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use rustc_errors::{Applicability, Diag};
use rustc_hir::def::{CtorOf, DefKind, Res};
use rustc_hir::def_id::LocalDefId;
use rustc_hir::{self as hir, ExprKind, PatKind};
use rustc_hir_pretty::ty_to_string;
use rustc_middle::ty::{self, Ty};
use rustc_span::Span;
use rustc_trait_selection::traits::{
IfExpressionCause, MatchExpressionArmCause, ObligationCause, ObligationCauseCode,
};
use tracing::{debug, instrument};
use crate::coercion::{AsCoercionSite, CoerceMany};
use crate::{Diverges, Expectation, FnCtxt, Needs};
impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
#[instrument(skip(self), level = "debug", ret)]
pub fn check_match(
&self,
expr: &'tcx hir::Expr<'tcx>,
scrut: &'tcx hir::Expr<'tcx>,
arms: &'tcx [hir::Arm<'tcx>],
orig_expected: Expectation<'tcx>,
match_src: hir::MatchSource,
) -> Ty<'tcx> {
let tcx = self.tcx;
let acrb = arms_contain_ref_bindings(arms);
let scrutinee_ty = self.demand_scrutinee_type(scrut, acrb, arms.is_empty());
debug!(?scrutinee_ty);
// If there are no arms, that is a diverging match; a special case.
if arms.is_empty() {
self.diverges.set(self.diverges.get() | Diverges::always(expr.span));
return tcx.types.never;
}
self.warn_arms_when_scrutinee_diverges(arms);
// Otherwise, we have to union together the types that the arms produce and so forth.
let scrut_diverges = self.diverges.replace(Diverges::Maybe);
// #55810: Type check patterns first so we get types for all bindings.
let scrut_span = scrut.span.find_ancestor_inside(expr.span).unwrap_or(scrut.span);
for arm in arms {
self.check_pat_top(arm.pat, scrutinee_ty, Some(scrut_span), Some(scrut), None);
}
// Now typecheck the blocks.
//
// The result of the match is the common supertype of all the
// arms. Start out the value as bottom, since it's the, well,
// bottom the type lattice, and we'll be moving up the lattice as
// we process each arm. (Note that any match with 0 arms is matching
// on any empty type and is therefore unreachable; should the flow
// of execution reach it, we will panic, so bottom is an appropriate
// type in that case)
let mut all_arms_diverge = Diverges::WarnedAlways;
let expected = orig_expected.adjust_for_branches(self);
debug!(?expected);
let mut coercion = {
let coerce_first = match expected {
// We don't coerce to `()` so that if the match expression is a
// statement it's branches can have any consistent type. That allows
// us to give better error messages (pointing to a usually better
// arm for inconsistent arms or to the whole match when a `()` type
// is required).
Expectation::ExpectHasType(ety) if ety != tcx.types.unit => ety,
_ => self.next_ty_var(expr.span),
};
CoerceMany::with_coercion_sites(coerce_first, arms)
};
let mut prior_non_diverging_arms = vec![]; // Used only for diagnostics.
let mut prior_arm = None;
for arm in arms {
if let Some(e) = &arm.guard {
self.diverges.set(Diverges::Maybe);
self.check_expr_has_type_or_error(e, tcx.types.bool, |_| {});
}
self.diverges.set(Diverges::Maybe);
let arm_ty = self.check_expr_with_expectation(arm.body, expected);
all_arms_diverge &= self.diverges.get();
let tail_defines_return_position_impl_trait =
self.return_position_impl_trait_from_match_expectation(orig_expected);
let (arm_block_id, arm_span) = if let hir::ExprKind::Block(blk, _) = arm.body.kind {
(Some(blk.hir_id), self.find_block_span(blk))
} else {
(None, arm.body.span)
};
let (span, code) = match prior_arm {
// The reason for the first arm to fail is not that the match arms diverge,
// but rather that there's a prior obligation that doesn't hold.
None => {
(arm_span, ObligationCauseCode::BlockTailExpression(arm.body.hir_id, match_src))
}
Some((prior_arm_block_id, prior_arm_ty, prior_arm_span)) => (
expr.span,
ObligationCauseCode::MatchExpressionArm(Box::new(MatchExpressionArmCause {
arm_block_id,
arm_span,
arm_ty,
prior_arm_block_id,
prior_arm_ty,
prior_arm_span,
scrut_span: scrut.span,
source: match_src,
prior_non_diverging_arms: prior_non_diverging_arms.clone(),
tail_defines_return_position_impl_trait,
})),
),
};
let cause = self.cause(span, code);
// This is the moral equivalent of `coercion.coerce(self, cause, arm.body, arm_ty)`.
// We use it this way to be able to expand on the potential error and detect when a
// `match` tail statement could be a tail expression instead. If so, we suggest
// removing the stray semicolon.
coercion.coerce_inner(
self,
&cause,
Some(arm.body),
arm_ty,
|err| {
self.explain_never_type_coerced_to_unit(err, arm, arm_ty, prior_arm, expr);
},
false,
);
if !arm_ty.is_never() {
// When a match arm has type `!`, then it doesn't influence the expected type for
// the following arm. If all of the prior arms are `!`, then the influence comes
// from elsewhere and we shouldn't point to any previous arm.
prior_arm = Some((arm_block_id, arm_ty, arm_span));
prior_non_diverging_arms.push(arm_span);
if prior_non_diverging_arms.len() > 5 {
prior_non_diverging_arms.remove(0);
}
}
}
// If all of the arms in the `match` diverge,
// and we're dealing with an actual `match` block
// (as opposed to a `match` desugared from something else'),
// we can emit a better note. Rather than pointing
// at a diverging expression in an arbitrary arm,
// we can point at the entire `match` expression
if let (Diverges::Always { .. }, hir::MatchSource::Normal) = (all_arms_diverge, match_src) {
all_arms_diverge = Diverges::Always {
span: expr.span,
custom_note: Some(
"any code following this `match` expression is unreachable, as all arms diverge",
),
};
}
// We won't diverge unless the scrutinee or all arms diverge.
self.diverges.set(scrut_diverges | all_arms_diverge);
coercion.complete(self)
}
fn explain_never_type_coerced_to_unit(
&self,
err: &mut Diag<'_>,
arm: &hir::Arm<'tcx>,
arm_ty: Ty<'tcx>,
prior_arm: Option<(Option<hir::HirId>, Ty<'tcx>, Span)>,
expr: &hir::Expr<'tcx>,
) {
if let hir::ExprKind::Block(block, _) = arm.body.kind
&& let Some(expr) = block.expr
&& let arm_tail_ty = self.node_ty(expr.hir_id)
&& arm_tail_ty.is_never()
&& !arm_ty.is_never()
{
err.span_label(
expr.span,
format!(
"this expression is of type `!`, but it is coerced to `{arm_ty}` due to its \
surrounding expression",
),
);
self.suggest_mismatched_types_on_tail(
err,
expr,
arm_ty,
prior_arm.map_or(arm_tail_ty, |(_, ty, _)| ty),
expr.hir_id,
);
}
self.suggest_removing_semicolon_for_coerce(err, expr, arm_ty, prior_arm)
}
fn suggest_removing_semicolon_for_coerce(
&self,
diag: &mut Diag<'_>,
expr: &hir::Expr<'tcx>,
arm_ty: Ty<'tcx>,
prior_arm: Option<(Option<hir::HirId>, Ty<'tcx>, Span)>,
) {
let hir = self.tcx.hir();
// First, check that we're actually in the tail of a function.
let Some(body) = hir.maybe_body_owned_by(self.body_id) else {
return;
};
let hir::ExprKind::Block(block, _) = body.value.kind else {
return;
};
let Some(hir::Stmt { kind: hir::StmtKind::Semi(last_expr), span: semi_span, .. }) =
block.innermost_block().stmts.last()
else {
return;
};
if last_expr.hir_id != expr.hir_id {
return;
}
// Next, make sure that we have no type expectation.
let Some(ret) =
self.tcx.hir_node_by_def_id(self.body_id).fn_decl().map(|decl| decl.output.span())
else {
return;
};
let can_coerce_to_return_ty = match self.ret_coercion.as_ref() {
Some(ret_coercion) => {
let ret_ty = ret_coercion.borrow().expected_ty();
let ret_ty = self.infcx.shallow_resolve(ret_ty);
self.can_coerce(arm_ty, ret_ty)
&& prior_arm.is_none_or(|(_, ty, _)| self.can_coerce(ty, ret_ty))
// The match arms need to unify for the case of `impl Trait`.
&& !matches!(ret_ty.kind(), ty::Alias(ty::Opaque, ..))
}
_ => false,
};
if !can_coerce_to_return_ty {
return;
}
let semi = expr.span.shrink_to_hi().with_hi(semi_span.hi());
let sugg = crate::errors::RemoveSemiForCoerce { expr: expr.span, ret, semi };
diag.subdiagnostic(sugg);
}
/// When the previously checked expression (the scrutinee) diverges,
/// warn the user about the match arms being unreachable.
fn warn_arms_when_scrutinee_diverges(&self, arms: &'tcx [hir::Arm<'tcx>]) {
for arm in arms {
self.warn_if_unreachable(arm.body.hir_id, arm.body.span, "arm");
}
}
/// Handle the fallback arm of a desugared if(-let) like a missing else.
///
/// Returns `true` if there was an error forcing the coercion to the `()` type.
pub(super) fn if_fallback_coercion<T>(
&self,
if_span: Span,
cond_expr: &'tcx hir::Expr<'tcx>,
then_expr: &'tcx hir::Expr<'tcx>,
coercion: &mut CoerceMany<'tcx, '_, T>,
) -> bool
where
T: AsCoercionSite,
{
// If this `if` expr is the parent's function return expr,
// the cause of the type coercion is the return type, point at it. (#25228)
let hir_id = self.tcx.parent_hir_id(self.tcx.parent_hir_id(then_expr.hir_id));
let ret_reason = self.maybe_get_coercion_reason(hir_id, if_span);
let cause = self.cause(if_span, ObligationCauseCode::IfExpressionWithNoElse);
let mut error = false;
coercion.coerce_forced_unit(
self,
&cause,
|err| self.explain_if_expr(err, ret_reason, if_span, cond_expr, then_expr, &mut error),
false,
);
error
}
/// Explain why `if` expressions without `else` evaluate to `()` and detect likely irrefutable
/// `if let PAT = EXPR {}` expressions that could be turned into `let PAT = EXPR;`.
fn explain_if_expr(
&self,
err: &mut Diag<'_>,
ret_reason: Option<(Span, String)>,
if_span: Span,
cond_expr: &'tcx hir::Expr<'tcx>,
then_expr: &'tcx hir::Expr<'tcx>,
error: &mut bool,
) {
if let Some((if_span, msg)) = ret_reason {
err.span_label(if_span, msg);
} else if let ExprKind::Block(block, _) = then_expr.kind
&& let Some(expr) = block.expr
{
err.span_label(expr.span, "found here");
}
err.note("`if` expressions without `else` evaluate to `()`");
err.help("consider adding an `else` block that evaluates to the expected type");
*error = true;
if let ExprKind::Let(hir::LetExpr { span, pat, init, .. }) = cond_expr.kind
&& let ExprKind::Block(block, _) = then_expr.kind
// Refutability checks occur on the MIR, so we approximate it here by checking
// if we have an enum with a single variant or a struct in the pattern.
&& let PatKind::TupleStruct(qpath, ..) | PatKind::Struct(qpath, ..) = pat.kind
&& let hir::QPath::Resolved(_, path) = qpath
{
match path.res {
Res::Def(DefKind::Ctor(CtorOf::Struct, _), _) => {
// Structs are always irrefutable. Their fields might not be, but we
// don't check for that here, it's only an approximation.
}
Res::Def(DefKind::Ctor(CtorOf::Variant, _), def_id)
if self
.tcx
.adt_def(self.tcx.parent(self.tcx.parent(def_id)))
.variants()
.len()
== 1 =>
{
// There's only a single variant in the `enum`, so we can suggest the
// irrefutable `let` instead of `if let`.
}
_ => return,
}
let mut sugg = vec![
// Remove the `if`
(if_span.until(*span), String::new()),
];
match (block.stmts, block.expr) {
([first, ..], Some(expr)) => {
let padding = self
.tcx
.sess
.source_map()
.indentation_before(first.span)
.unwrap_or_else(|| String::new());
sugg.extend([
(init.span.between(first.span), format!(";\n{padding}")),
(expr.span.shrink_to_hi().with_hi(block.span.hi()), String::new()),
]);
}
([], Some(expr)) => {
let padding = self
.tcx
.sess
.source_map()
.indentation_before(expr.span)
.unwrap_or_else(|| String::new());
sugg.extend([
(init.span.between(expr.span), format!(";\n{padding}")),
(expr.span.shrink_to_hi().with_hi(block.span.hi()), String::new()),
]);
}
// If there's no value in the body, then the `if` expression would already
// be of type `()`, so checking for those cases is unnecessary.
(_, None) => return,
}
err.multipart_suggestion(
"consider using an irrefutable `let` binding instead",
sugg,
Applicability::MaybeIncorrect,
);
}
}
pub(crate) fn maybe_get_coercion_reason(
&self,
hir_id: hir::HirId,
sp: Span,
) -> Option<(Span, String)> {
let node = self.tcx.hir_node(hir_id);
if let hir::Node::Block(block) = node {
// check that the body's parent is an fn
let parent = self.tcx.parent_hir_node(self.tcx.parent_hir_id(block.hir_id));
if let (Some(expr), hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(..), .. })) =
(&block.expr, parent)
{
// check that the `if` expr without `else` is the fn body's expr
if expr.span == sp {
return self.get_fn_decl(hir_id).map(|(_, fn_decl, _)| {
let (ty, span) = match fn_decl.output {
hir::FnRetTy::DefaultReturn(span) => ("()".to_string(), span),
hir::FnRetTy::Return(ty) => (ty_to_string(&self.tcx, ty), ty.span),
};
(span, format!("expected `{ty}` because of this return type"))
});
}
}
}
if let hir::Node::LetStmt(hir::LetStmt { ty: Some(_), pat, .. }) = node {
return Some((pat.span, "expected because of this assignment".to_string()));
}
None
}
pub(crate) fn if_cause(
&self,
span: Span,
cond_span: Span,
then_expr: &'tcx hir::Expr<'tcx>,
else_expr: &'tcx hir::Expr<'tcx>,
then_ty: Ty<'tcx>,
else_ty: Ty<'tcx>,
tail_defines_return_position_impl_trait: Option<LocalDefId>,
) -> ObligationCause<'tcx> {
let mut outer_span = if self.tcx.sess.source_map().is_multiline(span) {
// The `if`/`else` isn't in one line in the output, include some context to make it
// clear it is an if/else expression:
// ```
// LL | let x = if true {
// | _____________-
// LL || 10i32
// || ----- expected because of this
// LL || } else {
// LL || 10u32
// || ^^^^^ expected `i32`, found `u32`
// LL || };
// ||_____- `if` and `else` have incompatible types
// ```
Some(span)
} else {
// The entire expression is in one line, only point at the arms
// ```
// LL | let x = if true { 10i32 } else { 10u32 };
// | ----- ^^^^^ expected `i32`, found `u32`
// | |
// | expected because of this
// ```
None
};
let (error_sp, else_id) = if let ExprKind::Block(block, _) = &else_expr.kind {
let block = block.innermost_block();
// Avoid overlapping spans that aren't as readable:
// ```
// 2 | let x = if true {
// | _____________-
// 3 | | 3
// | | - expected because of this
// 4 | | } else {
// | |____________^
// 5 | ||
// 6 | || };
// | || ^
// | ||_____|
// | |______if and else have incompatible types
// | expected integer, found `()`
// ```
// by not pointing at the entire expression:
// ```
// 2 | let x = if true {
// | ------- `if` and `else` have incompatible types
// 3 | 3
// | - expected because of this
// 4 | } else {
// | ____________^
// 5 | |
// 6 | | };
// | |_____^ expected integer, found `()`
// ```
if block.expr.is_none()
&& block.stmts.is_empty()
&& let Some(outer_span) = &mut outer_span
&& let Some(cond_span) = cond_span.find_ancestor_inside(*outer_span)
{
*outer_span = outer_span.with_hi(cond_span.hi())
}
(self.find_block_span(block), block.hir_id)
} else {
(else_expr.span, else_expr.hir_id)
};
let then_id = if let ExprKind::Block(block, _) = &then_expr.kind {
let block = block.innermost_block();
// Exclude overlapping spans
if block.expr.is_none() && block.stmts.is_empty() {
outer_span = None;
}
block.hir_id
} else {
then_expr.hir_id
};
// Finally construct the cause:
self.cause(
error_sp,
ObligationCauseCode::IfExpression(Box::new(IfExpressionCause {
else_id,
then_id,
then_ty,
else_ty,
outer_span,
tail_defines_return_position_impl_trait,
})),
)
}
pub(super) fn demand_scrutinee_type(
&self,
scrut: &'tcx hir::Expr<'tcx>,
contains_ref_bindings: Option<hir::Mutability>,
no_arms: bool,
) -> Ty<'tcx> {
// Not entirely obvious: if matches may create ref bindings, we want to
// use the *precise* type of the scrutinee, *not* some supertype, as
// the "scrutinee type" (issue #23116).
//
// arielb1 [writes here in this comment thread][c] that there
// is certainly *some* potential danger, e.g., for an example
// like:
//
// [c]: https://github.com/rust-lang/rust/pull/43399#discussion_r130223956
//
// ```
// let Foo(x) = f()[0];
// ```
//
// Then if the pattern matches by reference, we want to match
// `f()[0]` as a lexpr, so we can't allow it to be
// coerced. But if the pattern matches by value, `f()[0]` is
// still syntactically a lexpr, but we *do* want to allow
// coercions.
//
// However, *likely* we are ok with allowing coercions to
// happen if there are no explicit ref mut patterns - all
// implicit ref mut patterns must occur behind a reference, so
// they will have the "correct" variance and lifetime.
//
// This does mean that the following pattern would be legal:
//
// ```
// struct Foo(Bar);
// struct Bar(u32);
// impl Deref for Foo {
// type Target = Bar;
// fn deref(&self) -> &Bar { &self.0 }
// }
// impl DerefMut for Foo {
// fn deref_mut(&mut self) -> &mut Bar { &mut self.0 }
// }
// fn foo(x: &mut Foo) {
// {
// let Bar(z): &mut Bar = x;
// *z = 42;
// }
// assert_eq!(foo.0.0, 42);
// }
// ```
//
// FIXME(tschottdorf): don't call contains_explicit_ref_binding, which
// is problematic as the HIR is being scraped, but ref bindings may be
// implicit after #42640. We need to make sure that pat_adjustments
// (once introduced) is populated by the time we get here.
//
// See #44848.
if let Some(m) = contains_ref_bindings {
self.check_expr_with_needs(scrut, Needs::maybe_mut_place(m))
} else if no_arms {
self.check_expr(scrut)
} else {
// ...but otherwise we want to use any supertype of the
// scrutinee. This is sort of a workaround, see note (*) in
// `check_pat` for some details.
let scrut_ty = self.next_ty_var(scrut.span);
self.check_expr_has_type_or_error(scrut, scrut_ty, |_| {});
scrut_ty
}
}
// Does the expectation of the match define an RPIT?
// (e.g. we're in the tail of a function body)
//
// Returns the `LocalDefId` of the RPIT, which is always identity-substituted.
pub(crate) fn return_position_impl_trait_from_match_expectation(
&self,
expectation: Expectation<'tcx>,
) -> Option<LocalDefId> {
let expected_ty = expectation.to_option(self)?;
let (def_id, args) = match *expected_ty.kind() {
// FIXME: Could also check that the RPIT is not defined
ty::Alias(ty::Opaque, alias_ty) => (alias_ty.def_id.as_local()?, alias_ty.args),
// FIXME(-Znext-solver): Remove this branch once `replace_opaque_types_with_infer` is gone.
ty::Infer(ty::TyVar(_)) => self
.inner
.borrow()
.iter_opaque_types()
.find(|(_, v)| v.ty == expected_ty)
.map(|(k, _)| (k.def_id, k.args))?,
_ => return None,
};
let hir::OpaqueTyOrigin::FnReturn(parent_def_id) = self.tcx.opaque_type_origin(def_id)
else {
return None;
};
if &args[0..self.tcx.generics_of(parent_def_id).count()]
!= ty::GenericArgs::identity_for_item(self.tcx, parent_def_id).as_slice()
{
return None;
}
Some(def_id)
}
}
fn arms_contain_ref_bindings<'tcx>(arms: &'tcx [hir::Arm<'tcx>]) -> Option<hir::Mutability> {
arms.iter().filter_map(|a| a.pat.contains_explicit_ref_binding()).max()
}