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Auto merge of #60155 - davidtwco:issue-59819, r=oli-obk
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Suggest dereferencing when `Deref` is implemented.

Fixes #59819.

r? @oli-obk
cc @estebank
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bors committed Apr 23, 2019
2 parents fe0a415 + 7ab1bfd commit 4eff852
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Showing 4 changed files with 175 additions and 55 deletions.
121 changes: 66 additions & 55 deletions src/librustc_typeck/check/demand.rs
Original file line number Diff line number Diff line change
@@ -1,6 +1,6 @@
use crate::check::FnCtxt;
use rustc::infer::InferOk;
use rustc::traits::{ObligationCause, ObligationCauseCode};
use rustc::traits::{self, ObligationCause, ObligationCauseCode};

use syntax::util::parser::PREC_POSTFIX;
use syntax_pos::Span;
Expand Down Expand Up @@ -324,8 +324,12 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
sp,
);

match (&expected.sty, &checked_ty.sty) {
(&ty::Ref(_, exp, _), &ty::Ref(_, check, _)) => match (&exp.sty, &check.sty) {
// Check the `expn_info()` to see if this is a macro; if so, it's hard to
// extract the text and make a good suggestion, so don't bother.
let is_macro = sp.ctxt().outer().expn_info().is_some();

match (&expr.node, &expected.sty, &checked_ty.sty) {
(_, &ty::Ref(_, exp, _), &ty::Ref(_, check, _)) => match (&exp.sty, &check.sty) {
(&ty::Str, &ty::Array(arr, _)) |
(&ty::Str, &ty::Slice(arr)) if arr == self.tcx.types.u8 => {
if let hir::ExprKind::Lit(_) = expr.node {
Expand All @@ -352,7 +356,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
}
_ => {}
},
(&ty::Ref(_, _, mutability), _) => {
(_, &ty::Ref(_, _, mutability), _) => {
// Check if it can work when put into a ref. For example:
//
// ```
Expand Down Expand Up @@ -407,60 +411,67 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
});
}
}
}
(_, &ty::Ref(_, checked, _)) => {
},
(hir::ExprKind::AddrOf(_, ref expr), _, &ty::Ref(_, checked, _)) if {
self.infcx.can_sub(self.param_env, checked, &expected).is_ok() && !is_macro
} => {
// We have `&T`, check if what was expected was `T`. If so,
// we may want to suggest adding a `*`, or removing
// a `&`.
//
// (But, also check the `expn_info()` to see if this is
// a macro; if so, it's hard to extract the text and make a good
// suggestion, so don't bother.)
if self.infcx.can_sub(self.param_env, checked, &expected).is_ok() &&
sp.ctxt().outer().expn_info().is_none() {
match expr.node {
// Maybe remove `&`?
hir::ExprKind::AddrOf(_, ref expr) => {
if !cm.span_to_filename(expr.span).is_real() {
if let Ok(code) = cm.span_to_snippet(sp) {
if code.chars().next() == Some('&') {
return Some((
sp,
"consider removing the borrow",
code[1..].to_string()),
);
}
}
return None;
}
if let Ok(code) = cm.span_to_snippet(expr.span) {
return Some((sp, "consider removing the borrow", code));
}
}

// Maybe add `*`? Only if `T: Copy`.
_ => {
if self.infcx.type_is_copy_modulo_regions(self.param_env,
checked,
sp) {
// do not suggest if the span comes from a macro (#52783)
if let (Ok(code), true) = (
cm.span_to_snippet(sp),
sp == expr.span,
) {
return Some((
sp,
"consider dereferencing the borrow",
if is_struct_pat_shorthand_field {
format!("{}: *{}", code, code)
} else {
format!("*{}", code)
},
));
}
}
// we may want to suggest removing a `&`.
if !cm.span_to_filename(expr.span).is_real() {
if let Ok(code) = cm.span_to_snippet(sp) {
if code.chars().next() == Some('&') {
return Some((
sp,
"consider removing the borrow",
code[1..].to_string(),
));
}
}
return None;
}
if let Ok(code) = cm.span_to_snippet(expr.span) {
return Some((sp, "consider removing the borrow", code));
}
},
_ if sp == expr.span && !is_macro => {
// Check for `Deref` implementations by constructing a predicate to
// prove: `<T as Deref>::Output == U`
let deref_trait = self.tcx.lang_items().deref_trait().unwrap();
let item_def_id = self.tcx.associated_items(deref_trait).next().unwrap().def_id;
let predicate = ty::Predicate::Projection(ty::Binder::bind(ty::ProjectionPredicate {
// `<T as Deref>::Output`
projection_ty: ty::ProjectionTy {
// `T`
substs: self.tcx.mk_substs_trait(
checked_ty,
self.fresh_substs_for_item(sp, item_def_id),
),
// `Deref::Output`
item_def_id,
},
// `U`
ty: expected,
}));
let obligation = traits::Obligation::new(self.misc(sp), self.param_env, predicate);
let impls_deref = self.infcx.predicate_may_hold(&obligation);

// For a suggestion to make sense, the type would need to be `Copy`.
let is_copy = self.infcx.type_is_copy_modulo_regions(self.param_env, expected, sp);

if is_copy && impls_deref {
if let Ok(code) = cm.span_to_snippet(sp) {
let message = if checked_ty.is_region_ptr() {
"consider dereferencing the borrow"
} else {
"consider dereferencing the type"
};
let suggestion = if is_struct_pat_shorthand_field {
format!("{}: *{}", code, code)
} else {
format!("*{}", code)
};
return Some((sp, message, suggestion));
}
}
}
_ => {}
Expand Down
35 changes: 35 additions & 0 deletions src/test/ui/suggestions/issue-59819.fixed
Original file line number Diff line number Diff line change
@@ -0,0 +1,35 @@
// run-rustfix

#![allow(warnings)]

// Test that suggestion to add `*` characters applies to implementations of `Deref` as well as
// references.

struct Foo(i32);

struct Bar(String);

impl std::ops::Deref for Foo {
type Target = i32;
fn deref(&self) -> &Self::Target {
&self.0
}
}

impl std::ops::Deref for Bar {
type Target = String;
fn deref(&self) -> &Self::Target {
&self.0
}
}

fn main() {
let x = Foo(42);
let y: i32 = *x; //~ ERROR mismatched types
let a = &42;
let b: i32 = *a; //~ ERROR mismatched types

// Do not make a suggestion when adding a `*` wouldn't actually fix the issue:
let f = Bar("bar".to_string());
let g: String = f.to_string(); //~ ERROR mismatched types
}
35 changes: 35 additions & 0 deletions src/test/ui/suggestions/issue-59819.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,35 @@
// run-rustfix

#![allow(warnings)]

// Test that suggestion to add `*` characters applies to implementations of `Deref` as well as
// references.

struct Foo(i32);

struct Bar(String);

impl std::ops::Deref for Foo {
type Target = i32;
fn deref(&self) -> &Self::Target {
&self.0
}
}

impl std::ops::Deref for Bar {
type Target = String;
fn deref(&self) -> &Self::Target {
&self.0
}
}

fn main() {
let x = Foo(42);
let y: i32 = x; //~ ERROR mismatched types
let a = &42;
let b: i32 = a; //~ ERROR mismatched types

// Do not make a suggestion when adding a `*` wouldn't actually fix the issue:
let f = Bar("bar".to_string());
let g: String = f; //~ ERROR mismatched types
}
39 changes: 39 additions & 0 deletions src/test/ui/suggestions/issue-59819.stderr
Original file line number Diff line number Diff line change
@@ -0,0 +1,39 @@
error[E0308]: mismatched types
--> $DIR/issue-59819.rs:28:18
|
LL | let y: i32 = x;
| ^
| |
| expected i32, found struct `Foo`
| help: consider dereferencing the type: `*x`
|
= note: expected type `i32`
found type `Foo`

error[E0308]: mismatched types
--> $DIR/issue-59819.rs:30:18
|
LL | let b: i32 = a;
| ^
| |
| expected i32, found &{integer}
| help: consider dereferencing the borrow: `*a`
|
= note: expected type `i32`
found type `&{integer}`

error[E0308]: mismatched types
--> $DIR/issue-59819.rs:34:21
|
LL | let g: String = f;
| ^
| |
| expected struct `std::string::String`, found struct `Bar`
| help: try using a conversion method: `f.to_string()`
|
= note: expected type `std::string::String`
found type `Bar`

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0308`.

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