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Improve error msgs when found type is deref of expected

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This improves help messages in two cases:

- When expected type is `T` and found type is `&T`, we now look through blocks
  and suggest dereferencing the expression of the block, rather than the whole
  block.

- In the above case, if the expression is an `&`, we not suggest removing the
  `&` instead of adding `*`.

Both of these are demonstrated in the regression test. Before this patch the
first error in the test would be:

    error[E0308]: `if` and `else` have incompatible types
     --> test.rs:8:9
      |
    5 | /     if true {
    6 | |         a
      | |         - expected because of this
    7 | |     } else {
    8 | |         b
      | |         ^ expected `usize`, found `&usize`
    9 | |     };
      | |_____- `if` and `else` have incompatible types
      |
    help: consider dereferencing the borrow
      |
    7 |     } else *{
    8 |         b
    9 |     };
      |

Now:

    error[E0308]: `if` and `else` have incompatible types
     --> test.rs:8:9
      |
    5 | /     if true {
    6 | |         a
      | |         - expected because of this
    7 | |     } else {
    8 | |         b
      | |         ^
      | |         |
      | |         expected `usize`, found `&usize`
      | |         help: consider dereferencing the borrow: `*b`
    9 | |     };
      | |_____- `if` and `else` have incompatible types

The second error:

    error[E0308]: `if` and `else` have incompatible types
      --> test.rs:14:9
       |
    11 | /     if true {
    12 | |         1
       | |         - expected because of this
    13 | |     } else {
    14 | |         &1
       | |         ^^ expected integer, found `&{integer}`
    15 | |     };
       | |_____- `if` and `else` have incompatible types
       |
    help: consider dereferencing the borrow
       |
    13 |     } else *{
    14 |         &1
    15 |     };
       |

now:

    error[E0308]: `if` and `else` have incompatible types
      --> test.rs:14:9
       |
    11 | /     if true {
    12 | |         1
       | |         - expected because of this
    13 | |     } else {
    14 | |         &1
       | |         ^-
       | |         ||
       | |         |help: consider removing the `&`: `1`
       | |         expected integer, found `&{integer}`
    15 | |     };
       | |_____- `if` and `else` have incompatible types

Fixes #82361
This commit is contained in:
Ömer Sinan Ağacan 2021-02-21 13:29:43 +03:00
parent a4e595db8f
commit fa74d489a2
5 changed files with 128 additions and 4 deletions
Download patch file Download diff file Expand all files Collapse all files

28
compiler/rustc_typeck/src/check/demand.rs
View file

@ -616,10 +616,30 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
}
_ if sp == expr.span && !is_macro => {
if let Some(steps) = self.deref_steps(checked_ty, expected) {
let expr = expr.peel_blocks();
if steps == 1 {
// For a suggestion to make sense, the type would need to be `Copy`.
if self.infcx.type_is_copy_modulo_regions(self.param_env, expected, sp) {
if let Ok(code) = sm.span_to_snippet(sp) {
if let hir::ExprKind::AddrOf(_, mutbl, inner) = expr.kind {
// If the expression has `&`, removing it would fix the error
let prefix_span = expr.span.with_hi(inner.span.lo());
let message = match mutbl {
hir::Mutability::Not => "consider removing the `&`",
hir::Mutability::Mut => "consider removing the `&mut`",
};
let suggestion = String::new();
return Some((
prefix_span,
message,
suggestion,
Applicability::MachineApplicable,
));
} else if self.infcx.type_is_copy_modulo_regions(
self.param_env,
expected,
sp,
) {
// For this suggestion to make sense, the type would need to be `Copy`.
if let Ok(code) = sm.span_to_snippet(expr.span) {
let message = if checked_ty.is_region_ptr() {
"consider dereferencing the borrow"
} else {
@ -631,7 +651,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
format!("*{}", code)
};
return Some((
sp,
expr.span,
message,
suggestion,
Applicability::MachineApplicable,