Fix cycle error when emitting suggestion for mismatched `fn` type

Fixes #66667

Previously, we called `tcx.typeck_tables_of` when determining whether or
not to emit a suggestion for a type error. However, we might already be
type-checking the `DefId` we pass to `typeck_tables_of` (it could be
anywhere in the query stack).

Fortunately, we only need the function signature, not the entire
`TypeckTables`. By using `tcx.fn_sig`, we avoid the possibility of cycle
errors while retaining the ability to emit a suggestion.

2 files changed, 71 insertions, 0 deletions

diff --git a/src/test/ui/issues/issue-66667-function-cmp-cycle.rs b/src/test/ui/issues/issue-66667-function-cmp-cycle.rs
new file mode 100644
index 00000000000..7b025be11a0
--- /dev/null
+++ b/src/test/ui/issues/issue-66667-function-cmp-cycle.rs
@@ -0,0 +1,16 @@
+fn first() {
+    second == 1 //~ ERROR binary operation
+    //~^ ERROR mismatched types
+}
+
+fn second() {
+    first == 1 //~ ERROR binary operation
+    //~^ ERROR mismatched types
+}
+
+fn bar() {
+    bar == 1 //~ ERROR binary operation
+    //~^ ERROR mismatched types
+}
+
+fn main() {}
diff --git a/src/test/ui/issues/issue-66667-function-cmp-cycle.stderr b/src/test/ui/issues/issue-66667-function-cmp-cycle.stderr
new file mode 100644
index 00000000000..887699ef5ce
--- /dev/null
+++ b/src/test/ui/issues/issue-66667-function-cmp-cycle.stderr
@@ -0,0 +1,55 @@
+error[E0369]: binary operation `==` cannot be applied to type `fn() {second}`
+  --> $DIR/issue-66667-function-cmp-cycle.rs:2:12
+   |
+LL |     second == 1
+   |     ------ ^^ - {integer}
+   |     |
+   |     fn() {second}
+
+error[E0308]: mismatched types
+  --> $DIR/issue-66667-function-cmp-cycle.rs:2:15
+   |
+LL |     second == 1
+   |               ^ expected fn item, found integer
+   |
+   = note: expected fn item `fn() {second}`
+                 found type `{integer}`
+
+error[E0369]: binary operation `==` cannot be applied to type `fn() {first}`
+  --> $DIR/issue-66667-function-cmp-cycle.rs:7:11
+   |
+LL |     first == 1
+   |     ----- ^^ - {integer}
+   |     |
+   |     fn() {first}
+
+error[E0308]: mismatched types
+  --> $DIR/issue-66667-function-cmp-cycle.rs:7:14
+   |
+LL |     first == 1
+   |              ^ expected fn item, found integer
+   |
+   = note: expected fn item `fn() {first}`
+                 found type `{integer}`
+
+error[E0369]: binary operation `==` cannot be applied to type `fn() {bar}`
+  --> $DIR/issue-66667-function-cmp-cycle.rs:12:9
+   |
+LL |     bar == 1
+   |     --- ^^ - {integer}
+   |     |
+   |     fn() {bar}
+
+error[E0308]: mismatched types
+  --> $DIR/issue-66667-function-cmp-cycle.rs:12:12
+   |
+LL |     bar == 1
+   |            ^ expected fn item, found integer
+   |
+   = note: expected fn item `fn() {bar}`
+                 found type `{integer}`
+
+error: aborting due to 6 previous errors
+
+Some errors have detailed explanations: E0308, E0369.
+For more information about an error, try `rustc --explain E0308`.