Rollup merge of #91272 - FabianWolff:issue-90870-const-fn-eq, r=wesleywiser

Print a suggestion when comparing references to primitive types in `const fn`

Fixes #90870.

src/test/ui/consts/issue-90870.fixed

@@ -0,0 +1,34 @@
+// Regression test for issue #90870.
+
+// run-rustfix
+
+#![allow(dead_code)]
+
+const fn f(a: &u8, b: &u8) -> bool {
+    *a == *b
+    //~^ ERROR: calls in constant functions are limited to constant functions, tuple structs and tuple variants [E0015]
+    //~| HELP: consider dereferencing here
+}
+
+const fn g(a: &&&&i64, b: &&&&i64) -> bool {
+    ****a == ****b
+    //~^ ERROR: calls in constant functions are limited to constant functions, tuple structs and tuple variants [E0015]
+    //~| HELP: consider dereferencing here
+}
+
+const fn h(mut a: &[u8], mut b: &[u8]) -> bool {
+    while let ([l, at @ ..], [r, bt @ ..]) = (a, b) {
+        if *l == *r {
+        //~^ ERROR: calls in constant functions are limited to constant functions, tuple structs and tuple variants [E0015]
+        //~| HELP: consider dereferencing here
+            a = at;
+            b = bt;
+        } else {
+            return false;
+        }
+    }
+
+    a.is_empty() && b.is_empty()
+}
+
+fn main() {}

src/test/ui/consts/issue-90870.rs

@@ -0,0 +1,34 @@
+// Regression test for issue #90870.
+
+// run-rustfix
+
+#![allow(dead_code)]
+
+const fn f(a: &u8, b: &u8) -> bool {
+    a == b
+    //~^ ERROR: calls in constant functions are limited to constant functions, tuple structs and tuple variants [E0015]
+    //~| HELP: consider dereferencing here
+}
+
+const fn g(a: &&&&i64, b: &&&&i64) -> bool {
+    a == b
+    //~^ ERROR: calls in constant functions are limited to constant functions, tuple structs and tuple variants [E0015]
+    //~| HELP: consider dereferencing here
+}
+
+const fn h(mut a: &[u8], mut b: &[u8]) -> bool {
+    while let ([l, at @ ..], [r, bt @ ..]) = (a, b) {
+        if l == r {
+        //~^ ERROR: calls in constant functions are limited to constant functions, tuple structs and tuple variants [E0015]
+        //~| HELP: consider dereferencing here
+            a = at;
+            b = bt;
+        } else {
+            return false;
+        }
+    }
+
+    a.is_empty() && b.is_empty()
+}
+
+fn main() {}

src/test/ui/consts/issue-90870.stderr

@@ -0,0 +1,36 @@
+error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+  --> $DIR/issue-90870.rs:8:5
+   |
+LL |     a == b
+   |     ^^^^^^
+   |
+help: consider dereferencing here
+   |
+LL |     *a == *b
+   |     +     +
+
+error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+  --> $DIR/issue-90870.rs:14:5
+   |
+LL |     a == b
+   |     ^^^^^^
+   |
+help: consider dereferencing here
+   |
+LL |     ****a == ****b
+   |     ++++     ++++
+
+error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+  --> $DIR/issue-90870.rs:21:12
+   |
+LL |         if l == r {
+   |            ^^^^^^
+   |
+help: consider dereferencing here
+   |
+LL |         if *l == *r {
+   |            +     +
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0015`.