Point at appropriate arm on type error on if/else/match with one non-! arm

src/librustc_typeck/check/mod.rs

@@ -3687,6 +3687,40 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  }
  }
 
+ /// If `expr` is a `match` expression that has only one non-`!` arm, use that arm's tail
+ /// expression's `Span`, otherwise return `expr.span`. This is done to give bettern errors
+ /// when given code like the following:
+ /// ```text
+ /// if false { return 0i32; } else { 1u32 }
+ /// // ^^^^ point at this instead of the whole `if` expression
+ /// ```
+ fn get_expr_coercion_span(&self, expr: &hir::Expr) -> syntax_pos::Span {
+ if let hir::ExprKind::Match(_, arms, _) = &expr.node {
+ let arm_spans: Vec<Span> = arms.iter().filter_map(|arm| {
+ self.in_progress_tables
+ .and_then(|tables| tables.borrow().node_type_opt(arm.body.hir_id))
+ .and_then(|arm_ty| {
+ if arm_ty.is_never() {
+ None
+ } else {
+ Some(match &arm.body.node {
+ // Point at the tail expression when possible.
+ hir::ExprKind::Block(block, _) => block.expr
+ .as_ref()
+ .map(|e| e.span)
+ .unwrap_or(block.span),
+ _ => arm.body.span,
+ })
+ }
+ })
+ }).collect();
+ if arm_spans.len() == 1 {
+ return arm_spans[0];
+ }
+ }
+ expr.span
+ }
+
  fn check_block_with_expected(
  &self,
  blk: &'tcx hir::Block,
@@ -3746,12 +3780,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  let coerce = ctxt.coerce.as_mut().unwrap();
  if let Some(tail_expr_ty) = tail_expr_ty {
  let tail_expr = tail_expr.unwrap();
- let cause = self.cause(tail_expr.span,
- ObligationCauseCode::BlockTailExpression(blk.hir_id));
- coerce.coerce(self,
- &cause,
- tail_expr,
- tail_expr_ty);
+ let span = self.get_expr_coercion_span(tail_expr);
+ let cause = self.cause(span, ObligationCauseCode::BlockTailExpression(blk.hir_id));
+ coerce.coerce(self, &cause, tail_expr, tail_expr_ty);
  } else {
  // Subtle: if there is no explicit tail expression,
  // that is typically equivalent to a tail expression

src/test/ui/point-to-type-err-cause-on-impl-trait-return.rs

@@ -17,10 +17,10 @@ fn bar() -> impl std::fmt::Display {
 
 fn baz() -> impl std::fmt::Display {
  if false {
- //~^ ERROR mismatched types
  return 0i32;
  } else {
  1u32
+ //~^ ERROR mismatched types
  }
 }
 

src/test/ui/point-to-type-err-cause-on-impl-trait-return.stderr

@@ -29,18 +29,16 @@ LL | return 1u32;
  found type `u32`
 
 error[E0308]: mismatched types
- --> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:19:5
+ --> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:22:9
  |
-LL | fn baz() -> impl std::fmt::Display {
- | ---------------------- expected because this return type...
-LL | / if false {
-LL | |
-LL | | return 0i32;
- | | ---- ...is found to be `i32` here
-LL | | } else {
-LL | | 1u32
-LL | | }
- | |_____^ expected i32, found u32
+LL | fn baz() -> impl std::fmt::Display {
+ | ---------------------- expected because this return type...
+LL | if false {
+LL | return 0i32;
+ | ---- ...is found to be `i32` here
+LL | } else {
+LL | 1u32
+ | ^^^^ expected i32, found u32
  |
  = note: expected type `i32`
  found type `u32`