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Rollup merge of #127987 - estebank:impl-trait-sugg, r=cjgillot

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More accurate suggestion for `-> Box<dyn Trait>` or `-> impl Trait`

When encountering `-> Trait`, suggest `-> Box<dyn Trait>` (instead of `-> Box<Trait>`.

If there's a single returned type within the `fn`, suggest `-> impl Trait`.
This commit is contained in:
Matthias Krüger 2024-07-20 07:13:46 +02:00 • committed by GitHub
commit 89798e9064
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9 changed files with 72 additions and 65 deletions
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50
compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
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@ -1793,25 +1793,42 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
err.children.clear();
let span = obligation.cause.span;
if let Ok(snip) = self.tcx.sess.source_map().span_to_snippet(span)
&& snip.starts_with("dyn ")
{
err.span_suggestion(
span.with_hi(span.lo() + BytePos(4)),
"return an `impl Trait` instead of a `dyn Trait`, \
if all returned values are the same type",
"impl ",
Applicability::MaybeIncorrect,
);
}
let body = self.tcx.hir().body_owned_by(obligation.cause.body_id);
let mut visitor = ReturnsVisitor::default();
visitor.visit_body(&body);
let mut sugg =
vec![(span.shrink_to_lo(), "Box<".to_string()), (span.shrink_to_hi(), ">".to_string())];
let (pre, impl_span) = if let Ok(snip) = self.tcx.sess.source_map().span_to_snippet(span)
&& snip.starts_with("dyn ")
{
("", span.with_hi(span.lo() + BytePos(4)))
} else {
("dyn ", span.shrink_to_lo())
};
let alternatively = if visitor
.returns
.iter()
.map(|expr| self.typeck_results.as_ref().unwrap().expr_ty_adjusted_opt(expr))
.collect::<FxHashSet<_>>()
.len()
<= 1
{
err.span_suggestion_verbose(
impl_span,
"consider returning an `impl Trait` instead of a `dyn Trait`",
"impl ",
Applicability::MaybeIncorrect,
);
"alternatively, "
} else {
err.help("if there were a single returned type, you could use `impl Trait` instead");
""
};
let mut sugg = vec![
(span.shrink_to_lo(), format!("Box<{pre}")),
(span.shrink_to_hi(), ">".to_string()),
];
sugg.extend(visitor.returns.into_iter().flat_map(|expr| {
let span =
expr.span.find_ancestor_in_same_ctxt(obligation.cause.span).unwrap_or(expr.span);
@ -1837,7 +1854,10 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
}));
err.multipart_suggestion(
"box the return type, and wrap all of the returned values in `Box::new`",
format!(
"{alternatively}box the return type, and wrap all of the returned values in \
`Box::new`",
),
sugg,
Applicability::MaybeIncorrect,
);