Build complete usable type from a type-relative prefix

Instead of looking for angle brackets in the source code, use the HIR
and Ty interfaces to either copy the original type, or complete it with
`_` placeholders if all type and const generic arguments are inferred.

3 files changed, 141 insertions, 1 deletions

diff --git a/tests/ui/from_iter_instead_of_collect.fixed b/tests/ui/from_iter_instead_of_collect.fixed
index 8618004efb8..be98b227795 100644
--- a/tests/ui/from_iter_instead_of_collect.fixed
+++ b/tests/ui/from_iter_instead_of_collect.fixed
@@ -73,3 +73,46 @@ fn main() {
     for _i in [1, 2, 3].iter().collect::<Vec<&i32>>() {}
     //~^ from_iter_instead_of_collect
 }
+
+fn issue14581() {
+    let nums = [0, 1, 2];
+    let _ = &nums.iter().map(|&num| char::from_u32(num).unwrap()).collect::<String>();
+    //~^ from_iter_instead_of_collect
+}
+
+fn test_implicit_generic_args(iter: impl Iterator<Item = &'static i32> + Copy) {
+    struct S<'l, T = i32, const A: usize = 3, const B: usize = 3> {
+        a: [&'l T; A],
+        b: [&'l T; B],
+    }
+
+    impl<'l, T, const A: usize, const B: usize> FromIterator<&'l T> for S<'l, T, A, B> {
+        fn from_iter<I: IntoIterator<Item = &'l T>>(_: I) -> Self {
+            todo!()
+        }
+    }
+
+    let _ = iter.collect::<S<'static, i32, 7>>();
+    //~^ from_iter_instead_of_collect
+
+    let _ = iter.collect::<S<'static, i32>>();
+    //~^ from_iter_instead_of_collect
+
+    let _ = iter.collect::<S<'static, _, 7>>();
+    //~^ from_iter_instead_of_collect
+
+    let _ = iter.collect::<S<'static, _, 7, 8>>();
+    //~^ from_iter_instead_of_collect
+
+    let _ = iter.collect::<S<_, 7, 8>>();
+    //~^ from_iter_instead_of_collect
+
+    let _ = iter.collect::<S<i32>>();
+    //~^ from_iter_instead_of_collect
+
+    let _ = iter.collect::<S<i32>>();
+    //~^ from_iter_instead_of_collect
+
+    let _ = iter.collect::<S>();
+    //~^ from_iter_instead_of_collect
+}
diff --git a/tests/ui/from_iter_instead_of_collect.rs b/tests/ui/from_iter_instead_of_collect.rs
index c46397e8ff5..ce20fef2ac3 100644
--- a/tests/ui/from_iter_instead_of_collect.rs
+++ b/tests/ui/from_iter_instead_of_collect.rs
@@ -73,3 +73,46 @@ fn main() {
     for _i in Vec::<&i32>::from_iter([1, 2, 3].iter()) {}
     //~^ from_iter_instead_of_collect
 }
+
+fn issue14581() {
+    let nums = [0, 1, 2];
+    let _ = &String::from_iter(nums.iter().map(|&num| char::from_u32(num).unwrap()));
+    //~^ from_iter_instead_of_collect
+}
+
+fn test_implicit_generic_args(iter: impl Iterator<Item = &'static i32> + Copy) {
+    struct S<'l, T = i32, const A: usize = 3, const B: usize = 3> {
+        a: [&'l T; A],
+        b: [&'l T; B],
+    }
+
+    impl<'l, T, const A: usize, const B: usize> FromIterator<&'l T> for S<'l, T, A, B> {
+        fn from_iter<I: IntoIterator<Item = &'l T>>(_: I) -> Self {
+            todo!()
+        }
+    }
+
+    let _ = <S<'static, i32, 7>>::from_iter(iter);
+    //~^ from_iter_instead_of_collect
+
+    let _ = <S<'static, i32>>::from_iter(iter);
+    //~^ from_iter_instead_of_collect
+
+    let _ = <S<'static, _, 7>>::from_iter(iter);
+    //~^ from_iter_instead_of_collect
+
+    let _ = <S<'static, _, 7, 8>>::from_iter(iter);
+    //~^ from_iter_instead_of_collect
+
+    let _ = <S<'_, _, 7, 8>>::from_iter(iter);
+    //~^ from_iter_instead_of_collect
+
+    let _ = <S<i32>>::from_iter(iter);
+    //~^ from_iter_instead_of_collect
+
+    let _ = <S<'_, i32>>::from_iter(iter);
+    //~^ from_iter_instead_of_collect
+
+    let _ = <S>::from_iter(iter);
+    //~^ from_iter_instead_of_collect
+}
diff --git a/tests/ui/from_iter_instead_of_collect.stderr b/tests/ui/from_iter_instead_of_collect.stderr
index b46d97af152..ec11a375c0d 100644
--- a/tests/ui/from_iter_instead_of_collect.stderr
+++ b/tests/ui/from_iter_instead_of_collect.stderr
@@ -91,5 +91,59 @@ error: usage of `FromIterator::from_iter`
 LL |     for _i in Vec::<&i32>::from_iter([1, 2, 3].iter()) {}
    |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `[1, 2, 3].iter().collect::<Vec<&i32>>()`
 
-error: aborting due to 15 previous errors
+error: usage of `FromIterator::from_iter`
+  --> tests/ui/from_iter_instead_of_collect.rs:79:14
+   |
+LL |     let _ = &String::from_iter(nums.iter().map(|&num| char::from_u32(num).unwrap()));
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `nums.iter().map(|&num| char::from_u32(num).unwrap()).collect::<String>()`
+
+error: usage of `FromIterator::from_iter`
+  --> tests/ui/from_iter_instead_of_collect.rs:95:13
+   |
+LL |     let _ = <S<'static, i32, 7>>::from_iter(iter);
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `iter.collect::<S<'static, i32, 7>>()`
+
+error: usage of `FromIterator::from_iter`
+  --> tests/ui/from_iter_instead_of_collect.rs:98:13
+   |
+LL |     let _ = <S<'static, i32>>::from_iter(iter);
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `iter.collect::<S<'static, i32>>()`
+
+error: usage of `FromIterator::from_iter`
+  --> tests/ui/from_iter_instead_of_collect.rs:101:13
+   |
+LL |     let _ = <S<'static, _, 7>>::from_iter(iter);
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `iter.collect::<S<'static, _, 7>>()`
+
+error: usage of `FromIterator::from_iter`
+  --> tests/ui/from_iter_instead_of_collect.rs:104:13
+   |
+LL |     let _ = <S<'static, _, 7, 8>>::from_iter(iter);
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `iter.collect::<S<'static, _, 7, 8>>()`
+
+error: usage of `FromIterator::from_iter`
+  --> tests/ui/from_iter_instead_of_collect.rs:107:13
+   |
+LL |     let _ = <S<'_, _, 7, 8>>::from_iter(iter);
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `iter.collect::<S<_, 7, 8>>()`
+
+error: usage of `FromIterator::from_iter`
+  --> tests/ui/from_iter_instead_of_collect.rs:110:13
+   |
+LL |     let _ = <S<i32>>::from_iter(iter);
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `iter.collect::<S<i32>>()`
+
+error: usage of `FromIterator::from_iter`
+  --> tests/ui/from_iter_instead_of_collect.rs:113:13
+   |
+LL |     let _ = <S<'_, i32>>::from_iter(iter);
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `iter.collect::<S<i32>>()`
+
+error: usage of `FromIterator::from_iter`
+  --> tests/ui/from_iter_instead_of_collect.rs:116:13
+   |
+LL |     let _ = <S>::from_iter(iter);
+   |             ^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `iter.collect::<S>()`
+
+error: aborting due to 24 previous errors