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JonathanBrouwerJonathanBrouwer
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Rollup merge of rust-lang#151954 - Muhtasim-Rasheed:issue-109829-help-message, r=eholk
Add help message suggesting explicit reference cast for From/TryFrom Closes rust-lang#109829 Improves E0277 diagnostics when a `From` or `TryFrom` implementation is expected, but the provided type is a reference that can be explicitly cast to a type the trait can convert from.
2 parents f21b4c0 + ac159dd commit 37d354a

3 files changed

Lines changed: 203 additions & 0 deletions

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‎compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs‎

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@@ -277,6 +277,28 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
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};
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let mut err = struct_span_code_err!(self.dcx(), span, E0277, "{}", err_msg);
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let trait_def_id = main_trait_predicate.def_id();
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if self.tcx.is_diagnostic_item(sym::From, trait_def_id)
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|| self.tcx.is_diagnostic_item(sym::TryFrom, trait_def_id)
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{
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let found_ty = leaf_trait_predicate.skip_binder().trait_ref.args.type_at(1);
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let ty = main_trait_predicate.skip_binder().self_ty();
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if let Some(cast_ty) = self.find_explicit_cast_type(
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obligation.param_env,
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found_ty,
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ty,
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) {
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let found_ty_str = self.tcx.short_string(found_ty, &mut long_ty_file);
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let cast_ty_str = self.tcx.short_string(cast_ty, &mut long_ty_file);
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err.help(
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format!(
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"consider casting the `{found_ty_str}` value to `{cast_ty_str}`",
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),
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);
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}
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}
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*err.long_ty_path() = long_ty_file;
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let mut suggested = false;
@@ -2932,6 +2954,69 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
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})
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}
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/// If `found_ty` is a reference that can be explicitly cast to another reference type for which
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/// a `From` / `TryFrom` impl exists for `self_ty`, return that type.
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fn find_explicit_cast_type(
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&self,
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param_env: ty::ParamEnv<'tcx>,
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found_ty: Ty<'tcx>,
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self_ty: Ty<'tcx>,
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) -> Option<Ty<'tcx>> {
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let ty::Ref(region, inner_ty, mutbl) = *found_ty.kind() else {
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return None;
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};
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let mut derefs = (self.autoderef_steps)(inner_ty).into_iter();
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derefs.next(); // skip the first one, which is inner_ty itself
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let deref_target = derefs.into_iter().next()?.0;
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let cast_ty = Ty::new_ref(self.tcx, region, deref_target, mutbl);
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let Some(from_def_id) = self.tcx.get_diagnostic_item(sym::From) else {
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return None;
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};
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let Some(try_from_def_id) = self.tcx.get_diagnostic_item(sym::TryFrom) else {
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return None;
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};
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if self.has_impl_for_type(
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param_env,
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ty::TraitRef::new(
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self.tcx,
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from_def_id,
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self.tcx.mk_args(&[self_ty.into(), cast_ty.into()]),
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),
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) {
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Some(cast_ty)
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} else if self.has_impl_for_type(
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param_env,
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ty::TraitRef::new(
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self.tcx,
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try_from_def_id,
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self.tcx.mk_args(&[self_ty.into(), cast_ty.into()]),
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),
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) {
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Some(cast_ty)
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} else {
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None
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}
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}
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fn has_impl_for_type(
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&self,
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param_env: ty::ParamEnv<'tcx>,
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trait_ref: ty::TraitRef<'tcx>,
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) -> bool {
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let obligation = Obligation::new(
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self.tcx,
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ObligationCause::dummy(),
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param_env,
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ty::TraitPredicate { trait_ref, polarity: ty::PredicatePolarity::Positive },
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);
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self.predicate_must_hold_modulo_regions(&obligation)
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}
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29353020
fn add_tuple_trait_message(
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&self,
29373022
obligation_cause_code: &ObligationCauseCode<'tcx>,

‎tests/ui/traits/explicit-reference-cast.rs‎

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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,50 @@
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// compile-fail
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use std::convert::TryFrom;
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use std::path::{Path, PathBuf};
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pub struct ToolA(PathBuf);
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//~^ HELP the trait `From<&PathBuf>` is not implemented for `ToolA`
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impl From<&Path> for ToolA {
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//~^ HELP the following other types implement trait `From<T>`
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fn from(p: &Path) -> ToolA {
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ToolA(p.to_path_buf())
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}
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}
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// Add a different From<T> impl to ensure we suggest the correct cast
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impl From<&str> for ToolA {
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fn from(s: &str) -> ToolA {
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ToolA(PathBuf::from(s))
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}
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}
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pub struct ToolB(PathBuf);
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//~^ HELP the trait `From<&PathBuf>` is not implemented for `ToolB`
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//~| HELP the trait `From<&PathBuf>` is not implemented for `ToolB`
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impl TryFrom<&Path> for ToolB {
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//~^ HELP the trait `TryFrom<&PathBuf>` is not implemented for `ToolB`
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//~| HELP the trait `TryFrom<&PathBuf>` is not implemented for `ToolB`
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type Error = ();
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fn try_from(p: &Path) -> Result<ToolB, ()> {
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Ok(ToolB(p.to_path_buf()))
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}
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}
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fn main() {
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let path = PathBuf::new();
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let _ = ToolA::from(&path);
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//~^ ERROR the trait bound `ToolA: From<&PathBuf>` is not satisfied
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//~| HELP consider casting the `&PathBuf` value to `&Path`
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let _ = ToolB::try_from(&path);
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//~^ ERROR the trait bound `ToolB: TryFrom<&PathBuf>` is not satisfied
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//~| ERROR the trait bound `ToolB: From<&PathBuf>` is not satisfied
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//~| HELP consider casting the `&PathBuf` value to `&Path`
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//~| HELP consider casting the `&PathBuf` value to `&Path`
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//~| HELP for that trait implementation, expected `Path`, found `PathBuf`
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//~| HELP for that trait implementation, expected `Path`, found `PathBuf`
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}

‎tests/ui/traits/explicit-reference-cast.stderr‎

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@@ -0,0 +1,68 @@
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error[E0277]: the trait bound `ToolA: From<&PathBuf>` is not satisfied
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--> $DIR/explicit-reference-cast.rs:40:13
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|
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LL | let _ = ToolA::from(&path);
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| ^^^^^ unsatisfied trait bound
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|
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= help: consider casting the `&PathBuf` value to `&Path`
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help: the trait `From<&PathBuf>` is not implemented for `ToolA`
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--> $DIR/explicit-reference-cast.rs:6:1
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|
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LL | pub struct ToolA(PathBuf);
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| ^^^^^^^^^^^^^^^^
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help: the following other types implement trait `From<T>`
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--> $DIR/explicit-reference-cast.rs:9:1
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|
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LL | impl From<&Path> for ToolA {
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^ `ToolA` implements `From<&Path>`
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...
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LL | impl From<&str> for ToolA {
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| ^^^^^^^^^^^^^^^^^^^^^^^^^ `ToolA` implements `From<&str>`
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error[E0277]: the trait bound `ToolB: TryFrom<&PathBuf>` is not satisfied
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--> $DIR/explicit-reference-cast.rs:43:13
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|
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LL | let _ = ToolB::try_from(&path);
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| ^^^^^ unsatisfied trait bound
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|
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= help: consider casting the `&PathBuf` value to `&Path`
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help: the trait `From<&PathBuf>` is not implemented for `ToolB`
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--> $DIR/explicit-reference-cast.rs:23:1
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|
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LL | pub struct ToolB(PathBuf);
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| ^^^^^^^^^^^^^^^^
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help: the trait `TryFrom<&PathBuf>` is not implemented for `ToolB`
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but trait `TryFrom<&Path>` is implemented for it
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--> $DIR/explicit-reference-cast.rs:27:1
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|
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LL | impl TryFrom<&Path> for ToolB {
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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= help: for that trait implementation, expected `Path`, found `PathBuf`
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= note: required for `&PathBuf` to implement `Into<ToolB>`
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= note: required for `ToolB` to implement `TryFrom<&PathBuf>`
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error[E0277]: the trait bound `ToolB: From<&PathBuf>` is not satisfied
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--> $DIR/explicit-reference-cast.rs:43:13
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|
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LL | let _ = ToolB::try_from(&path);
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| ^^^^^^^^^^^^^^^^^^^^^^ unsatisfied trait bound
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|
50+
= help: consider casting the `&PathBuf` value to `&Path`
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help: the trait `From<&PathBuf>` is not implemented for `ToolB`
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--> $DIR/explicit-reference-cast.rs:23:1
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|
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LL | pub struct ToolB(PathBuf);
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| ^^^^^^^^^^^^^^^^
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help: the trait `TryFrom<&PathBuf>` is not implemented for `ToolB`
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but trait `TryFrom<&Path>` is implemented for it
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--> $DIR/explicit-reference-cast.rs:27:1
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|
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LL | impl TryFrom<&Path> for ToolB {
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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= help: for that trait implementation, expected `Path`, found `PathBuf`
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= note: required for `&PathBuf` to implement `Into<ToolB>`
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= note: required for `ToolB` to implement `TryFrom<&PathBuf>`
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error: aborting due to 3 previous errors
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For more information about this error, try `rustc --explain E0277`.

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