resolve: fix error title regarding private constructors

[da-x]

One reason is that constructors can be private while their types can be
public.

Idea credit to @petrochenkov, discussed at #65153

[rust-highfive]

r? @varkor

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[da-x]

r? @petrochenkov

[petrochenkov]

ping @rust-lang/wg-diagnostics on whether the new wording looks better on average or not, I'm not sure.
(Ignoring "struct variant constructor", which is a clear regression #65250 (comment).)

Perhaps a flag parameter can be added to DefKind::descr to sometimes add "constructor" and sometimes not, and all uses of it will need to be audited, but maybe that's an overkill.

[rust-highfive]

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2019-10-09T20:38:08.1523022Z ##[command]git remote add origin https://github.com/rust-lang/rust
2019-10-09T20:38:08.1616667Z ##[command]git config gc.auto 0
2019-10-09T20:38:08.1697636Z ##[command]git config --get-all http.https://github.com/rust-lang/rust.extraheader
2019-10-09T20:38:08.1757983Z ##[command]git config --get-all http.proxy
2019-10-09T20:38:08.1907886Z ##[command]git -c http.extraheader="AUTHORIZATION: basic ***" fetch --force --tags --prune --progress --no-recurse-submodules --depth=2 origin +refs/heads/*:refs/remotes/origin/* +refs/pull/65250/merge:refs/remotes/pull/65250/merge
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2019-10-09T20:43:49.8575369Z    Compiling serde_json v1.0.40
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2019-10-09T20:44:05.8701926Z ##[error]Bash exited with code '1'.
2019-10-09T20:44:05.8745025Z ##[section]Starting: Checkout
2019-10-09T20:44:05.8746785Z ==============================================================================
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[estebank]

IMO, there are a handful of places where mentioning "constructor" would look good, namely when differentiating between the ADT itself and its constructor in the same message (type mismatch, "expected struct of type foo, found struct constructor foo", but those already look reasonable), but the great majority of these changes look worse/less understandable to me.

The only case I think needs fixing is

error[E0308]: mismatched types
 --> src/main.rs:4:18
  |
1 | struct Foo();
  | ------------- fn() -> Foo {Foo} defined here
...
4 |     let _: Foo = Foo;
  |                  ^^^
  |                  |
  |                  expected struct `Foo`, found fn item
  |                  help: use parentheses to instantiate this tuple struct: `Foo()`
  |
  = note: expected type `Foo`
             found type `fn() -> Foo {Foo}`

which should probably look closer to

error[E0308]: mismatched types
 --> src/main.rs:4:18
  |
1 | struct Foo();
  | ------------- `Foo` defined here
...
4 |     let _: Foo = Foo;
  |                  ^^^
  |                  |
  |                  expected struct `Foo`, found struct `Foo`'s constructor
  |                  help: use parentheses to instantiate this tuple struct: `Foo()`
  |
  = note: expected type `Foo`
             found constructor for type `Foo`

[petrochenkov]

So, I looked through the diff and it's indeed mostly a regression.
It's useful to mention "constructor" only the type and the constructor are opposed to each other.
The "x is private" is the only context where I found that to be the case.

So, I think, let's keep hir/def.rs alone and add "constructor" to the primary message wording only for the privacy errors (same code as in 48f8bed), and only if we are adding the "tuple struct constructor is private" label.

[da-x]

Fixed.

[petrochenkov]

Thanks!
@bors r+

[bors]

📌 Commit 9d11bda has been approved by petrochenkov

[petrochenkov]

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