Better suggestions for Fn trait selection errors

5 files changed, 114 insertions, 0 deletions

diff --git a/src/test/ui/higher-rank-trait-bounds/normalize-under-binder/issue-62529-3.stderr b/src/test/ui/higher-rank-trait-bounds/normalize-under-binder/issue-62529-3.stderr
index b1107346421..066bf431a83 100644
--- a/src/test/ui/higher-rank-trait-bounds/normalize-under-binder/issue-62529-3.stderr
+++ b/src/test/ui/higher-rank-trait-bounds/normalize-under-binder/issue-62529-3.stderr
@@ -4,6 +4,8 @@ error[E0277]: expected a `Fn<(<_ as ATC<'a>>::Type,)>` closure, found `F`
 LL |         call(f, ());
    |         ^^^^ expected an `Fn<(<_ as ATC<'a>>::Type,)>` closure, found `F`
    |
+   = note: expected a closure with arguments `((),)`
+              found a closure with arguments `(<_ as ATC<'a>>::Type,)`
 note: required by a bound in `call`
   --> $DIR/issue-62529-3.rs:9:36
    |
diff --git a/src/test/ui/issues/issue-59494.stderr b/src/test/ui/issues/issue-59494.stderr
index a9284535e4d..8b542bb69de 100644
--- a/src/test/ui/issues/issue-59494.stderr
+++ b/src/test/ui/issues/issue-59494.stderr
@@ -7,6 +7,8 @@ LL |     let t8 = t8n(t7, t7p(f, g));
    |              required by a bound introduced by this call
    |
    = help: the trait `Fn<(_,)>` is not implemented for `impl Fn(((_, _), _))`
+   = note: expected a closure with arguments `(((_, _), _),)`
+              found a closure with arguments `(_,)`
 note: required by a bound in `t8n`
   --> $DIR/issue-59494.rs:5:45
    |
diff --git a/src/test/ui/trait-bounds/mismatch-fn-trait.rs b/src/test/ui/trait-bounds/mismatch-fn-trait.rs
new file mode 100644
index 00000000000..0ed64043a9a
--- /dev/null
+++ b/src/test/ui/trait-bounds/mismatch-fn-trait.rs
@@ -0,0 +1,28 @@
+fn take(_f: impl FnMut(i32)) {}
+
+fn test1(f: impl FnMut(u32)) {
+    take(f)
+    //~^ ERROR [E0277]
+}
+
+fn test2(f: impl FnMut(i32, i32)) {
+    take(f)
+    //~^ ERROR [E0277]
+}
+
+fn test3(f: impl FnMut()) {
+    take(f)
+    //~^ ERROR [E0277]
+}
+
+fn test4(f: impl FnOnce(i32)) {
+    take(f)
+    //~^ ERROR [E0277]
+}
+
+fn test5(f: impl FnOnce(u32)) {
+    take(f)
+    //~^ ERROR [E0277]
+}
+
+fn main() {}
diff --git a/src/test/ui/trait-bounds/mismatch-fn-trait.stderr b/src/test/ui/trait-bounds/mismatch-fn-trait.stderr
new file mode 100644
index 00000000000..961e6d88fbe
--- /dev/null
+++ b/src/test/ui/trait-bounds/mismatch-fn-trait.stderr
@@ -0,0 +1,81 @@
+error[E0277]: expected a `FnMut<(i32,)>` closure, found `impl FnMut(u32)`
+  --> $DIR/mismatch-fn-trait.rs:4:10
+   |
+LL |     take(f)
+   |     ---- ^ expected an `FnMut<(i32,)>` closure, found `impl FnMut(u32)`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = note: expected a closure with arguments `(u32,)`
+              found a closure with arguments `(i32,)`
+note: required by a bound in `take`
+  --> $DIR/mismatch-fn-trait.rs:1:18
+   |
+LL | fn take(_f: impl FnMut(i32)) {}
+   |                  ^^^^^^^^^^ required by this bound in `take`
+
+error[E0277]: expected a `FnMut<(i32,)>` closure, found `impl FnMut(i32, i32)`
+  --> $DIR/mismatch-fn-trait.rs:9:10
+   |
+LL |     take(f)
+   |     ---- ^ expected an `FnMut<(i32,)>` closure, found `impl FnMut(i32, i32)`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = note: expected a closure taking 2 arguments, but one taking 1 argument was given
+note: required by a bound in `take`
+  --> $DIR/mismatch-fn-trait.rs:1:18
+   |
+LL | fn take(_f: impl FnMut(i32)) {}
+   |                  ^^^^^^^^^^ required by this bound in `take`
+
+error[E0277]: expected a `FnMut<(i32,)>` closure, found `impl FnMut()`
+  --> $DIR/mismatch-fn-trait.rs:14:10
+   |
+LL |     take(f)
+   |     ---- ^ expected an `FnMut<(i32,)>` closure, found `impl FnMut()`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = note: expected a closure taking 0 arguments, but one taking 1 argument was given
+note: required by a bound in `take`
+  --> $DIR/mismatch-fn-trait.rs:1:18
+   |
+LL | fn take(_f: impl FnMut(i32)) {}
+   |                  ^^^^^^^^^^ required by this bound in `take`
+
+error[E0277]: expected a `FnMut<(i32,)>` closure, found `impl FnOnce(i32)`
+  --> $DIR/mismatch-fn-trait.rs:19:10
+   |
+LL |     take(f)
+   |     ---- ^ expected an `FnMut<(i32,)>` closure, found `impl FnOnce(i32)`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = note: `impl FnOnce(i32)` implements `FnOnce`, but it must implement `FnMut`, which is more general
+note: required by a bound in `take`
+  --> $DIR/mismatch-fn-trait.rs:1:18
+   |
+LL | fn take(_f: impl FnMut(i32)) {}
+   |                  ^^^^^^^^^^ required by this bound in `take`
+
+error[E0277]: expected a `FnMut<(i32,)>` closure, found `impl FnOnce(u32)`
+  --> $DIR/mismatch-fn-trait.rs:24:10
+   |
+LL |     take(f)
+   |     ---- ^ expected an `FnMut<(i32,)>` closure, found `impl FnOnce(u32)`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = note: `impl FnOnce(u32)` implements `FnOnce`, but it must implement `FnMut`, which is more general
+   = note: expected a closure with arguments `(u32,)`
+              found a closure with arguments `(i32,)`
+note: required by a bound in `take`
+  --> $DIR/mismatch-fn-trait.rs:1:18
+   |
+LL | fn take(_f: impl FnMut(i32)) {}
+   |                  ^^^^^^^^^^ required by this bound in `take`
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/src/test/ui/unboxed-closures/unboxed-closures-fnmut-as-fn.stderr b/src/test/ui/unboxed-closures/unboxed-closures-fnmut-as-fn.stderr
index f379d73eecf..0ea1c1dcd5b 100644
--- a/src/test/ui/unboxed-closures/unboxed-closures-fnmut-as-fn.stderr
+++ b/src/test/ui/unboxed-closures/unboxed-closures-fnmut-as-fn.stderr
@@ -7,6 +7,7 @@ LL |     let x = call_it(&S, 22);
    |             required by a bound introduced by this call
    |
    = help: the trait `Fn<(isize,)>` is not implemented for `S`
+   = note: `S` implements `FnMut`, but it must implement `Fn`, which is more general
 note: required by a bound in `call_it`
   --> $DIR/unboxed-closures-fnmut-as-fn.rs:22:14
    |