diagnostics: Note capturing closures can't be coerced to fns

compiler/rustc_typeck/src/check/demand.rs

@@ -32,6 +32,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         if self.suggest_calling_boxed_future_when_appropriate(err, expr, expected, expr_ty) {
             return;
         }
+        self.suggest_no_capture_closure(err, expected, expr_ty);
         self.suggest_boxing_when_appropriate(err, expr, expected, expr_ty);
         self.suggest_missing_parentheses(err, expr);
         self.note_need_for_fn_pointer(err, expected, expr_ty);

compiler/rustc_typeck/src/check/fn_ctxt/suggestions.rs

@@ -2,7 +2,7 @@ use super::FnCtxt;
 use crate::astconv::AstConv;
 
 use rustc_ast::util::parser::ExprPrecedence;
-use rustc_span::{self, Span};
+use rustc_span::{self, MultiSpan, Span};
 
 use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc_hir as hir;
@@ -287,6 +287,38 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
+    /// When encountering a closure that captures variables, where a FnPtr is expected,
+    /// suggest a non-capturing closure
+    pub(in super::super) fn suggest_no_capture_closure(
+        &self,
+        err: &mut DiagnosticBuilder<'_>,
+        expected: Ty<'tcx>,
+        found: Ty<'tcx>,
+    ) {
+        if let (ty::FnPtr(_), ty::Closure(def_id, _)) = (expected.kind(), found.kind()) {
+            if let Some(upvars) = self.tcx.upvars_mentioned(*def_id) {
+                // Report upto four upvars being captured to reduce the amount error messages
+                // reported back to the user.
+                let spans_and_labels = upvars
+                    .iter()
+                    .take(4)
+                    .map(|(var_hir_id, upvar)| {
+                        let var_name = self.tcx.hir().name(*var_hir_id).to_string();
+                        let msg = format!("`{}` captured here", var_name);
+                        (upvar.span, msg)
+                    })
+                    .collect::<Vec<_>>();
+
+                let mut multi_span: MultiSpan =
+                    spans_and_labels.iter().map(|(sp, _)| *sp).collect::<Vec<_>>().into();
+                for (sp, label) in spans_and_labels {
+                    multi_span.push_span_label(sp, label);
+                }
+                err.span_note(multi_span, "closures can only be coerced to `fn` types if they do not capture any variables");
+            }
+        }
+    }
+
     /// When encountering an `impl Future` where `BoxFuture` is expected, suggest `Box::pin`.
     pub(in super::super) fn suggest_calling_boxed_future_when_appropriate(
         &self,

src/test/ui/closures/closure-no-fn-1.stderr

@@ -8,6 +8,11 @@ LL |     let foo: fn(u8) -> u8 = |v: u8| { a += v; a };
    |
    = note: expected fn pointer `fn(u8) -> u8`
                  found closure `[closure@$DIR/closure-no-fn-1.rs:6:29: 6:50]`
+note: closures can only be coerced to `fn` types if they do not capture any variables
+  --> $DIR/closure-no-fn-1.rs:6:39
+   |
+LL |     let foo: fn(u8) -> u8 = |v: u8| { a += v; a };
+   |                                       ^ `a` captured here
 
 error: aborting due to previous error
 

src/test/ui/closures/closure-no-fn-2.stderr

@@ -8,6 +8,11 @@ LL |     let bar: fn() -> u8 = || { b };
    |
    = note: expected fn pointer `fn() -> u8`
                  found closure `[closure@$DIR/closure-no-fn-2.rs:6:27: 6:35]`
+note: closures can only be coerced to `fn` types if they do not capture any variables
+  --> $DIR/closure-no-fn-2.rs:6:32
+   |
+LL |     let bar: fn() -> u8 = || { b };
+   |                                ^ `b` captured here
 
 error: aborting due to previous error
 

src/test/ui/closures/closure-no-fn-4.rs

@@ -0,0 +1,8 @@
+fn main() {
+    let b = 2;
+    let _: fn(usize) -> usize = match true {
+        true => |a| a + 1,
+        false => |a| a - b,
+        //~^ ERROR `match` arms have incompatible types
+    };
+}

src/test/ui/closures/closure-no-fn-4.stderr

@@ -0,0 +1,24 @@
+error[E0308]: `match` arms have incompatible types
+  --> $DIR/closure-no-fn-4.rs:5:18
+   |
+LL |       let _: fn(usize) -> usize = match true {
+   |  _________________________________-
+LL | |         true => |a| a + 1,
+   | |                 --------- this is found to be of type `fn(usize) -> usize`
+LL | |         false => |a| a - b,
+   | |                  ^^^^^^^^^ expected fn pointer, found closure
+LL | |
+LL | |     };
+   | |_____- `match` arms have incompatible types
+   |
+   = note: expected fn pointer `fn(usize) -> usize`
+                 found closure `[closure@$DIR/closure-no-fn-4.rs:5:18: 5:27]`
+note: closures can only be coerced to `fn` types if they do not capture any variables
+  --> $DIR/closure-no-fn-4.rs:5:26
+   |
+LL |         false => |a| a - b,
+   |                          ^ `b` captured here
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.

src/test/ui/closures/closure-no-fn-5.rs

@@ -0,0 +1,12 @@
+// When providing diagnostics about not being able to coerce a capturing-closure
+// to fn type, we want to report only upto 4 captures.
+
+fn main() {
+    let a = 0u8;
+    let b = 0u8;
+    let c = 0u8;
+    let d = 0u8;
+    let e = 0u8;
+    let bar: fn() -> u8 = || { a; b; c; d; e };
+    //~^ ERROR mismatched types
+}

src/test/ui/closures/closure-no-fn-5.stderr

@@ -0,0 +1,23 @@
+error[E0308]: mismatched types
+  --> $DIR/closure-no-fn-5.rs:10:27
+   |
+LL |     let bar: fn() -> u8 = || { a; b; c; d; e };
+   |              ----------   ^^^^^^^^^^^^^^^^^^^^ expected fn pointer, found closure
+   |              |
+   |              expected due to this
+   |
+   = note: expected fn pointer `fn() -> u8`
+                 found closure `[closure@$DIR/closure-no-fn-5.rs:10:27: 10:47]`
+note: closures can only be coerced to `fn` types if they do not capture any variables
+  --> $DIR/closure-no-fn-5.rs:10:32
+   |
+LL |     let bar: fn() -> u8 = || { a; b; c; d; e };
+   |                                ^  ^  ^  ^ `d` captured here
+   |                                |  |  |
+   |                                |  |  `c` captured here
+   |                                |  `b` captured here
+   |                                `a` captured here
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.

src/test/ui/closures/closure-reform-bad.stderr

@@ -8,6 +8,11 @@ LL |     call_bare(f)
    |
    = note: expected fn pointer `for<'r> fn(&'r str)`
                  found closure `[closure@$DIR/closure-reform-bad.rs:10:13: 10:50]`
+note: closures can only be coerced to `fn` types if they do not capture any variables
+  --> $DIR/closure-reform-bad.rs:10:43
+   |
+LL |     let f = |s: &str| println!("{}{}", s, string);
+   |                                           ^^^^^^ `string` captured here
 
 error: aborting due to previous error
 

src/test/ui/closures/print/closure-print-verbose.stderr

@@ -8,6 +8,11 @@ LL |     let foo: fn(u8) -> u8 = |v: u8| { a += v; a };
    |
    = note: expected fn pointer `fn(u8) -> u8`
                  found closure `[main::{closure#0} closure_substs=(unavailable)]`
+note: closures can only be coerced to `fn` types if they do not capture any variables
+  --> $DIR/closure-print-verbose.rs:10:39
+   |
+LL |     let foo: fn(u8) -> u8 = |v: u8| { a += v; a };
+   |                                       ^ `a` captured here
 
 error: aborting due to previous error