Add better turbofish extractor

4 files changed, 120 insertions, 23 deletions

diff --git a/clippy_lints/src/methods/mod.rs b/clippy_lints/src/methods/mod.rs
index 3e356afa2a4..d9f906619a0 100644
--- a/clippy_lints/src/methods/mod.rs
+++ b/clippy_lints/src/methods/mod.rs
@@ -4095,7 +4095,8 @@ fn lint_from_iter(cx: &LateContext<'_>, expr: &hir::Expr<'_>, args: &[hir::Expr<
         then {
             // `expr` implements `FromIterator` trait
             let iter_expr = sugg::Sugg::hir(cx, &args[0], "..").maybe_par();
-            let sugg = format!("{}.collect::<{}>()", iter_expr, ty);
+            let turbofish = extract_turbofish(cx, expr, ty);
+            let sugg = format!("{}.collect::<{}>()", iter_expr, turbofish);
             span_lint_and_sugg(
                 cx,
                 FROM_ITER_INSTEAD_OF_COLLECT,
@@ -4109,6 +4110,38 @@ fn lint_from_iter(cx: &LateContext<'_>, expr: &hir::Expr<'_>, args: &[hir::Expr<
     }
 }
 
+fn extract_turbofish(cx: &LateContext<'_>, expr: &hir::Expr<'_>, ty: Ty<'tcx>) -> String {
+    if_chain! {
+        let call_site = expr.span.source_callsite();
+        if let Ok(snippet) = cx.sess().source_map().span_to_snippet(call_site);
+        let snippet_split = snippet.split("::").collect::<Vec<_>>();
+        if let Some((_, elements)) = snippet_split.split_last();
+
+        then {
+            // is there a type specifier? (i.e.: like `<u32>` in `collections::BTreeSet::<u32>::`)
+            if let Some(type_specifier) = snippet_split.iter().find(|e| e.starts_with('<') && e.ends_with('>')) {
+                // remove the type specifier from the path elements
+                let without_ts = elements.iter().filter_map(|e| {
+                    if e == type_specifier { None } else { Some((*e).to_string()) }
+                }).collect::<Vec<_>>();
+                // join and add the type specifier at the end (i.e.: `collections::BTreeSet<u32>`)
+                format!("{}{}", without_ts.join("::"), type_specifier)
+            } else {
+                // type is not explicitly specified so wildcards are needed
+                // i.e.: 2 wildcards in `std::collections::BTreeMap<&i32, &char>`
+                let ty_str = ty.to_string();
+                let start = ty_str.find('<').unwrap_or(0);
+                let end = ty_str.find('>').unwrap_or_else(|| ty_str.len());
+                let nb_wildcard = ty_str[start..end].split(',').count();
+                let wildcards = format!("_{}", ", _".repeat(nb_wildcard - 1));
+                format!("{}<{}>", elements.join("::"), wildcards)
+            }
+        } else {
+            ty.to_string()
+        }
+    }
+}
+
 fn fn_header_equals(expected: hir::FnHeader, actual: hir::FnHeader) -> bool {
     expected.constness == actual.constness
         && expected.unsafety == actual.unsafety
diff --git a/tests/ui/from_iter_instead_of_collect.fixed b/tests/ui/from_iter_instead_of_collect.fixed
index 96701e86395..b5f548810e6 100644
--- a/tests/ui/from_iter_instead_of_collect.fixed
+++ b/tests/ui/from_iter_instead_of_collect.fixed
@@ -8,27 +8,41 @@ use std::iter::FromIterator;
 
 fn main() {
     let iter_expr = std::iter::repeat(5).take(5);
-    let _ = iter_expr.collect::<std::vec::Vec<i32>>();
+    let _ = iter_expr.collect::<Vec<_>>();
 
-    let _ = vec![5, 5, 5, 5].iter().enumerate().collect::<std::collections::HashMap<usize, &i8>>();
+    let _ = vec![5, 5, 5, 5].iter().enumerate().collect::<HashMap<usize, &i8>>();
 
     Vec::from_iter(vec![42u32]);
 
     let a = vec![0, 1, 2];
-    assert_eq!(a, (0..3).collect::<std::vec::Vec<i32>>());
+    assert_eq!(a, (0..3).collect::<Vec<_>>());
+    assert_eq!(a, (0..3).collect::<Vec<i32>>());
 
-    let mut b = (0..3).collect::<std::collections::VecDeque<i32>>();
+    let mut b = (0..3).collect::<VecDeque<_>>();
     b.push_back(4);
 
+    let mut b = (0..3).collect::<VecDeque<i32>>();
+    b.push_back(4);
+
+    {
+        use std::collections;
+        let mut b = (0..3).collect::<collections::VecDeque<i32>>();
+        b.push_back(4);
+    }
+
     let values = [(0, 'a'), (1, 'b'), (2, 'c'), (3, 'd')];
-    let bm = values.iter().cloned().collect::<std::collections::BTreeMap<i32, char>>();
-    let mut bar = bm.range(0..2).collect::<std::collections::BTreeMap<&i32, &char>>();
+    let bm = values.iter().cloned().collect::<BTreeMap<_, _>>();
+    let mut bar = bm.range(0..2).collect::<BTreeMap<_, _>>();
     bar.insert(&4, &'e');
 
-    let mut bts = (0..3).collect::<std::collections::BTreeSet<i32>>();
+    let mut bts = (0..3).collect::<BTreeSet<_>>();
     bts.insert(2);
     {
         use std::collections;
-        let _ = (0..3).collect::<std::collections::BTreeSet<i32>>();
+        let _ = (0..3).collect::<collections::BTreeSet<_>>();
+        let _ = (0..3).collect::<collections::BTreeSet<u32>>();
     }
+
+    for _i in [1, 2, 3].iter().collect::<Vec<_>>() {}
+    for _i in [1, 2, 3].iter().collect::<Vec<&i32>>() {}
 }
diff --git a/tests/ui/from_iter_instead_of_collect.rs b/tests/ui/from_iter_instead_of_collect.rs
index 211f57bc537..b842b5451d1 100644
--- a/tests/ui/from_iter_instead_of_collect.rs
+++ b/tests/ui/from_iter_instead_of_collect.rs
@@ -16,10 +16,20 @@ fn main() {
 
     let a = vec![0, 1, 2];
     assert_eq!(a, Vec::from_iter(0..3));
+    assert_eq!(a, Vec::<i32>::from_iter(0..3));
 
     let mut b = VecDeque::from_iter(0..3);
     b.push_back(4);
 
+    let mut b = VecDeque::<i32>::from_iter(0..3);
+    b.push_back(4);
+
+    {
+        use std::collections;
+        let mut b = collections::VecDeque::<i32>::from_iter(0..3);
+        b.push_back(4);
+    }
+
     let values = [(0, 'a'), (1, 'b'), (2, 'c'), (3, 'd')];
     let bm = BTreeMap::from_iter(values.iter().cloned());
     let mut bar = BTreeMap::from_iter(bm.range(0..2));
@@ -30,5 +40,9 @@ fn main() {
     {
         use std::collections;
         let _ = collections::BTreeSet::from_iter(0..3);
+        let _ = collections::BTreeSet::<u32>::from_iter(0..3);
     }
+
+    for _i in Vec::from_iter([1, 2, 3].iter()) {}
+    for _i in Vec::<&i32>::from_iter([1, 2, 3].iter()) {}
 }
diff --git a/tests/ui/from_iter_instead_of_collect.stderr b/tests/ui/from_iter_instead_of_collect.stderr
index 336e25a8adf..434734c9a21 100644
--- a/tests/ui/from_iter_instead_of_collect.stderr
+++ b/tests/ui/from_iter_instead_of_collect.stderr
@@ -2,7 +2,7 @@ error: usage of `FromIterator::from_iter`
   --> $DIR/from_iter_instead_of_collect.rs:11:13
    |
 LL |     let _ = Vec::from_iter(iter_expr);
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `iter_expr.collect::<std::vec::Vec<i32>>()`
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `iter_expr.collect::<Vec<_>>()`
    |
    = note: `-D clippy::from-iter-instead-of-collect` implied by `-D warnings`
 
@@ -10,43 +10,79 @@ error: usage of `FromIterator::from_iter`
   --> $DIR/from_iter_instead_of_collect.rs:13:13
    |
 LL |     let _ = HashMap::<usize, &i8>::from_iter(vec![5, 5, 5, 5].iter().enumerate());
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `vec![5, 5, 5, 5].iter().enumerate().collect::<std::collections::HashMap<usize, &i8>>()`
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `vec![5, 5, 5, 5].iter().enumerate().collect::<HashMap<usize, &i8>>()`
 
 error: usage of `FromIterator::from_iter`
   --> $DIR/from_iter_instead_of_collect.rs:18:19
    |
 LL |     assert_eq!(a, Vec::from_iter(0..3));
-   |                   ^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `(0..3).collect::<std::vec::Vec<i32>>()`
+   |                   ^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `(0..3).collect::<Vec<_>>()`
 
 error: usage of `FromIterator::from_iter`
-  --> $DIR/from_iter_instead_of_collect.rs:20:17
+  --> $DIR/from_iter_instead_of_collect.rs:19:19
+   |
+LL |     assert_eq!(a, Vec::<i32>::from_iter(0..3));
+   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `(0..3).collect::<Vec<i32>>()`
+
+error: usage of `FromIterator::from_iter`
+  --> $DIR/from_iter_instead_of_collect.rs:21:17
    |
 LL |     let mut b = VecDeque::from_iter(0..3);
-   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `(0..3).collect::<std::collections::VecDeque<i32>>()`
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `(0..3).collect::<VecDeque<_>>()`
+
+error: usage of `FromIterator::from_iter`
+  --> $DIR/from_iter_instead_of_collect.rs:24:17
+   |
+LL |     let mut b = VecDeque::<i32>::from_iter(0..3);
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `(0..3).collect::<VecDeque<i32>>()`
+
+error: usage of `FromIterator::from_iter`
+  --> $DIR/from_iter_instead_of_collect.rs:29:21
+   |
+LL |         let mut b = collections::VecDeque::<i32>::from_iter(0..3);
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `(0..3).collect::<collections::VecDeque<i32>>()`
 
 error: usage of `FromIterator::from_iter`
-  --> $DIR/from_iter_instead_of_collect.rs:24:14
+  --> $DIR/from_iter_instead_of_collect.rs:34:14
    |
 LL |     let bm = BTreeMap::from_iter(values.iter().cloned());
-   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `values.iter().cloned().collect::<std::collections::BTreeMap<i32, char>>()`
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `values.iter().cloned().collect::<BTreeMap<_, _>>()`
 
 error: usage of `FromIterator::from_iter`
-  --> $DIR/from_iter_instead_of_collect.rs:25:19
+  --> $DIR/from_iter_instead_of_collect.rs:35:19
    |
 LL |     let mut bar = BTreeMap::from_iter(bm.range(0..2));
-   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `bm.range(0..2).collect::<std::collections::BTreeMap<&i32, &char>>()`
+   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `bm.range(0..2).collect::<BTreeMap<_, _>>()`
 
 error: usage of `FromIterator::from_iter`
-  --> $DIR/from_iter_instead_of_collect.rs:28:19
+  --> $DIR/from_iter_instead_of_collect.rs:38:19
    |
 LL |     let mut bts = BTreeSet::from_iter(0..3);
-   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `(0..3).collect::<std::collections::BTreeSet<i32>>()`
+   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `(0..3).collect::<BTreeSet<_>>()`
 
 error: usage of `FromIterator::from_iter`
-  --> $DIR/from_iter_instead_of_collect.rs:32:17
+  --> $DIR/from_iter_instead_of_collect.rs:42:17
    |
 LL |         let _ = collections::BTreeSet::from_iter(0..3);
-   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `(0..3).collect::<std::collections::BTreeSet<i32>>()`
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `(0..3).collect::<collections::BTreeSet<_>>()`
+
+error: usage of `FromIterator::from_iter`
+  --> $DIR/from_iter_instead_of_collect.rs:43:17
+   |
+LL |         let _ = collections::BTreeSet::<u32>::from_iter(0..3);
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `(0..3).collect::<collections::BTreeSet<u32>>()`
+
+error: usage of `FromIterator::from_iter`
+  --> $DIR/from_iter_instead_of_collect.rs:46:15
+   |
+LL |     for _i in Vec::from_iter([1, 2, 3].iter()) {}
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `.collect()` instead of `::from_iter()`: `[1, 2, 3].iter().collect::<Vec<_>>()`
+
+error: usage of `FromIterator::from_iter`
+  --> $DIR/from_iter_instead_of_collect.rs:47:15
+   |
+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 8 previous errors
+error: aborting due to 14 previous errors