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Remove note about iteration count in coerce
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and replace it with a simple note suggesting
returning a value.

The type mismatch error was never due to
how many times the loop iterates. It is more
because of the peculiar structure of what the for
loop desugars to. So the note talking about
iteration count didn't make sense
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@gurry
gurry committed Apr 9, 2024
1 parent 2805aed commit fa73e37
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Showing 13 changed files with 545 additions and 205 deletions.
155 changes: 17 additions & 138 deletions compiler/rustc_hir_typeck/src/coercion.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -37,11 +37,9 @@
use crate::errors::SuggestBoxingForReturnImplTrait;
use crate::FnCtxt;
use rustc_errors::{codes::*, struct_span_code_err, Applicability, Diag, MultiSpan};
use rustc_errors::{codes::*, struct_span_code_err, 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::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
use rustc_infer::infer::{Coercion, DefineOpaqueTypes, InferOk, InferResult};
Expand Down Expand Up @@ -95,22 +93,6 @@ 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>) {
match expr.kind {
hir::ExprKind::Ret(_) => self.ret_exprs.push(expr),
// `return` in closures does not return from the outer function
hir::ExprKind::Closure(_) => return,
_ => {}
}
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>(
Expand Down Expand Up @@ -1607,7 +1589,6 @@ 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_code_err!(
Expand All @@ -1633,11 +1614,6 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
if !fcx.tcx.features().unsized_locals {
unsized_return = self.is_return_ty_definitely_unsized(fcx);
}
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(
Expand Down Expand Up @@ -1738,6 +1714,22 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
augment_error(&mut err);

if let Some(expr) = expression {
if let hir::ExprKind::Loop(
_,
_,
loop_src @ (hir::LoopSource::While | hir::LoopSource::ForLoop),
_,
) = expr.kind
{
let loop_type = if loop_src == hir::LoopSource::While {
"`while` loops"
} else {
"`for` loops"
};

err.note(format!("{loop_type} evaluate to unit type `()`"));
}

fcx.emit_coerce_suggestions(
&mut err,
expr,
Expand All @@ -1746,15 +1738,6 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
None,
Some(coercion_error),
);
if visitor.ret_exprs.len() > 0 {
self.note_unreachable_loop_return(
&mut err,
fcx.tcx,
&expr,
&visitor.ret_exprs,
expected,
);
}
}

let reported = err.emit_unless(unsized_return);
Expand Down Expand Up @@ -1828,110 +1811,6 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
);
}

fn note_unreachable_loop_return(
&self,
err: &mut Diag<'_>,
tcx: TyCtxt<'tcx>,
expr: &hir::Expr<'tcx>,
ret_exprs: &Vec<&'tcx hir::Expr<'tcx>>,
ty: Ty<'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");
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",
);
}
err.span_note(
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
));
}
let hir = tcx.hir();
let item = hir.get_parent_item(expr.hir_id);
let ret_msg = "return a value for the case when the loop has zero elements to iterate on";
let ret_ty_msg =
"otherwise consider changing the return type to account for that possibility";
let node = tcx.hir_node(item.into());
if let Some(body_id) = node.body_id()
&& let Some(sig) = node.fn_sig()
&& let hir::ExprKind::Block(block, _) = hir.body(body_id).value.kind
&& !ty.is_never()
{
let indentation = if let None = block.expr
&& let [.., last] = &block.stmts
{
tcx.sess.source_map().indentation_before(last.span).unwrap_or_else(String::new)
} else if let Some(expr) = block.expr {
tcx.sess.source_map().indentation_before(expr.span).unwrap_or_else(String::new)
} else {
String::new()
};
if let None = block.expr
&& let [.., last] = &block.stmts
{
err.span_suggestion_verbose(
last.span.shrink_to_hi(),
ret_msg,
format!("\n{indentation}/* `{ty}` value */"),
Applicability::MaybeIncorrect,
);
} else if let Some(expr) = block.expr {
err.span_suggestion_verbose(
expr.span.shrink_to_hi(),
ret_msg,
format!("\n{indentation}/* `{ty}` value */"),
Applicability::MaybeIncorrect,
);
}
let mut sugg = match sig.decl.output {
hir::FnRetTy::DefaultReturn(span) => {
vec![(span, " -> Option<()>".to_string())]
}
hir::FnRetTy::Return(ty) => {
vec![
(ty.span.shrink_to_lo(), "Option<".to_string()),
(ty.span.shrink_to_hi(), ">".to_string()),
]
}
};
for ret_expr in ret_exprs {
match ret_expr.kind {
hir::ExprKind::Ret(Some(expr)) => {
sugg.push((expr.span.shrink_to_lo(), "Some(".to_string()));
sugg.push((expr.span.shrink_to_hi(), ")".to_string()));
}
hir::ExprKind::Ret(None) => {
sugg.push((ret_expr.span.shrink_to_hi(), " Some(())".to_string()));
}
_ => {}
}
}
if let None = block.expr
&& let [.., last] = &block.stmts
{
sugg.push((last.span.shrink_to_hi(), format!("\n{indentation}None")));
} else if let Some(expr) = block.expr {
sugg.push((expr.span.shrink_to_hi(), format!("\n{indentation}None")));
}
err.multipart_suggestion(ret_ty_msg, sugg, Applicability::MaybeIncorrect);
} else {
err.help(format!("{ret_msg}, {ret_ty_msg}"));
}
}

fn report_return_mismatched_types<'a>(
&self,
cause: &ObligationCause<'tcx>,
Expand Down
3 changes: 2 additions & 1 deletion compiler/rustc_hir_typeck/src/demand.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -57,7 +57,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
|| self.suggest_into(err, expr, expr_ty, expected)
|| self.suggest_floating_point_literal(err, expr, expected)
|| self.suggest_null_ptr_for_literal_zero_given_to_ptr_arg(err, expr, expected)
|| self.suggest_coercing_result_via_try_operator(err, expr, expected, expr_ty);
|| self.suggest_coercing_result_via_try_operator(err, expr, expected, expr_ty)
|| self.suggest_returning_value_after_loop(err, expr, expected);

if !suggested {
self.note_source_of_type_mismatch_constraint(
Expand Down
89 changes: 87 additions & 2 deletions compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -17,8 +17,8 @@ use rustc_hir::def::Res;
use rustc_hir::def::{CtorKind, CtorOf, DefKind};
use rustc_hir::lang_items::LangItem;
use rustc_hir::{
CoroutineDesugaring, CoroutineKind, CoroutineSource, Expr, ExprKind, GenericBound, HirId, Node,
Path, QPath, Stmt, StmtKind, TyKind, WherePredicate,
Arm, CoroutineDesugaring, CoroutineKind, CoroutineSource, Expr, ExprKind, GenericBound, HirId,
Node, Path, QPath, Stmt, StmtKind, TyKind, WherePredicate,
};
use rustc_hir_analysis::hir_ty_lowering::HirTyLowerer;
use rustc_infer::traits::{self};
Expand Down Expand Up @@ -1955,6 +1955,91 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
false
}

// If the expr is a while or for loop and is the tail expr of its
// enclosing body suggest returning a value right after it
pub fn suggest_returning_value_after_loop(
&self,
err: &mut Diag<'_>,
expr: &hir::Expr<'tcx>,
expected: Ty<'tcx>,
) -> bool {
let hir = self.tcx.hir();
let enclosing_scope =
hir.get_enclosing_scope(expr.hir_id).map(|hir_id| self.tcx.hir_node(hir_id));

// Get tail expr of the enclosing block or body
let tail_expr = if let Some(Node::Block(hir::Block { expr, .. })) = enclosing_scope
&& expr.is_some()
{
*expr
} else {
let body_def_id = hir.enclosing_body_owner(expr.hir_id);
let body_id = hir.body_owned_by(body_def_id);
let body = hir.body(body_id);

// Get tail expr of the body
match body.value.kind {
// Regular function body etc.
hir::ExprKind::Block(block, _) => block.expr,
// Anon const body (there's no block in this case)
hir::ExprKind::DropTemps(expr) => Some(expr),
_ => None,
}
};

let Some(tail_expr) = tail_expr else {
return false; // Body doesn't have a tail expr we can compare with
};

// Get the loop expr within the tail expr
let loop_expr_in_tail = match expr.kind {
hir::ExprKind::Loop(_, _, hir::LoopSource::While, _) => tail_expr,
hir::ExprKind::Loop(_, _, hir::LoopSource::ForLoop, _) => {
match tail_expr.peel_drop_temps() {
Expr { kind: ExprKind::Match(_, [Arm { body, .. }], _), .. } => body,
_ => return false, // Not really a for loop
}
}
_ => return false, // Not a while or a for loop
};

// If the expr is the loop expr in the tail
// then make the suggestion
if expr.hir_id == loop_expr_in_tail.hir_id {
let span = expr.span;

let (msg, suggestion) = if expected.is_never() {
(
"consider adding a diverging expression here",
"`loop {}` or `panic!(\"...\")`".to_string(),
)
} else {
("consider returning a value here", format!("`{expected}` value"))
};

let src_map = self.tcx.sess.source_map();
let suggestion = if src_map.is_multiline(expr.span) {
let indentation = src_map.indentation_before(span).unwrap_or_else(String::new);
format!("\n{indentation}/* {suggestion} */")
} else {
// If the entire expr is on a single line
// put the suggestion also on the same line
format!(" /* {suggestion} */")
};

err.span_suggestion_verbose(
span.shrink_to_hi(),
msg,
suggestion,
Applicability::MaybeIncorrect,
);

true
} else {
false
}
}

/// If the expected type is an enum (Issue #55250) with any variants whose
/// sole field is of the found type, suggest such variants. (Issue #42764)
pub(crate) fn suggest_compatible_variants(
Expand Down
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