when there are 3 or more return statements in the loop

emit the first 3 errors and duplicated diagnostic information
using take of iterator for the first third return
	modified:   compiler/rustc_typeck/src/check/coercion.rs
	new file:   src/test/ui/typeck/issue-100285.rs
	new file:   src/test/ui/typeck/issue-100285.stderr

Author: Yiming Lei

compiler/rustc_typeck/src/check/coercion.rs

@@ -1590,7 +1590,9 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
         let hir::ExprKind::Loop(_, _, _, loop_span) = expr.kind else { return;};
         let mut span: MultiSpan = vec![loop_span].into();
         span.push_span_label(loop_span, "this might have zero elements to iterate on");
-        for ret_expr in ret_exprs {
+        const MAXITER: usize = 3;
+        let iter = ret_exprs.iter().take(MAXITER);
+        for ret_expr in iter {
             span.push_span_label(
                 ret_expr.span,
                 "if the loop doesn't execute, this value would never get returned",
@@ -1600,6 +1602,12 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
             span,
             "the function expects a value to always be returned, but loops might run zero times",
         );
+        if MAXITER < ret_exprs.len() {
+            err.note(&format!(
+                "if the loop doesn't execute, {} other values would never get returned",
+                ret_exprs.len() - MAXITER
+            ));
+        }
         err.help(
             "return a value for the case when the loop has zero elements to iterate on, or \
            consider changing the return type to account for that possibility",

src/test/ui/typeck/issue-100285.rs

@@ -0,0 +1,22 @@
+fn foo(n: i32) -> i32 {
+    for i in 0..0 {
+    //~^ ERROR: mismatched types [E0308]
+       if n < 0 {
+        return i;
+        } else if n < 10 {
+          return 1;
+        } else if n < 20 {
+          return 2;
+        } else if n < 30 {
+          return 3;
+        } else if n < 40 {
+          return 4;
+        } else {
+          return 5;
+        }
+
+    }
+    //~| help: return a value for the case when the loop has zero elements to iterate on, or consider changing the return type to account for that possibility
+}
+
+fn main() {}

src/test/ui/typeck/issue-100285.stderr

@@ -0,0 +1,34 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-100285.rs:2:5
+   |
+LL |   fn foo(n: i32) -> i32 {
+   |                     --- expected `i32` because of return type
+LL | /     for i in 0..0 {
+LL | |
+LL | |        if n < 0 {
+LL | |         return i;
+...  |
+LL | |
+LL | |     }
+   | |_____^ expected `i32`, found `()`
+   |
+note: the function expects a value to always be returned, but loops might run zero times
+  --> $DIR/issue-100285.rs:2:5
+   |
+LL |     for i in 0..0 {
+   |     ^^^^^^^^^^^^^ this might have zero elements to iterate on
+...
+LL |         return i;
+   |         -------- if the loop doesn't execute, this value would never get returned
+LL |         } else if n < 10 {
+LL |           return 1;
+   |           -------- if the loop doesn't execute, this value would never get returned
+LL |         } else if n < 20 {
+LL |           return 2;
+   |           -------- if the loop doesn't execute, this value would never get returned
+   = note: if the loop doesn't execute, 3 other values would never get returned
+   = help: return a value for the case when the loop has zero elements to iterate on, or consider changing the return type to account for that possibility
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.