Add diagnostics to "while loop" and "for loop" that note that it is always determined that it might iterate zero times.

compiler/rustc_hir_typeck/src/coercion.rs

@@ -40,6 +40,8 @@ use crate::FnCtxt;
 use rustc_errors::{codes::*, struct_span_code_err, Applicability, Diag};
 use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_hir::intravisit::{self, Visitor};
+use rustc_hir::Expr;
 use rustc_hir_analysis::hir_ty_lowering::HirTyLowerer;
 use rustc_infer::infer::relate::RelateResult;
 use rustc_infer::infer::{Coercion, DefineOpaqueTypes, InferOk, InferResult};
@@ -1702,10 +1704,10 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
 
                 if let Some(expr) = expression {
                     if let hir::ExprKind::Loop(
-                        _,
+                        loop_blk,
                         _,
                         loop_src @ (hir::LoopSource::While | hir::LoopSource::ForLoop),
-                        _,
+                        loop_span,
                     ) = expr.kind
                     {
                         let loop_type = if loop_src == hir::LoopSource::While {
@@ -1715,6 +1717,33 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
                         };
 
                         err.note(format!("{loop_type} evaluate to unit type `()`"));
+
+                        struct RetExprChecker {
+                            contains_ret_expr: bool,
+                        }
+                        impl<'v> Visitor<'v> for RetExprChecker {
+                            fn visit_expr(&mut self, ex: &'v Expr<'v>) {
+                                if let hir::ExprKind::Ret(_) = ex.kind {
+                                    self.contains_ret_expr = true;
+                                    return;
+                                }
+                                intravisit::walk_expr(self, ex);
+                            }
+                        }
+                        let mut visitor = RetExprChecker { contains_ret_expr: false };
+                        intravisit::walk_block(&mut visitor, loop_blk);
+                        if visitor.contains_ret_expr {
+                            let loop_header = if loop_src == hir::LoopSource::While {
+                                "loop condition expression or pattern"
+                            } else {
+                                "iterator expression"
+                            };
+                            err.span_note(
+                                    loop_span,
+                                    format!("It is determined that this might iterate zero times, regardless of the {loop_header}.")
+                                );
+                            err.span_help(loop_span, "If you are assuming that it will iterate at least once, consider using a `loop` expression instead.");
+                        }
                     }
 
                     fcx.emit_coerce_suggestions(

tests/ui/coercion/coerce-loop-issue-122561.stderr

@@ -98,6 +98,16 @@ LL | |     }
    | |_____^ expected `bool`, found `()`
    |
    = note: `for` loops evaluate to unit type `()`
+note: It is determined that this might iterate zero times, regardless of the iterator expression.
+  --> $DIR/coerce-loop-issue-122561.rs:4:5
+   |
+LL |     for i in 0.. {
+   |     ^^^^^^^^^^^^
+help: If you are assuming that it will iterate at least once, consider using a `loop` expression instead.
+  --> $DIR/coerce-loop-issue-122561.rs:4:5
+   |
+LL |     for i in 0.. {
+   |     ^^^^^^^^^^^^
 help: consider returning a value here
    |
 LL ~     }
@@ -116,6 +126,16 @@ LL | |     }
    | |_____^ expected `String`, found `()`
    |
    = note: `for` loops evaluate to unit type `()`
+note: It is determined that this might iterate zero times, regardless of the iterator expression.
+  --> $DIR/coerce-loop-issue-122561.rs:11:5
+   |
+LL |     for i in 0..5 {
+   |     ^^^^^^^^^^^^^
+help: If you are assuming that it will iterate at least once, consider using a `loop` expression instead.
+  --> $DIR/coerce-loop-issue-122561.rs:11:5
+   |
+LL |     for i in 0..5 {
+   |     ^^^^^^^^^^^^^
 help: consider returning a value here
    |
 LL ~     }
@@ -134,6 +154,16 @@ LL | |     }
    | |_____^ expected `bool`, found `()`
    |
    = note: `for` loops evaluate to unit type `()`
+note: It is determined that this might iterate zero times, regardless of the iterator expression.
+  --> $DIR/coerce-loop-issue-122561.rs:18:5
+   |
+LL |     for i in 0..0 {
+   |     ^^^^^^^^^^^^^
+help: If you are assuming that it will iterate at least once, consider using a `loop` expression instead.
+  --> $DIR/coerce-loop-issue-122561.rs:18:5
+   |
+LL |     for i in 0..0 {
+   |     ^^^^^^^^^^^^^
 help: consider returning a value here
    |
 LL ~     }
@@ -168,6 +198,16 @@ LL | fn for_single_line() -> bool { for i in 0.. { return false; } }
    |                         expected `bool` because of return type
    |
    = note: `for` loops evaluate to unit type `()`
+note: It is determined that this might iterate zero times, regardless of the iterator expression.
+  --> $DIR/coerce-loop-issue-122561.rs:33:32
+   |
+LL | fn for_single_line() -> bool { for i in 0.. { return false; } }
+   |                                ^^^^^^^^^^^^
+help: If you are assuming that it will iterate at least once, consider using a `loop` expression instead.
+  --> $DIR/coerce-loop-issue-122561.rs:33:32
+   |
+LL | fn for_single_line() -> bool { for i in 0.. { return false; } }
+   |                                ^^^^^^^^^^^^
 help: consider returning a value here
    |
 LL | fn for_single_line() -> bool { for i in 0.. { return false; } /* `bool` value */ }
@@ -186,6 +226,16 @@ LL | |     }
    | |_____^ expected `bool`, found `()`
    |
    = note: `while` loops evaluate to unit type `()`
+note: It is determined that this might iterate zero times, regardless of the loop condition expression or pattern.
+  --> $DIR/coerce-loop-issue-122561.rs:48:5
+   |
+LL |     while true {
+   |     ^^^^^^^^^^
+help: If you are assuming that it will iterate at least once, consider using a `loop` expression instead.
+  --> $DIR/coerce-loop-issue-122561.rs:48:5
+   |
+LL |     while true {
+   |     ^^^^^^^^^^
 help: consider returning a value here
    |
 LL ~     }
@@ -206,6 +256,16 @@ LL | |     }
    | |_____^ expected `bool`, found `()`
    |
    = note: `while` loops evaluate to unit type `()`
+note: It is determined that this might iterate zero times, regardless of the loop condition expression or pattern.
+  --> $DIR/coerce-loop-issue-122561.rs:57:5
+   |
+LL |     while i < 3 {
+   |     ^^^^^^^^^^^
+help: If you are assuming that it will iterate at least once, consider using a `loop` expression instead.
+  --> $DIR/coerce-loop-issue-122561.rs:57:5
+   |
+LL |     while i < 3 {
+   |     ^^^^^^^^^^^
 help: consider returning a value here
    |
 LL ~     }
@@ -224,6 +284,16 @@ LL | |     }
    | |_____^ expected `bool`, found `()`
    |
    = note: `while` loops evaluate to unit type `()`
+note: It is determined that this might iterate zero times, regardless of the loop condition expression or pattern.
+  --> $DIR/coerce-loop-issue-122561.rs:65:5
+   |
+LL |     while false {
+   |     ^^^^^^^^^^^
+help: If you are assuming that it will iterate at least once, consider using a `loop` expression instead.
+  --> $DIR/coerce-loop-issue-122561.rs:65:5
+   |
+LL |     while false {
+   |     ^^^^^^^^^^^
 help: consider returning a value here
    |
 LL ~     }

tests/ui/for-loop-while/break-while-condition.stderr

@@ -38,6 +38,16 @@ LL | |             }
    = note:   expected type `!`
            found unit type `()`
    = note: `while` loops evaluate to unit type `()`
+note: It is determined that this might iterate zero times, regardless of the loop condition expression or pattern.
+  --> $DIR/break-while-condition.rs:24:13
+   |
+LL |             while false {
+   |             ^^^^^^^^^^^
+help: If you are assuming that it will iterate at least once, consider using a `loop` expression instead.
+  --> $DIR/break-while-condition.rs:24:13
+   |
+LL |             while false {
+   |             ^^^^^^^^^^^
 help: consider adding a diverging expression here
    |
 LL ~             }

tests/ui/typeck/irrefutable-while-let-pattern.rs

@@ -0,0 +1,8 @@
+fn main() {}
+
+fn foo() -> bool {
+    while let x = 0 {
+        //~^ 4:5: 7:6: mismatched types [E0308]
+        return true;
+    }
+}

tests/ui/typeck/irrefutable-while-let-pattern.stderr

@@ -0,0 +1,31 @@
+error[E0308]: mismatched types
+  --> $DIR/irrefutable-while-let-pattern.rs:4:5
+   |
+LL |   fn foo() -> bool {
+   |               ---- expected `bool` because of return type
+LL | /     while let x = 0 {
+LL | |
+LL | |         return true;
+LL | |     }
+   | |_____^ expected `bool`, found `()`
+   |
+   = note: `while` loops evaluate to unit type `()`
+note: It is determined that this might iterate zero times, regardless of the loop condition expression or pattern.
+  --> $DIR/irrefutable-while-let-pattern.rs:4:5
+   |
+LL |     while let x = 0 {
+   |     ^^^^^^^^^^^^^^^
+help: If you are assuming that it will iterate at least once, consider using a `loop` expression instead.
+  --> $DIR/irrefutable-while-let-pattern.rs:4:5
+   |
+LL |     while let x = 0 {
+   |     ^^^^^^^^^^^^^^^
+help: consider returning a value here
+   |
+LL ~     }
+LL +     /* `bool` value */
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/typeck/issue-100285.rs

@@ -13,8 +13,7 @@ fn foo(n: i32) -> i32 {
         } else {
           return 5;
         }
-
-    } //~ HELP consider returning a value here
+    }
 }
 
 fn main() {}

tests/ui/typeck/issue-100285.stderr

@@ -8,15 +8,25 @@ LL | |        if n < 0 {
 LL | |         return i;
 LL | |         } else if n < 10 {
 ...  |
-LL | |
+LL | |         }
 LL | |     }
    | |_____^ expected `i32`, found `()`
    |
    = note: `for` loops evaluate to unit type `()`
+note: It is determined that this might iterate zero times, regardless of the iterator expression.
+  --> $DIR/issue-100285.rs:2:5
+   |
+LL |     for i in 0..0 {
+   |     ^^^^^^^^^^^^^
+help: If you are assuming that it will iterate at least once, consider using a `loop` expression instead.
+  --> $DIR/issue-100285.rs:2:5
+   |
+LL |     for i in 0..0 {
+   |     ^^^^^^^^^^^^^
 help: consider returning a value here
    |
 LL ~     }
-LL ~     /* `i32` value */
+LL +     /* `i32` value */
    |
 
 error: aborting due to 1 previous error

tests/ui/typeck/issue-98982.rs

@@ -1,7 +1,7 @@
 fn foo() -> i32 {
     for i in 0..0 { //~ ERROR mismatched types [E0308]
         return i;
-    } //~ HELP consider returning a value here
+    }
 }
 
 fn main() {}

tests/ui/typeck/issue-98982.stderr

@@ -9,10 +9,20 @@ LL | |     }
    | |_____^ expected `i32`, found `()`
    |
    = note: `for` loops evaluate to unit type `()`
+note: It is determined that this might iterate zero times, regardless of the iterator expression.
+  --> $DIR/issue-98982.rs:2:5
+   |
+LL |     for i in 0..0 {
+   |     ^^^^^^^^^^^^^
+help: If you are assuming that it will iterate at least once, consider using a `loop` expression instead.
+  --> $DIR/issue-98982.rs:2:5
+   |
+LL |     for i in 0..0 {
+   |     ^^^^^^^^^^^^^
 help: consider returning a value here
    |
 LL ~     }
-LL ~     /* `i32` value */
+LL +     /* `i32` value */
    |
 
 error: aborting due to 1 previous error