Rollup merge of #106172 - estebank:suggest-impl-trait, r=compiler-errors

Suggest `impl Iterator` when possible for `_` return type

Address #106096.

3 files changed, 32 insertions, 3 deletions

diff --git a/src/test/ui/suggestions/unnamable-types.stderr b/src/test/ui/suggestions/unnamable-types.stderr
index ede3ebfa739..24bedb5297b 100644
--- a/src/test/ui/suggestions/unnamable-types.stderr
+++ b/src/test/ui/suggestions/unnamable-types.stderr
@@ -19,7 +19,7 @@ error[E0121]: the placeholder `_` is not allowed within types on item signatures
 LL | const C: _ = || 42;
    |          ^ not allowed in type signatures
    |
-note: however, the inferred type `[closure@$DIR/unnamable-types.rs:17:14: 17:16]` cannot be named
+note: however, the inferred type `[closure@unnamable-types.rs:17:14]` cannot be named
   --> $DIR/unnamable-types.rs:17:14
    |
 LL | const C: _ = || 42;
@@ -31,7 +31,7 @@ error: missing type for `const` item
 LL | const D = S { t: { let i = 0; move || -> i32 { i } } };
    |        ^
    |
-note: however, the inferred type `S<[closure@$DIR/unnamable-types.rs:23:31: 23:45]>` cannot be named
+note: however, the inferred type `S<[closure@unnamable-types.rs:23:31]>` cannot be named
   --> $DIR/unnamable-types.rs:23:11
    |
 LL | const D = S { t: { let i = 0; move || -> i32 { i } } };
diff --git a/src/test/ui/typeck/typeck_type_placeholder_item.rs b/src/test/ui/typeck/typeck_type_placeholder_item.rs
index 22fedb22d66..b96c5271339 100644
--- a/src/test/ui/typeck/typeck_type_placeholder_item.rs
+++ b/src/test/ui/typeck/typeck_type_placeholder_item.rs
@@ -220,3 +220,11 @@ fn value() -> Option<&'static _> {
 
 const _: Option<_> = map(value);
 //~^ ERROR the placeholder `_` is not allowed within types on item signatures for constants
+
+fn evens_squared(n: usize) -> _ {
+//~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types
+    (1..n).filter(|x| x % 2 == 0).map(|x| x * x)
+}
+
+const _: _ = (1..10).filter(|x| x % 2 == 0).map(|x| x * x);
+//~^ ERROR the placeholder `_` is not allowed within types on item signatures for constants
diff --git a/src/test/ui/typeck/typeck_type_placeholder_item.stderr b/src/test/ui/typeck/typeck_type_placeholder_item.stderr
index c57f71b8057..bc02547c65e 100644
--- a/src/test/ui/typeck/typeck_type_placeholder_item.stderr
+++ b/src/test/ui/typeck/typeck_type_placeholder_item.stderr
@@ -428,6 +428,27 @@ LL | const _: Option<_> = map(value);
    |          not allowed in type signatures
    |          help: replace with the correct type: `Option<u8>`
 
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
+  --> $DIR/typeck_type_placeholder_item.rs:224:31
+   |
+LL | fn evens_squared(n: usize) -> _ {
+   |                               ^
+   |                               |
+   |                               not allowed in type signatures
+   |                               help: replace with an appropriate return type: `impl Iterator<Item = usize>`
+
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for constants
+  --> $DIR/typeck_type_placeholder_item.rs:229:10
+   |
+LL | const _: _ = (1..10).filter(|x| x % 2 == 0).map(|x| x * x);
+   |          ^ not allowed in type signatures
+   |
+note: however, the inferred type `Map<Filter<Range<i32>, [closure@typeck_type_placeholder_item.rs:229:29]>, [closure@typeck_type_placeholder_item.rs:229:49]>` cannot be named
+  --> $DIR/typeck_type_placeholder_item.rs:229:14
+   |
+LL | const _: _ = (1..10).filter(|x| x % 2 == 0).map(|x| x * x);
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
 error[E0121]: the placeholder `_` is not allowed within types on item signatures for functions
   --> $DIR/typeck_type_placeholder_item.rs:140:31
    |
@@ -636,7 +657,7 @@ LL |     const D: _ = 42;
    |              not allowed in type signatures
    |              help: replace with the correct type: `i32`
 
-error: aborting due to 69 previous errors
+error: aborting due to 71 previous errors
 
 Some errors have detailed explanations: E0121, E0282, E0403.
 For more information about an error, try `rustc --explain E0121`.