From 9815667b8b59e5351c257f7a62f6c0874fabd7ca Mon Sep 17 00:00:00 2001
From: Yiming Lei <yiming.lei@futurewei.com>
Date: Tue, 2 Aug 2022 21:46:07 -0700
Subject: [PATCH] implement #98982 when loop as tail expression for miss match
type E0308 error, recursively get the return statement and add diagnostic
information on it use rustc_hir::intravisit to collect the return expression
modified: compiler/rustc_typeck/src/check/coercion.rs new file:
src/test/ui/typeck/issue-98982.rs new file:
src/test/ui/typeck/issue-98982.stderr
---
compiler/rustc_typeck/src/check/coercion.rs | 49 ++++++++++++++++++-
.../break-while-condition.stderr | 8 +++
src/test/ui/typeck/issue-98982.rs | 9 ++++
src/test/ui/typeck/issue-98982.stderr | 24 +++++++++
4 files changed, 89 insertions(+), 1 deletion(-)
create mode 100644 src/test/ui/typeck/issue-98982.rs
create mode 100644 src/test/ui/typeck/issue-98982.stderr
diff --git a/compiler/rustc_typeck/src/check/coercion.rs b/compiler/rustc_typeck/src/check/coercion.rs
index 639cab98f1741..fc7ace0e6aa4c 100644
--- a/compiler/rustc_typeck/src/check/coercion.rs
+++ b/compiler/rustc_typeck/src/check/coercion.rs
@@ -38,10 +38,12 @@
use crate::astconv::AstConv;
use crate::check::FnCtxt;
use rustc_errors::{
- struct_span_err, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed,
+ struct_span_err, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed, MultiSpan,
};
use rustc_hir as hir;
use rustc_hir::def_id::DefId;
+use rustc_hir::intravisit::{self, Visitor};
+use rustc_hir::Expr;
use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
use rustc_infer::infer::{Coercion, InferOk, InferResult};
use rustc_infer::traits::{Obligation, TraitEngine, TraitEngineExt};
@@ -87,6 +89,19 @@ impl<'a, 'tcx> Deref for Coerce<'a, 'tcx> {
type CoerceResult<'tcx> = InferResult<'tcx, (Vec<Adjustment<'tcx>>, Ty<'tcx>)>;
+struct CollectRetsVisitor<'tcx> {
+ ret_exprs: Vec<&'tcx hir::Expr<'tcx>>,
+}
+
+impl<'tcx> Visitor<'tcx> for CollectRetsVisitor<'tcx> {
+ fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
+ if let hir::ExprKind::Ret(_) = expr.kind {
+ self.ret_exprs.push(expr);
+ }
+ intravisit::walk_expr(self, expr);
+ }
+}
+
/// Coercing a mutable reference to an immutable works, while
/// coercing `&T` to `&mut T` should be forbidden.
fn coerce_mutbls<'tcx>(
@@ -1481,6 +1496,7 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
let mut err;
let mut unsized_return = false;
+ let mut visitor = CollectRetsVisitor { ret_exprs: vec![] };
match *cause.code() {
ObligationCauseCode::ReturnNoExpression => {
err = struct_span_err!(
@@ -1506,6 +1522,10 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
if !fcx.tcx.features().unsized_locals {
unsized_return = self.is_return_ty_unsized(fcx, blk_id);
}
+ if let Some(expression) = expression
+ && let hir::ExprKind::Loop(loop_blk, ..) = expression.kind {
+ intravisit::walk_block(& mut visitor, loop_blk);
+ }
}
ObligationCauseCode::ReturnValue(id) => {
err = self.report_return_mismatched_types(
@@ -1551,12 +1571,39 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
);
}
+ if visitor.ret_exprs.len() > 0 && let Some(expr) = expression {
+ self.note_unreachable_loop_return(&mut err, &expr, &visitor.ret_exprs);
+ }
err.emit_unless(unsized_return);
self.final_ty = Some(fcx.tcx.ty_error());
}
}
}
+ fn note_unreachable_loop_return<'a>(
+ &self,
+ err: &mut DiagnosticBuilder<'a, ErrorGuaranteed>,
+ expr: &hir::Expr<'tcx>,
+ ret_exprs: &Vec<&'tcx hir::Expr<'tcx>>,
+ ) {
+ 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".to_string());
+ for ret_expr in ret_exprs {
+ span.push_span_label(
+ ret_expr.span,
+ "if the loop doesn't execute, this value would never get returned".to_string(),
+ );
+ }
+ err.span_note(
+ span,
+ "the function expects a value to always be returned, but loops might run zero times",
+ );
+ 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",
+ );
+ }
fn report_return_mismatched_types<'a>(
&self,
diff --git a/src/test/ui/for-loop-while/break-while-condition.stderr b/src/test/ui/for-loop-while/break-while-condition.stderr
index 6960c4fd86735..e79f6a75fdec5 100644
--- a/src/test/ui/for-loop-while/break-while-condition.stderr
+++ b/src/test/ui/for-loop-while/break-while-condition.stderr
@@ -31,6 +31,14 @@ LL | | }
|
= note: expected type `!`
found unit type `()`
+note: the function expects a value to always be returned, but loops might run zero times
+ --> $DIR/break-while-condition.rs:24:13
+ |
+LL | while false {
+ | ^^^^^^^^^^^ this might have zero elements to iterate on
+LL | return
+ | ------ if the loop doesn't execute, this value 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 3 previous errors
diff --git a/src/test/ui/typeck/issue-98982.rs b/src/test/ui/typeck/issue-98982.rs
new file mode 100644
index 0000000000000..2553824bbfebb
--- /dev/null
+++ b/src/test/ui/typeck/issue-98982.rs
@@ -0,0 +1,9 @@
+fn foo() -> i32 {
+ for i in 0..0 {
+ //~^ ERROR: mismatched types [E0308]
+ return i;
+ }
+ //~| 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-98982.stderr b/src/test/ui/typeck/issue-98982.stderr
new file mode 100644
index 0000000000000..3c9806ac965fb
--- /dev/null
+++ b/src/test/ui/typeck/issue-98982.stderr
@@ -0,0 +1,24 @@
+error[E0308]: mismatched types
+ --> $DIR/issue-98982.rs:2:5
+ |
+LL | fn foo() -> i32 {
+ | --- expected `i32` because of return type
+LL | / for i in 0..0 {
+LL | |
+LL | | return i;
+LL | | }
+ | |_____^ expected `i32`, found `()`
+ |
+note: the function expects a value to always be returned, but loops might run zero times
+ --> $DIR/issue-98982.rs:2:5
+ |
+LL | for i in 0..0 {
+ | ^^^^^^^^^^^^^ this might have zero elements to iterate on
+LL |
+LL | return i;
+ | -------- if the loop doesn't execute, this value 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`.