+note: required by a bound in `Trader::<'a>::set_closure`
+  --> $DIR/late-bound-in-borrow-closure-sugg.rs:15:50
+   |
+LL |     pub fn set_closure(&mut self, function: impl Fn(&mut Trader) + 'a) {
+   |                                                  ^^^^^^^^^^^^^^^ required by this bound in `Trader::<'a>::set_closure`
+help: consider borrowing the argument
+   |
+LL |     let closure = |trader : &mut Trader| {
+   |                             ++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0631`.