Rollup merge of #135965 - estebank:shorten-ty-sugg, r=lcnr

In "specify type" suggestion, skip type params that are already known

When we suggest specifying a type for an expression or pattern, like in a `let` binding, we previously would print the entire type as the type system knew it. We now look at the params that have *no* inference variables, so they are fully known to the type system which means that they don't need to be specified.

This helps in suggestions for types that are really long, because we can usually skip most of the type params and make the annotation as short as possible:

```
error[E0282]: type annotations needed for `Result<_, ((..., ..., ..., ...), ..., ..., ...)>`
  --> $DIR/really-long-type-in-let-binding-without-sufficient-type-info.rs:7:9
   |
LL |     let y = Err(x);
   |         ^   ------ type must be known at this point
   |
help: consider giving `y` an explicit type, where the type for type parameter `T` is specified
   |
LL |     let y: Result<T, _> = Err(x);
   |          ++++++++++++++
```

Fix #135919.

6 files changed, 34 insertions, 10 deletions

diff --git a/tests/ui/inference/cannot-infer-closure-circular.stderr b/tests/ui/inference/cannot-infer-closure-circular.stderr
index a16e832f8ef..ee17f7737cf 100644
--- a/tests/ui/inference/cannot-infer-closure-circular.stderr
+++ b/tests/ui/inference/cannot-infer-closure-circular.stderr
@@ -9,8 +9,8 @@ LL |         Ok(v)
    |
 help: consider giving this closure parameter an explicit type, where the type for type parameter `E` is specified
    |
-LL |     let x = |r: Result<(), E>| {
-   |               +++++++++++++++
+LL |     let x = |r: Result<_, E>| {
+   |               ++++++++++++++
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/inference/erase-type-params-in-label.stderr b/tests/ui/inference/erase-type-params-in-label.stderr
index 4e9a74c1e40..1ec8a33eb82 100644
--- a/tests/ui/inference/erase-type-params-in-label.stderr
+++ b/tests/ui/inference/erase-type-params-in-label.stderr
@@ -12,8 +12,8 @@ LL | fn foo<T, K, W: Default, Z: Default>(t: T, k: K) -> Foo<T, K, W, Z> {
    |                 ^^^^^^^ required by this bound in `foo`
 help: consider giving `foo` an explicit type, where the type for type parameter `W` is specified
    |
-LL |     let foo: Foo<i32, &str, W, Z> = foo(1, "");
-   |            ++++++++++++++++++++++
+LL |     let foo: Foo<_, &_, W, Z> = foo(1, "");
+   |            ++++++++++++++++++
 
 error[E0283]: type annotations needed for `Bar<i32, &str, _>`
   --> $DIR/erase-type-params-in-label.rs:5:9
@@ -29,8 +29,8 @@ LL | fn bar<T, K, Z: Default>(t: T, k: K) -> Bar<T, K, Z> {
    |                 ^^^^^^^ required by this bound in `bar`
 help: consider giving `bar` an explicit type, where the type for type parameter `Z` is specified
    |
-LL |     let bar: Bar<i32, &str, Z> = bar(1, "");
-   |            +++++++++++++++++++
+LL |     let bar: Bar<_, &_, Z> = bar(1, "");
+   |            +++++++++++++++
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/inference/issue-104649.stderr b/tests/ui/inference/issue-104649.stderr
index 391ed16f349..27382e30134 100644
--- a/tests/ui/inference/issue-104649.stderr
+++ b/tests/ui/inference/issue-104649.stderr
@@ -6,8 +6,8 @@ LL |     let a = A(Result::Ok(Result::Ok(())));
    |
 help: consider giving `a` an explicit type, where the type for type parameter `E` is specified
    |
-LL |     let a: A<std::result::Result<std::result::Result<(), E>, Error>> = A(Result::Ok(Result::Ok(())));
-   |          +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+LL |     let a: A<std::result::Result<std::result::Result<_, E>, _>> = A(Result::Ok(Result::Ok(())));
+   |          ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/inference/issue-83606.stderr b/tests/ui/inference/issue-83606.stderr
index 69d1d71ef3c..97ccad9e785 100644
--- a/tests/ui/inference/issue-83606.stderr
+++ b/tests/ui/inference/issue-83606.stderr
@@ -11,8 +11,8 @@ LL | fn foo<const N: usize>(_: impl std::fmt::Display) -> [usize; N] {
    |        ^^^^^^^^^^^^^^ required by this const generic parameter in `foo`
 help: consider giving this pattern a type, where the value of const parameter `N` is specified
    |
-LL |     let _: [usize; N] = foo("foo");
-   |          ++++++++++++
+LL |     let _: [_; N] = foo("foo");
+   |          ++++++++
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/inference/really-long-type-in-let-binding-without-sufficient-type-info.rs b/tests/ui/inference/really-long-type-in-let-binding-without-sufficient-type-info.rs
new file mode 100644
index 00000000000..4fd15eea9e0
--- /dev/null
+++ b/tests/ui/inference/really-long-type-in-let-binding-without-sufficient-type-info.rs
@@ -0,0 +1,10 @@
+type A = (i32, i32, i32, i32);
+type B = (A, A, A, A);
+type C = (B, B, B, B);
+type D = (C, C, C, C);
+
+fn foo(x: D) {
+    let y = Err(x); //~ ERROR type annotations needed for `Result<_
+}
+
+fn main() {}
diff --git a/tests/ui/inference/really-long-type-in-let-binding-without-sufficient-type-info.stderr b/tests/ui/inference/really-long-type-in-let-binding-without-sufficient-type-info.stderr
new file mode 100644
index 00000000000..65fe2ffcb7f
--- /dev/null
+++ b/tests/ui/inference/really-long-type-in-let-binding-without-sufficient-type-info.stderr
@@ -0,0 +1,14 @@
+error[E0282]: type annotations needed for `Result<_, ((((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32))), (((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32))), (((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32))), (((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32))))>`
+  --> $DIR/really-long-type-in-let-binding-without-sufficient-type-info.rs:7:9
+   |
+LL |     let y = Err(x);
+   |         ^   ------ type must be known at this point
+   |
+help: consider giving `y` an explicit type, where the type for type parameter `T` is specified
+   |
+LL |     let y: Result<T, _> = Err(x);
+   |          ++++++++++++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0282`.