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Improve suggestions for returning binding #99539

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Jul 22, 2022
Merged

Improve suggestions for returning binding #99539

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3d9dd68
Resolve vars in same_type_modulo_infer
compiler-errors Jul 20, 2022
99c3257
Do if-expression obligation stuff less eagerly
compiler-errors Jul 21, 2022
8926dac
And for patterns too
compiler-errors Jul 21, 2022
cd89978
Generalize same_type_modulo_infer
compiler-errors Jul 21, 2022
3eef023
Address more nits
compiler-errors Jul 21, 2022
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10 changes: 10 additions & 0 deletions compiler/rustc_hir/src/hir.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -954,6 +954,16 @@ pub struct Block<'hir> {
pub targeted_by_break: bool,
}

impl<'hir> Block<'hir> {
pub fn innermost_block(&self) -> &Block<'hir> {
let mut block = self;
while let Some(Expr { kind: ExprKind::Block(inner_block, _), .. }) = block.expr {
block = inner_block;
}
block
}
}

#[derive(Debug, HashStable_Generic)]
pub struct Pat<'hir> {
#[stable_hasher(ignore)]
Expand Down
539 changes: 422 additions & 117 deletions compiler/rustc_infer/src/infer/error_reporting/mod.rs
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23 changes: 14 additions & 9 deletions compiler/rustc_middle/src/traits/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -351,7 +351,7 @@ pub enum ObligationCauseCode<'tcx> {
ConstPatternStructural,

/// Computing common supertype in an if expression
IfExpression(Box<IfExpressionCause>),
IfExpression(Box<IfExpressionCause<'tcx>>),

/// Computing common supertype of an if expression with no else counter-part
IfExpressionWithNoElse,
Expand Down Expand Up @@ -488,22 +488,27 @@ impl<'tcx> ty::Lift<'tcx> for StatementAsExpression {

#[derive(Clone, Debug, PartialEq, Eq, Hash, Lift)]
pub struct MatchExpressionArmCause<'tcx> {
pub arm_block_id: Option<hir::HirId>,
pub arm_ty: Ty<'tcx>,
pub arm_span: Span,
pub prior_arm_block_id: Option<hir::HirId>,
pub prior_arm_ty: Ty<'tcx>,
pub prior_arm_span: Span,
pub scrut_span: Span,
pub semi_span: Option<(Span, StatementAsExpression)>,
pub source: hir::MatchSource,
pub prior_arms: Vec<Span>,
pub last_ty: Ty<'tcx>,
pub scrut_hir_id: hir::HirId,
pub opt_suggest_box_span: Option<Span>,
}

#[derive(Clone, Debug, PartialEq, Eq, Hash)]
pub struct IfExpressionCause {
pub then: Span,
pub else_sp: Span,
pub outer: Option<Span>,
pub semicolon: Option<(Span, StatementAsExpression)>,
#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
#[derive(Lift, TypeFoldable, TypeVisitable)]
pub struct IfExpressionCause<'tcx> {
pub then_id: hir::HirId,
pub else_id: hir::HirId,
pub then_ty: Ty<'tcx>,
pub else_ty: Ty<'tcx>,
pub outer_span: Option<Span>,
pub opt_suggest_box_span: Option<Span>,
}

Expand Down
1 change: 0 additions & 1 deletion compiler/rustc_middle/src/traits/structural_impls.rs
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Expand Up @@ -130,7 +130,6 @@ impl<N: fmt::Debug> fmt::Debug for traits::ImplSourceConstDestructData<N> {
// Lift implementations

TrivialTypeTraversalAndLiftImpls! {
super::IfExpressionCause,
super::ImplSourceDiscriminantKindData,
super::ImplSourcePointeeData,
}
3 changes: 1 addition & 2 deletions compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -22,7 +22,6 @@ use rustc_hir::intravisit::Visitor;
use rustc_hir::GenericParam;
use rustc_hir::Item;
use rustc_hir::Node;
use rustc_infer::infer::error_reporting::same_type_modulo_infer;
use rustc_infer::traits::TraitEngine;
use rustc_middle::traits::select::OverflowError;
use rustc_middle::ty::abstract_const::NotConstEvaluatable;
Expand Down Expand Up @@ -640,7 +639,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
if expected.len() == 1 { "" } else { "s" },
)
);
} else if !same_type_modulo_infer(given_ty, expected_ty) {
} else if !self.same_type_modulo_infer(given_ty, expected_ty) {
// Print type mismatch
let (expected_args, given_args) =
self.cmp(given_ty, expected_ty);
Expand Down
169 changes: 68 additions & 101 deletions compiler/rustc_typeck/src/check/_match.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -9,7 +9,6 @@ use rustc_span::Span;
use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
use rustc_trait_selection::traits::{
IfExpressionCause, MatchExpressionArmCause, ObligationCause, ObligationCauseCode,
StatementAsExpression,
};

impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
Expand Down Expand Up @@ -75,8 +74,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
};

let mut other_arms = vec![]; // Used only for diagnostics.
let mut prior_arm_ty = None;
for (i, arm) in arms.iter().enumerate() {
let mut prior_arm = None;
for arm in arms {
if let Some(g) = &arm.guard {
self.diverges.set(Diverges::Maybe);
match g {
Expand All @@ -96,21 +95,28 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {

let opt_suggest_box_span = self.opt_suggest_box_span(arm_ty, orig_expected);

let (arm_span, semi_span) =
self.get_appropriate_arm_semicolon_removal_span(&arms, i, prior_arm_ty, arm_ty);
let (span, code) = match i {
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.
0 => (arm_span, ObligationCauseCode::BlockTailExpression(arm.body.hir_id)),
_ => (
None => (arm_span, ObligationCauseCode::BlockTailExpression(arm.body.hir_id)),
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,
semi_span,
source: match_src,
prior_arms: other_arms.clone(),
last_ty: prior_arm_ty.unwrap(),
scrut_hir_id: scrut.hir_id,
opt_suggest_box_span,
})),
Expand Down Expand Up @@ -139,7 +145,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
let ret_ty = ret_coercion.borrow().expected_ty();
let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
self.can_coerce(arm_ty, ret_ty)
&& prior_arm_ty.map_or(true, |t| self.can_coerce(t, ret_ty))
&& prior_arm.map_or(true, |(_, t, _)| self.can_coerce(t, ret_ty))
// The match arms need to unify for the case of `impl Trait`.
&& !matches!(ret_ty.kind(), ty::Opaque(..))
}
Expand Down Expand Up @@ -181,7 +187,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
if other_arms.len() > 5 {
other_arms.remove(0);
}
prior_arm_ty = Some(arm_ty);

prior_arm = Some((arm_block_id, arm_ty, arm_span));
}

// If all of the arms in the `match` diverge,
Expand All @@ -207,28 +214,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
match_ty
}

fn get_appropriate_arm_semicolon_removal_span(
&self,
arms: &'tcx [hir::Arm<'tcx>],
i: usize,
prior_arm_ty: Option<Ty<'tcx>>,
arm_ty: Ty<'tcx>,
) -> (Span, Option<(Span, StatementAsExpression)>) {
let arm = &arms[i];
let (arm_span, mut semi_span) = if let hir::ExprKind::Block(blk, _) = &arm.body.kind {
self.find_block_span(blk, prior_arm_ty)
} else {
(arm.body.span, None)
};
if semi_span.is_none() && i > 0 {
if let hir::ExprKind::Block(blk, _) = &arms[i - 1].body.kind {
let (_, semi_span_prev) = self.find_block_span(blk, Some(arm_ty));
semi_span = semi_span_prev;
}
}
(arm_span, semi_span)
}

/// 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>]) {
Expand Down Expand Up @@ -313,7 +298,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
else_ty: Ty<'tcx>,
opt_suggest_box_span: Option<Span>,
) -> ObligationCause<'tcx> {
let mut outer_sp = if self.tcx.sess.source_map().is_multiline(span) {
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:
// ```
Expand All @@ -339,69 +324,67 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
None
};

let mut remove_semicolon = None;
let error_sp = if let ExprKind::Block(block, _) = &else_expr.kind {
let (error_sp, semi_sp) = self.find_block_span(block, Some(then_ty));
remove_semicolon = semi_sp;
if block.expr.is_none() && block.stmts.is_empty() {
// 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 outer_sp.is_some() {
outer_sp = Some(self.tcx.sess.source_map().guess_head_span(span));
}
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
{
*outer_span = self.tcx.sess.source_map().guess_head_span(*outer_span);
}
error_sp

(self.find_block_span(block), block.hir_id)
} else {
// shouldn't happen unless the parser has done something weird
else_expr.span
(else_expr.span, else_expr.hir_id)
};

// Compute `Span` of `then` part of `if`-expression.
let then_sp = if let ExprKind::Block(block, _) = &then_expr.kind {
let (then_sp, semi_sp) = self.find_block_span(block, Some(else_ty));
remove_semicolon = remove_semicolon.or(semi_sp);
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_sp = None; // same as in `error_sp`; cleanup output
outer_span = None;
}
then_sp
block.hir_id
} else {
// shouldn't happen unless the parser has done something weird
then_expr.span
then_expr.hir_id
};

// Finally construct the cause:
self.cause(
error_sp,
ObligationCauseCode::IfExpression(Box::new(IfExpressionCause {
then: then_sp,
else_sp: error_sp,
outer: outer_sp,
semicolon: remove_semicolon,
else_id,
then_id,
then_ty,
else_ty,
outer_span,
opt_suggest_box_span,
})),
)
Expand Down Expand Up @@ -482,22 +465,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
}
}

fn find_block_span(
&self,
block: &'tcx hir::Block<'tcx>,
expected_ty: Option<Ty<'tcx>>,
) -> (Span, Option<(Span, StatementAsExpression)>) {
if let Some(expr) = &block.expr {
(expr.span, None)
} else if let Some(stmt) = block.stmts.last() {
// possibly incorrect trailing `;` in the else arm
(stmt.span, expected_ty.and_then(|ty| self.could_remove_semicolon(block, ty)))
} else {
// empty block; point at its entirety
(block.span, None)
}
}

// When we have a `match` as a tail expression in a `fn` with a returned `impl Trait`
// we check if the different arms would work with boxed trait objects instead and
// provide a structured suggestion in that case.
Expand Down
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