Merge #7741

7741: Add convert_for_to_iter_for_each assist r=mattyhall a=mattyhall

Implements one direction of #7681 

I wonder if this tries to guess too much at the right thing here. A common pattern is:

```rust
let col = vec![1, 2, 3];
for v in &mut col {
  *v *= 2;
}
// equivalent to:
col.iter_mut().for_each(|v| *v *= 2);
```

I've tried to detect this case by checking if the expression after the `in` is a (mutable) reference and if not inserting iter()/iter_mut(). This is just a convention used in the stdlib however, so could sometimes be wrong. I'd be happy to make an improvement for this, but not sure what would be best. A few options spring to mind:
1. Only allow this for types that are known to have iter/iter_mut (ie stdlib types)
2. Try to check if iter/iter_mut exists and they return the right iterator type
3. Don't try to do this and just add `.into_iter()` to whatever is after `in`

Co-authored-by: Matt Hall <matthew@quickbeam.me.uk>

crates/hir_expand/src/name.rs

@@ -189,6 +189,7 @@ pub mod known {
         // Components of known path (function name)
         filter_map,
         next,
+        iter_mut,
         // Builtin macros
         file,
         column,

crates/ide_assists/src/handlers/convert_for_to_iter_for_each.rs

@@ -0,0 +1,301 @@
+use ast::LoopBodyOwner;
+use hir::known;
+use ide_db::helpers::FamousDefs;
+use stdx::format_to;
+use syntax::{ast, AstNode};
+
+use crate::{AssistContext, AssistId, AssistKind, Assists};
+
+// Assist: convert_for_to_iter_for_each
+//
+// Converts a for loop into a for_each loop on the Iterator.
+//
+// ```
+// fn main() {
+//     let x = vec![1, 2, 3];
+//     for $0v in x {
+//         let y = v * 2;
+//     }
+// }
+// ```
+// ->
+// ```
+// fn main() {
+//     let x = vec![1, 2, 3];
+//     x.into_iter().for_each(|v| {
+//         let y = v * 2;
+//     });
+// }
+// ```
+pub(crate) fn convert_for_to_iter_for_each(acc: &mut Assists, ctx: &AssistContext) -> Option<()> {
+    let for_loop = ctx.find_node_at_offset::<ast::ForExpr>()?;
+    let iterable = for_loop.iterable()?;
+    let pat = for_loop.pat()?;
+    let body = for_loop.loop_body()?;
+
+    acc.add(
+        AssistId("convert_for_to_iter_for_each", AssistKind::RefactorRewrite),
+        "Convert a for loop into an Iterator::for_each",
+        for_loop.syntax().text_range(),
+        |builder| {
+            let mut buf = String::new();
+
+            if let Some((expr_behind_ref, method)) =
+                is_ref_and_impls_iter_method(&ctx.sema, &iterable)
+            {
+                // We have either "for x in &col" and col implements a method called iter
+                //             or "for x in &mut col" and col implements a method called iter_mut
+                format_to!(buf, "{}.{}()", expr_behind_ref, method);
+            } else if impls_core_iter(&ctx.sema, &iterable) {
+                format_to!(buf, "{}", iterable);
+            } else {
+                if let ast::Expr::RefExpr(_) = iterable {
+                    format_to!(buf, "({}).into_iter()", iterable);
+                } else {
+                    format_to!(buf, "{}.into_iter()", iterable);
+                }
+            }
+
+            format_to!(buf, ".for_each(|{}| {});", pat, body);
+
+            builder.replace(for_loop.syntax().text_range(), buf)
+        },
+    )
+}
+
+/// If iterable is a reference where the expression behind the reference implements a method
+/// returning an Iterator called iter or iter_mut (depending on the type of reference) then return
+/// the expression behind the reference and the method name
+fn is_ref_and_impls_iter_method(
+    sema: &hir::Semantics<ide_db::RootDatabase>,
+    iterable: &ast::Expr,
+) -> Option<(ast::Expr, hir::Name)> {
+    let ref_expr = match iterable {
+        ast::Expr::RefExpr(r) => r,
+        _ => return None,
+    };
+    let wanted_method = if ref_expr.mut_token().is_some() { known::iter_mut } else { known::iter };
+    let expr_behind_ref = ref_expr.expr()?;
+    let typ = sema.type_of_expr(&expr_behind_ref)?;
+    let scope = sema.scope(iterable.syntax());
+    let krate = scope.module()?.krate();
+    let traits_in_scope = scope.traits_in_scope();
+    let iter_trait = FamousDefs(sema, Some(krate)).core_iter_Iterator()?;
+    let has_wanted_method = typ.iterate_method_candidates(
+        sema.db,
+        krate,
+        &traits_in_scope,
+        Some(&wanted_method),
+        |_, func| {
+            if func.ret_type(sema.db).impls_trait(sema.db, iter_trait, &[]) {
+                return Some(());
+            }
+            None
+        },
+    );
+    has_wanted_method.and(Some((expr_behind_ref, wanted_method)))
+}
+
+/// Whether iterable implements core::Iterator
+fn impls_core_iter(sema: &hir::Semantics<ide_db::RootDatabase>, iterable: &ast::Expr) -> bool {
+    let it_typ = if let Some(i) = sema.type_of_expr(iterable) {
+        i
+    } else {
+        return false;
+    };
+    let module = if let Some(m) = sema.scope(iterable.syntax()).module() {
+        m
+    } else {
+        return false;
+    };
+    let krate = module.krate();
+    if let Some(iter_trait) = FamousDefs(sema, Some(krate)).core_iter_Iterator() {
+        return it_typ.impls_trait(sema.db, iter_trait, &[]);
+    }
+    false
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::tests::{check_assist, check_assist_not_applicable};
+
+    use super::*;
+
+    const EMPTY_ITER_FIXTURE: &'static str = r"
+//- /lib.rs deps:core crate:empty_iter
+pub struct EmptyIter;
+impl Iterator for EmptyIter {
+    type Item = usize;
+    fn next(&mut self) -> Option<Self::Item> { None }
+}
+
+pub struct Empty;
+impl Empty {
+    pub fn iter(&self) -> EmptyIter { EmptyIter }
+    pub fn iter_mut(&self) -> EmptyIter { EmptyIter }
+}
+
+pub struct NoIterMethod;
+";
+
+    fn check_assist_with_fixtures(before: &str, after: &str) {
+        let before = &format!(
+            "//- /main.rs crate:main deps:core,empty_iter{}{}{}",
+            before,
+            FamousDefs::FIXTURE,
+            EMPTY_ITER_FIXTURE
+        );
+        check_assist(convert_for_to_iter_for_each, before, after);
+    }
+
+    #[test]
+    fn test_not_for() {
+        check_assist_not_applicable(
+            convert_for_to_iter_for_each,
+            r"
+let mut x = vec![1, 2, 3];
+x.iter_mut().$0for_each(|v| *v *= 2);
+        ",
+        )
+    }
+
+    #[test]
+    fn test_simple_for() {
+        check_assist(
+            convert_for_to_iter_for_each,
+            r"
+fn main() {
+    let x = vec![1, 2, 3];
+    for $0v in x {
+        v *= 2;
+    }
+}",
+            r"
+fn main() {
+    let x = vec![1, 2, 3];
+    x.into_iter().for_each(|v| {
+        v *= 2;
+    });
+}",
+        )
+    }
+
+    #[test]
+    fn test_for_borrowed() {
+        check_assist_with_fixtures(
+            r"
+use empty_iter::*;
+fn main() {
+    let x = Empty;
+    for $0v in &x {
+        let a = v * 2;
+    }
+}
+",
+            r"
+use empty_iter::*;
+fn main() {
+    let x = Empty;
+    x.iter().for_each(|v| {
+        let a = v * 2;
+    });
+}
+",
+        )
+    }
+
+    #[test]
+    fn test_for_borrowed_no_iter_method() {
+        check_assist_with_fixtures(
+            r"
+use empty_iter::*;
+fn main() {
+    let x = NoIterMethod;
+    for $0v in &x {
+        let a = v * 2;
+    }
+}
+",
+            r"
+use empty_iter::*;
+fn main() {
+    let x = NoIterMethod;
+    (&x).into_iter().for_each(|v| {
+        let a = v * 2;
+    });
+}
+",
+        )
+    }
+
+    #[test]
+    fn test_for_borrowed_mut() {
+        check_assist_with_fixtures(
+            r"
+use empty_iter::*;
+fn main() {
+    let x = Empty;
+    for $0v in &mut x {
+        let a = v * 2;
+    }
+}
+",
+            r"
+use empty_iter::*;
+fn main() {
+    let x = Empty;
+    x.iter_mut().for_each(|v| {
+        let a = v * 2;
+    });
+}
+",
+        )
+    }
+
+    #[test]
+    fn test_for_borrowed_mut_behind_var() {
+        check_assist(
+            convert_for_to_iter_for_each,
+            r"
+fn main() {
+    let x = vec![1, 2, 3];
+    let y = &mut x;
+    for $0v in y {
+        *v *= 2;
+    }
+}",
+            r"
+fn main() {
+    let x = vec![1, 2, 3];
+    let y = &mut x;
+    y.into_iter().for_each(|v| {
+        *v *= 2;
+    });
+}",
+        )
+    }
+
+    #[test]
+    fn test_already_impls_iterator() {
+        check_assist_with_fixtures(
+            r#"
+use empty_iter::*;
+fn main() {
+    let x = Empty;
+    for$0 a in x.iter().take(1) {
+        println!("{}", a);
+    }
+}
+"#,
+            r#"
+use empty_iter::*;
+fn main() {
+    let x = Empty;
+    x.iter().take(1).for_each(|a| {
+        println!("{}", a);
+    });
+}
+"#,
+        );
+    }
+}

crates/ide_assists/src/lib.rs

@@ -114,6 +114,7 @@ mod handlers {
     mod apply_demorgan;
     mod auto_import;
     mod change_visibility;
+    mod convert_for_to_iter_for_each;
     mod convert_integer_literal;
     mod early_return;
     mod expand_glob_import;
@@ -175,6 +176,7 @@ mod handlers {
             apply_demorgan::apply_demorgan,
             auto_import::auto_import,
             change_visibility::change_visibility,
+            convert_for_to_iter_for_each::convert_for_to_iter_for_each,
             convert_integer_literal::convert_integer_literal,
             early_return::convert_to_guarded_return,
             expand_glob_import::expand_glob_import,

crates/ide_assists/src/tests/generated.rs

@@ -192,6 +192,29 @@ pub(crate) fn frobnicate() {}
     )
 }
 
+#[test]
+fn doctest_convert_for_to_iter_for_each() {
+    check_doc_test(
+        "convert_for_to_iter_for_each",
+        r#####"
+fn main() {
+    let x = vec![1, 2, 3];
+    for $0v in x {
+        let y = v * 2;
+    }
+}
+"#####,
+        r#####"
+fn main() {
+    let x = vec![1, 2, 3];
+    x.into_iter().for_each(|v| {
+        let y = v * 2;
+    });
+}
+"#####,
+    )
+}
+
 #[test]
 fn doctest_convert_integer_literal() {
     check_doc_test(