From 0471e2780fe528dae8b67ea3448e8ab641e8510e Mon Sep 17 00:00:00 2001
From: Yiming Lei <yiming.lei@futurewei.com>
Date: Tue, 9 Aug 2022 11:27:04 -0700
Subject: [PATCH] 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

---
 compiler/rustc_typeck/src/check/coercion.rs | 10 +++++-
 src/test/ui/typeck/issue-100285.rs          | 22 +++++++++++++
 src/test/ui/typeck/issue-100285.stderr      | 34 +++++++++++++++++++++
 3 files changed, 65 insertions(+), 1 deletion(-)
 create mode 100644 src/test/ui/typeck/issue-100285.rs
 create mode 100644 src/test/ui/typeck/issue-100285.stderr

diff --git a/compiler/rustc_typeck/src/check/coercion.rs b/compiler/rustc_typeck/src/check/coercion.rs
index 540a8c3a83d..cc400e1ed75 100644
--- a/compiler/rustc_typeck/src/check/coercion.rs
+++ b/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",
diff --git a/src/test/ui/typeck/issue-100285.rs b/src/test/ui/typeck/issue-100285.rs
new file mode 100644
index 00000000000..e206469b85f
--- /dev/null
+++ b/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() {}
diff --git a/src/test/ui/typeck/issue-100285.stderr b/src/test/ui/typeck/issue-100285.stderr
new file mode 100644
index 00000000000..42c64b03918
--- /dev/null
+++ b/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`.