7 files changed, 307 insertions, 345 deletions

diff --git a/crates/hir_ty/src/tests/patterns.rs b/crates/hir_ty/src/tests/patterns.rs
index aa513c56d..5adbe9c45 100644
--- a/crates/hir_ty/src/tests/patterns.rs
+++ b/crates/hir_ty/src/tests/patterns.rs
@@ -571,48 +571,44 @@ fn main() {
 fn match_ergonomics_in_closure_params() {
     check_infer(
         r#"
-        #[lang = "fn_once"]
-        trait FnOnce<Args> {
-            type Output;
-        }
-
-        fn foo<T, U, F: FnOnce(T) -> U>(t: T, f: F) -> U { loop {} }
+//- minicore: fn
+fn foo<T, U, F: FnOnce(T) -> U>(t: T, f: F) -> U { loop {} }
 
-        fn test() {
-            foo(&(1, "a"), |&(x, y)| x); // normal, no match ergonomics
-            foo(&(1, "a"), |(x, y)| x);
-        }
-        "#,
+fn test() {
+    foo(&(1, "a"), |&(x, y)| x); // normal, no match ergonomics
+    foo(&(1, "a"), |(x, y)| x);
+}
+"#,
         expect![[r#"
-            93..94 't': T
-            99..100 'f': F
-            110..121 '{ loop {} }': U
-            112..119 'loop {}': !
-            117..119 '{}': ()
-            133..232 '{     ... x); }': ()
-            139..142 'foo': fn foo<&(i32, &str), i32, |&(i32, &str)| -> i32>(&(i32, &str), |&(i32, &str)| -> i32) -> i32
-            139..166 'foo(&(...y)| x)': i32
-            143..152 '&(1, "a")': &(i32, &str)
-            144..152 '(1, "a")': (i32, &str)
-            145..146 '1': i32
-            148..151 '"a"': &str
-            154..165 '|&(x, y)| x': |&(i32, &str)| -> i32
-            155..162 '&(x, y)': &(i32, &str)
-            156..162 '(x, y)': (i32, &str)
-            157..158 'x': i32
-            160..161 'y': &str
-            164..165 'x': i32
-            203..206 'foo': fn foo<&(i32, &str), &i32, |&(i32, &str)| -> &i32>(&(i32, &str), |&(i32, &str)| -> &i32) -> &i32
-            203..229 'foo(&(...y)| x)': &i32
-            207..216 '&(1, "a")': &(i32, &str)
-            208..216 '(1, "a")': (i32, &str)
-            209..210 '1': i32
-            212..215 '"a"': &str
-            218..228 '|(x, y)| x': |&(i32, &str)| -> &i32
-            219..225 '(x, y)': (i32, &str)
-            220..221 'x': &i32
-            223..224 'y': &&str
-            227..228 'x': &i32
+            32..33 't': T
+            38..39 'f': F
+            49..60 '{ loop {} }': U
+            51..58 'loop {}': !
+            56..58 '{}': ()
+            72..171 '{     ... x); }': ()
+            78..81 'foo': fn foo<&(i32, &str), i32, |&(i32, &str)| -> i32>(&(i32, &str), |&(i32, &str)| -> i32) -> i32
+            78..105 'foo(&(...y)| x)': i32
+            82..91 '&(1, "a")': &(i32, &str)
+            83..91 '(1, "a")': (i32, &str)
+            84..85 '1': i32
+            87..90 '"a"': &str
+            93..104 '|&(x, y)| x': |&(i32, &str)| -> i32
+            94..101 '&(x, y)': &(i32, &str)
+            95..101 '(x, y)': (i32, &str)
+            96..97 'x': i32
+            99..100 'y': &str
+            103..104 'x': i32
+            142..145 'foo': fn foo<&(i32, &str), &i32, |&(i32, &str)| -> &i32>(&(i32, &str), |&(i32, &str)| -> &i32) -> &i32
+            142..168 'foo(&(...y)| x)': &i32
+            146..155 '&(1, "a")': &(i32, &str)
+            147..155 '(1, "a")': (i32, &str)
+            148..149 '1': i32
+            151..154 '"a"': &str
+            157..167 '|(x, y)| x': |&(i32, &str)| -> &i32
+            158..164 '(x, y)': (i32, &str)
+            159..160 'x': &i32
+            162..163 'y': &&str
+            166..167 'x': &i32
         "#]],
     );
 }
diff --git a/crates/hir_ty/src/tests/regression.rs b/crates/hir_ty/src/tests/regression.rs
index abd9c385a..1edec1615 100644
--- a/crates/hir_ty/src/tests/regression.rs
+++ b/crates/hir_ty/src/tests/regression.rs
@@ -884,41 +884,37 @@ fn issue_4966() {
 fn issue_6628() {
     check_infer(
         r#"
-        #[lang = "fn_once"]
-        pub trait FnOnce<Args> {
-            type Output;
-        }
-
-        struct S<T>();
-        impl<T> S<T> {
-            fn f(&self, _t: T) {}
-            fn g<F: FnOnce(&T)>(&self, _f: F) {}
-        }
-        fn main() {
-            let s = S();
-            s.g(|_x| {});
-            s.f(10);
-        }
-        "#,
+//- minicore: fn
+struct S<T>();
+impl<T> S<T> {
+    fn f(&self, _t: T) {}
+    fn g<F: FnOnce(&T)>(&self, _f: F) {}
+}
+fn main() {
+    let s = S();
+    s.g(|_x| {});
+    s.f(10);
+}
+"#,
         expect![[r#"
-            105..109 'self': &S<T>
-            111..113 '_t': T
-            118..120 '{}': ()
-            146..150 'self': &S<T>
-            152..154 '_f': F
-            159..161 '{}': ()
-            174..225 '{     ...10); }': ()
-            184..185 's': S<i32>
-            188..189 'S': S<i32>() -> S<i32>
-            188..191 'S()': S<i32>
-            197..198 's': S<i32>
-            197..209 's.g(|_x| {})': ()
-            201..208 '|_x| {}': |&i32| -> ()
-            202..204 '_x': &i32
-            206..208 '{}': ()
-            215..216 's': S<i32>
-            215..222 's.f(10)': ()
-            219..221 '10': i32
+            40..44 'self': &S<T>
+            46..48 '_t': T
+            53..55 '{}': ()
+            81..85 'self': &S<T>
+            87..89 '_f': F
+            94..96 '{}': ()
+            109..160 '{     ...10); }': ()
+            119..120 's': S<i32>
+            123..124 'S': S<i32>() -> S<i32>
+            123..126 'S()': S<i32>
+            132..133 's': S<i32>
+            132..144 's.g(|_x| {})': ()
+            136..143 '|_x| {}': |&i32| -> ()
+            137..139 '_x': &i32
+            141..143 '{}': ()
+            150..151 's': S<i32>
+            150..157 's.f(10)': ()
+            154..156 '10': i32
         "#]],
     );
 }
diff --git a/crates/hir_ty/src/tests/traits.rs b/crates/hir_ty/src/tests/traits.rs
index 65fed02d2..065cca74f 100644
--- a/crates/hir_ty/src/tests/traits.rs
+++ b/crates/hir_ty/src/tests/traits.rs
@@ -1846,11 +1846,7 @@ fn test() {
 fn closure_1() {
     check_infer_with_mismatches(
         r#"
-#[lang = "fn_once"]
-trait FnOnce<Args> {
-    type Output;
-}
-
+//- minicore: fn
 enum Option<T> { Some(T), None }
 impl<T> Option<T> {
     fn map<U, F: FnOnce(T) -> U>(self, f: F) -> Option<U> { loop {} }
@@ -1863,34 +1859,34 @@ fn test() {
     let y: Option<i64> = x.map(|_v| 1);
 }"#,
         expect![[r#"
-            147..151 'self': Option<T>
-            153..154 'f': F
-            172..183 '{ loop {} }': Option<U>
-            174..181 'loop {}': !
-            179..181 '{}': ()
-            197..316 '{     ... 1); }': ()
-            207..208 'x': Option<u32>
-            211..223 'Option::Some': Some<u32>(u32) -> Option<u32>
-            211..229 'Option...(1u32)': Option<u32>
-            224..228 '1u32': u32
-            235..236 'x': Option<u32>
-            235..251 'x.map(...v + 1)': Option<u32>
-            241..250 '|v| v + 1': |u32| -> u32
-            242..243 'v': u32
-            245..246 'v': u32
-            245..250 'v + 1': u32
-            249..250 '1': u32
-            257..258 'x': Option<u32>
-            257..273 'x.map(... 1u64)': Option<u64>
-            263..272 '|_v| 1u64': |u32| -> u64
-            264..266 '_v': u32
-            268..272 '1u64': u64
-            283..284 'y': Option<i64>
-            300..301 'x': Option<u32>
-            300..313 'x.map(|_v| 1)': Option<i64>
-            306..312 '|_v| 1': |u32| -> i64
-            307..309 '_v': u32
-            311..312 '1': i64
+            86..90 'self': Option<T>
+            92..93 'f': F
+            111..122 '{ loop {} }': Option<U>
+            113..120 'loop {}': !
+            118..120 '{}': ()
+            136..255 '{     ... 1); }': ()
+            146..147 'x': Option<u32>
+            150..162 'Option::Some': Some<u32>(u32) -> Option<u32>
+            150..168 'Option...(1u32)': Option<u32>
+            163..167 '1u32': u32
+            174..175 'x': Option<u32>
+            174..190 'x.map(...v + 1)': Option<u32>
+            180..189 '|v| v + 1': |u32| -> u32
+            181..182 'v': u32
+            184..185 'v': u32
+            184..189 'v + 1': u32
+            188..189 '1': u32
+            196..197 'x': Option<u32>
+            196..212 'x.map(... 1u64)': Option<u64>
+            202..211 '|_v| 1u64': |u32| -> u64
+            203..205 '_v': u32
+            207..211 '1u64': u64
+            222..223 'y': Option<i64>
+            239..240 'x': Option<u32>
+            239..252 'x.map(|_v| 1)': Option<i64>
+            245..251 '|_v| 1': |u32| -> i64
+            246..248 '_v': u32
+            250..251 '1': i64
         "#]],
     );
 }
@@ -1964,11 +1960,7 @@ fn test<F: FnOnce(u32) -> u64>(f: F) {
 fn closure_as_argument_inference_order() {
     check_infer_with_mismatches(
         r#"
-#[lang = "fn_once"]
-trait FnOnce<Args> {
-    type Output;
-}
-
+//- minicore: fn
 fn foo1<T, U, F: FnOnce(T) -> U>(x: T, f: F) -> U { loop {} }
 fn foo2<T, U, F: FnOnce(T) -> U>(f: F, x: T) -> U { loop {} }
 
@@ -1987,62 +1979,62 @@ fn test() {
     let x4 = S.foo2(|s| s.method(), S);
 }"#,
         expect![[r#"
-            94..95 'x': T
-            100..101 'f': F
-            111..122 '{ loop {} }': U
-            113..120 'loop {}': !
-            118..120 '{}': ()
-            156..157 'f': F
-            162..163 'x': T
-            173..184 '{ loop {} }': U
-            175..182 'loop {}': !
-            180..182 '{}': ()
-            219..223 'self': S
-            271..275 'self': S
-            277..278 'x': T
-            283..284 'f': F
-            294..305 '{ loop {} }': U
-            296..303 'loop {}': !
-            301..303 '{}': ()
-            343..347 'self': S
-            349..350 'f': F
-            355..356 'x': T
-            366..377 '{ loop {} }': U
-            368..375 'loop {}': !
-            373..375 '{}': ()
-            391..550 '{     ... S); }': ()
-            401..403 'x1': u64
-            406..410 'foo1': fn foo1<S, u64, |S| -> u64>(S, |S| -> u64) -> u64
-            406..429 'foo1(S...hod())': u64
-            411..412 'S': S
-            414..428 '|s| s.method()': |S| -> u64
-            415..416 's': S
-            418..419 's': S
-            418..428 's.method()': u64
-            439..441 'x2': u64
-            444..448 'foo2': fn foo2<S, u64, |S| -> u64>(|S| -> u64, S) -> u64
-            444..467 'foo2(|...(), S)': u64
-            449..463 '|s| s.method()': |S| -> u64
-            450..451 's': S
-            453..454 's': S
-            453..463 's.method()': u64
-            465..466 'S': S
-            477..479 'x3': u64
-            482..483 'S': S
-            482..507 'S.foo1...hod())': u64
-            489..490 'S': S
-            492..506 '|s| s.method()': |S| -> u64
-            493..494 's': S
-            496..497 's': S
-            496..506 's.method()': u64
-            517..519 'x4': u64
-            522..523 'S': S
-            522..547 'S.foo2...(), S)': u64
-            529..543 '|s| s.method()': |S| -> u64
-            530..531 's': S
-            533..534 's': S
-            533..543 's.method()': u64
-            545..546 'S': S
+            33..34 'x': T
+            39..40 'f': F
+            50..61 '{ loop {} }': U
+            52..59 'loop {}': !
+            57..59 '{}': ()
+            95..96 'f': F
+            101..102 'x': T
+            112..123 '{ loop {} }': U
+            114..121 'loop {}': !
+            119..121 '{}': ()
+            158..162 'self': S
+            210..214 'self': S
+            216..217 'x': T
+            222..223 'f': F
+            233..244 '{ loop {} }': U
+            235..242 'loop {}': !
+            240..242 '{}': ()
+            282..286 'self': S
+            288..289 'f': F
+            294..295 'x': T
+            305..316 '{ loop {} }': U
+            307..314 'loop {}': !
+            312..314 '{}': ()
+            330..489 '{     ... S); }': ()
+            340..342 'x1': u64
+            345..349 'foo1': fn foo1<S, u64, |S| -> u64>(S, |S| -> u64) -> u64
+            345..368 'foo1(S...hod())': u64
+            350..351 'S': S
+            353..367 '|s| s.method()': |S| -> u64
+            354..355 's': S
+            357..358 's': S
+            357..367 's.method()': u64
+            378..380 'x2': u64
+            383..387 'foo2': fn foo2<S, u64, |S| -> u64>(|S| -> u64, S) -> u64
+            383..406 'foo2(|...(), S)': u64
+            388..402 '|s| s.method()': |S| -> u64
+            389..390 's': S
+            392..393 's': S
+            392..402 's.method()': u64
+            404..405 'S': S
+            416..418 'x3': u64
+            421..422 'S': S
+            421..446 'S.foo1...hod())': u64
+            428..429 'S': S
+            431..445 '|s| s.method()': |S| -> u64
+            432..433 's': S
+            435..436 's': S
+            435..445 's.method()': u64
+            456..458 'x4': u64
+            461..462 'S': S
+            461..486 'S.foo2...(), S)': u64
+            468..482 '|s| s.method()': |S| -> u64
+            469..470 's': S
+            472..473 's': S
+            472..482 's.method()': u64
+            484..485 'S': S
         "#]],
     );
 }
@@ -2051,11 +2043,7 @@ fn test() {
 fn fn_item_fn_trait() {
     check_types(
         r#"
-#[lang = "fn_once"]
-trait FnOnce<Args> {
-    type Output;
-}
-
+//- minicore: fn
 struct S;
 
 fn foo() -> S {}
@@ -2494,12 +2482,7 @@ fn test() -> impl Trait<i32> {
 fn assoc_types_from_bounds() {
     check_infer(
         r#"
-//- /main.rs
-#[lang = "fn_once"]
-trait FnOnce<Args> {
-    type Output;
-}
-
+//- minicore: fn
 trait T {
     type O;
 }
@@ -2518,15 +2501,15 @@ fn main() {
     f::<(), _>(|z| { z; });
 }"#,
         expect![[r#"
-            133..135 '_v': F
-            178..181 '{ }': ()
-            193..224 '{     ... }); }': ()
-            199..209 'f::<(), _>': fn f<(), |&()| -> ()>(|&()| -> ())
-            199..221 'f::<()... z; })': ()
-            210..220 '|z| { z; }': |&()| -> ()
-            211..212 'z': &()
-            214..220 '{ z; }': ()
-            216..217 'z': &()
+            72..74 '_v': F
+            117..120 '{ }': ()
+            132..163 '{     ... }); }': ()
+            138..148 'f::<(), _>': fn f<(), |&()| -> ()>(|&()| -> ())
+            138..160 'f::<()... z; })': ()
+            149..159 '|z| { z; }': |&()| -> ()
+            150..151 'z': &()
+            153..159 '{ z; }': ()
+            155..156 'z': &()
         "#]],
     );
 }
@@ -2599,17 +2582,7 @@ fn test() {
 fn iterator_chain() {
     check_infer_with_mismatches(
         r#"
-//- /main.rs
-#[lang = "fn_once"]
-trait FnOnce<Args> {
-    type Output;
-}
-#[lang = "fn_mut"]
-trait FnMut<Args>: FnOnce<Args> { }
-
-enum Option<T> { Some(T), None }
-use Option::*;
-
+//- minicore: fn, option
 pub trait Iterator {
     type Item;
 
@@ -2669,46 +2642,46 @@ fn main() {
     .for_each(|y| { y; });
 }"#,
         expect![[r#"
-            226..230 'self': Self
-            232..233 'f': F
-            317..328 '{ loop {} }': FilterMap<Self, F>
-            319..326 'loop {}': !
-            324..326 '{}': ()
-            349..353 'self': Self
-            355..356 'f': F
-            405..416 '{ loop {} }': ()
-            407..414 'loop {}': !
-            412..414 '{}': ()
-            525..529 'self': Self
-            854..858 'self': I
-            865..885 '{     ...     }': I
-            875..879 'self': I
-            944..955 '{ loop {} }': Vec<T>
-            946..953 'loop {}': !
-            951..953 '{}': ()
-            1142..1269 '{     ... }); }': ()
-            1148..1163 'Vec::<i32>::new': fn new<i32>() -> Vec<i32>
-            1148..1165 'Vec::<...:new()': Vec<i32>
-            1148..1177 'Vec::<...iter()': IntoIter<i32>
-            1148..1240 'Vec::<...one })': FilterMap<IntoIter<i32>, |i32| -> Option<u32>>
-            1148..1266 'Vec::<... y; })': ()
-            1194..1239 '|x| if...None }': |i32| -> Option<u32>
-            1195..1196 'x': i32
-            1198..1239 'if x >...None }': Option<u32>
-            1201..1202 'x': i32
-            1201..1206 'x > 0': bool
-            1205..1206 '0': i32
-            1207..1225 '{ Some...u32) }': Option<u32>
-            1209..1213 'Some': Some<u32>(u32) -> Option<u32>
-            1209..1223 'Some(x as u32)': Option<u32>
-            1214..1215 'x': i32
-            1214..1222 'x as u32': u32
-            1231..1239 '{ None }': Option<u32>
-            1233..1237 'None': Option<u32>
-            1255..1265 '|y| { y; }': |u32| -> ()
-            1256..1257 'y': u32
-            1259..1265 '{ y; }': ()
-            1261..1262 'y': u32
+            61..65 'self': Self
+            67..68 'f': F
+            152..163 '{ loop {} }': FilterMap<Self, F>
+            154..161 'loop {}': !
+            159..161 '{}': ()
+            184..188 'self': Self
+            190..191 'f': F
+            240..251 '{ loop {} }': ()
+            242..249 'loop {}': !
+            247..249 '{}': ()
+            360..364 'self': Self
+            689..693 'self': I
+            700..720 '{     ...     }': I
+            710..714 'self': I
+            779..790 '{ loop {} }': Vec<T>
+            781..788 'loop {}': !
+            786..788 '{}': ()
+            977..1104 '{     ... }); }': ()
+            983..998 'Vec::<i32>::new': fn new<i32>() -> Vec<i32>
+            983..1000 'Vec::<...:new()': Vec<i32>
+            983..1012 'Vec::<...iter()': IntoIter<i32>
+            983..1075 'Vec::<...one })': FilterMap<IntoIter<i32>, |i32| -> Option<u32>>
+            983..1101 'Vec::<... y; })': ()
+            1029..1074 '|x| if...None }': |i32| -> Option<u32>
+            1030..1031 'x': i32
+            1033..1074 'if x >...None }': Option<u32>
+            1036..1037 'x': i32
+            1036..1041 'x > 0': bool
+            1040..1041 '0': i32
+            1042..1060 '{ Some...u32) }': Option<u32>
+            1044..1048 'Some': Some<u32>(u32) -> Option<u32>
+            1044..1058 'Some(x as u32)': Option<u32>
+            1049..1050 'x': i32
+            1049..1057 'x as u32': u32
+            1066..1074 '{ None }': Option<u32>
+            1068..1072 'None': Option<u32>
+            1090..1100 '|y| { y; }': |u32| -> ()
+            1091..1092 'y': u32
+            1094..1100 '{ y; }': ()
+            1096..1097 'y': u32
         "#]],
     );
 }
@@ -2995,45 +2968,23 @@ fn foo() {
 fn infer_fn_trait_arg() {
     check_infer_with_mismatches(
         r#"
-        //- /lib.rs deps:std
-
-        #[lang = "fn_once"]
-        pub trait FnOnce<Args> {
-            type Output;
-
-            extern "rust-call" fn call_once(&self, args: Args) -> Self::Output;
-        }
-
-        #[lang = "fn"]
-        pub trait Fn<Args>:FnOnce<Args> {
-            extern "rust-call" fn call(&self, args: Args) -> Self::Output;
-        }
-
-        enum Option<T> {
-            None,
-            Some(T)
-        }
-
-        fn foo<F, T>(f: F) -> T
-        where
-            F: Fn(Option<i32>) -> T,
-        {
-            let s = None;
-            f(s)
-        }
-        "#,
+//- minicore: fn, option
+fn foo<F, T>(f: F) -> T
+where
+    F: Fn(Option<i32>) -> T,
+{
+    let s = None;
+    f(s)
+}
+"#,
         expect![[r#"
-            101..105 'self': &Self
-            107..111 'args': Args
-            220..224 'self': &Self
-            226..230 'args': Args
-            313..314 'f': F
-            359..389 '{     ...f(s) }': T
-            369..370 's': Option<i32>
-            373..377 'None': Option<i32>
-            383..384 'f': F
-            383..387 'f(s)': T
-            385..386 's': Option<i32>
+            13..14 'f': F
+            59..89 '{     ...f(s) }': T
+            69..70 's': Option<i32>
+            73..77 'None': Option<i32>
+            83..84 'f': F
+            83..87 'f(s)': T
+            85..86 's': Option<i32>
         "#]],
     );
 }
@@ -3112,17 +3063,7 @@ fn foo() {
 fn infer_dyn_fn_output() {
     check_types(
         r#"
-#[lang = "fn_once"]
-pub trait FnOnce<Args> {
-    type Output;
-    extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
-}
-
-#[lang = "fn"]
-pub trait Fn<Args>: FnOnce<Args> {
-    extern "rust-call" fn call(&self, args: Args) -> Self::Output;
-}
-
+//- minicore: fn
 fn foo() {
     let f: &dyn Fn() -> i32;
     f();
@@ -3135,12 +3076,7 @@ fn foo() {
 fn infer_dyn_fn_once_output() {
     check_types(
         r#"
-#[lang = "fn_once"]
-pub trait FnOnce<Args> {
-    type Output;
-    extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
-}
-
+//- minicore: fn
 fn foo() {
     let f: dyn FnOnce() -> i32;
     f();
@@ -3575,20 +3511,16 @@ fn main() {
 fn fn_returning_unit() {
     check_infer_with_mismatches(
         r#"
-#[lang = "fn_once"]
-trait FnOnce<Args> {
-    type Output;
-}
-
+//- minicore: fn
 fn test<F: FnOnce()>(f: F) {
     let _: () = f();
 }"#,
         expect![[r#"
-            82..83 'f': F
-            88..112 '{     ...f(); }': ()
-            98..99 '_': ()
-            106..107 'f': F
-            106..109 'f()': ()
+            21..22 'f': F
+            27..51 '{     ...f(); }': ()
+            37..38 '_': ()
+            45..46 'f': F
+            45..48 'f()': ()
         "#]],
     );
 }
diff --git a/crates/ide/src/hover.rs b/crates/ide/src/hover.rs
index 14cf94d60..529cf5f33 100644
--- a/crates/ide/src/hover.rs
+++ b/crates/ide/src/hover.rs
@@ -3016,8 +3016,8 @@ fn foo() {
                                     file_id: FileId(
                                         1,
                                     ),
-                                    full_range: 244..426,
-                                    focus_range: 283..289,
+                                    full_range: 245..427,
+                                    focus_range: 284..290,
                                     name: "Future",
                                     kind: Trait,
                                     description: "pub trait Future",
diff --git a/crates/ide_completion/src/render.rs b/crates/ide_completion/src/render.rs
index 3eb51e80b..fac83b650 100644
--- a/crates/ide_completion/src/render.rs
+++ b/crates/ide_completion/src/render.rs
@@ -1191,21 +1191,11 @@ fn main() {
     fn suggest_deref_mut() {
         check_relevance(
             r#"
-#[lang = "deref"]
-trait Deref {
-    type Target;
-    fn deref(&self) -> &Self::Target;
-}
-
-#[lang = "deref_mut"]
-pub trait DerefMut: Deref {
-    fn deref_mut(&mut self) -> &mut Self::Target;
-}
-
+//- minicore: deref_mut
 struct S;
 struct T(S);
 
-impl Deref for T {
+impl core::ops::Deref for T {
     type Target = S;
 
     fn deref(&self) -> &Self::Target {
@@ -1213,7 +1203,7 @@ impl Deref for T {
     }
 }
 
-impl DerefMut for T {
+impl core::ops::DerefMut for T {
     fn deref_mut(&mut self) -> &mut Self::Target {
         &mut self.0
     }
@@ -1232,12 +1222,12 @@ fn main() {
                 lc m [local]
                 lc t [local]
                 lc &mut t [type+local]
-                tt DerefMut []
-                tt Deref []
-                fn foo(…) []
                 st T []
                 st S []
                 fn main() []
+                fn foo(…) []
+                md core []
+                tt Sized []
             "#]],
         )
     }
diff --git a/crates/test_utils/src/fixture.rs b/crates/test_utils/src/fixture.rs
index 6ba112de8..005a5c092 100644
--- a/crates/test_utils/src/fixture.rs
+++ b/crates/test_utils/src/fixture.rs
@@ -129,8 +129,18 @@ impl Fixture {
             if line.starts_with("//-") {
                 let meta = Fixture::parse_meta_line(line);
                 res.push(meta)
-            } else if let Some(entry) = res.last_mut() {
-                entry.text.push_str(line);
+            } else {
+                if line.starts_with("// ")
+                    && line.contains(":")
+                    && !line.contains("::")
+                    && line.chars().all(|it| !it.is_uppercase())
+                {
+                    panic!("looks like invalid metadata line: {:?}", line)
+                }
+
+                if let Some(entry) = res.last_mut() {
+                    entry.text.push_str(line);
+                }
             }
         }
 
@@ -154,7 +164,9 @@ impl Fixture {
         let mut env = FxHashMap::default();
         let mut introduce_new_source_root = false;
         for component in components[1..].iter() {
-            let (key, value) = component.split_once(':').unwrap();
+            let (key, value) = component
+                .split_once(':')
+                .unwrap_or_else(|| panic!("invalid meta line: {:?}", meta));
             match key {
                 "crate" => krate = Some(value.to_string()),
                 "deps" => deps = value.split(',').map(|it| it.to_string()).collect(),
@@ -276,38 +288,44 @@ impl MiniCore {
             }
         }
 
-        let mut curr_region = "";
+        let mut active_regions = Vec::new();
         let mut seen_regions = Vec::new();
         for line in lines {
             let trimmed = line.trim();
             if let Some(region) = trimmed.strip_prefix("// region:") {
-                assert_eq!(curr_region, "");
-                curr_region = region;
+                active_regions.push(region);
                 continue;
             }
             if let Some(region) = trimmed.strip_prefix("// endregion:") {
-                assert_eq!(curr_region, region);
-                curr_region = "";
+                let prev = active_regions.pop().unwrap();
+                assert_eq!(prev, region);
                 continue;
             }
-            seen_regions.push(curr_region);
 
-            let mut flag = curr_region;
+            let mut line_region = false;
             if let Some(idx) = trimmed.find("// :") {
-                flag = &trimmed[idx + "// :".len()..];
+                line_region = true;
+                active_regions.push(&trimmed[idx + "// :".len()..]);
             }
 
-            let skip = if flag == "" {
-                false
-            } else {
-                assert!(!flag.starts_with(' '), "region marker starts with a space: {:?}", flag);
-                self.assert_valid_flag(flag);
-                !self.has_flag(flag)
-            };
+            let mut keep = true;
+            for &region in &active_regions {
+                assert!(
+                    !region.starts_with(' '),
+                    "region marker starts with a space: {:?}",
+                    region
+                );
+                self.assert_valid_flag(region);
+                seen_regions.push(region);
+                keep &= self.has_flag(region);
+            }
 
-            if !skip {
+            if keep {
                 buf.push_str(line)
             }
+            if line_region {
+                active_regions.pop().unwrap();
+            }
         }
 
         for flag in &self.valid_flags {
@@ -315,7 +333,7 @@ impl MiniCore {
                 panic!("unused minicore flag: {:?}", flag);
             }
         }
-
+        format!("{}", buf);
         buf
     }
 }
diff --git a/crates/test_utils/src/minicore.rs b/crates/test_utils/src/minicore.rs
index 5ff60178c..e04ca58d2 100644
--- a/crates/test_utils/src/minicore.rs
+++ b/crates/test_utils/src/minicore.rs
@@ -9,11 +9,13 @@
 //!
 //! Available flags:
 //!     sized:
+//!     unsize: sized
+//!     coerce_unsized: unsize
 //!     slice:
 //!     range:
-//!     unsize: sized
 //!     deref: sized
-//!     coerce_unsized: unsize
+//!     deref_mut: deref
+//!     fn:
 //!     pin:
 //!     future: pin
 //!     option:
@@ -64,9 +66,16 @@ pub mod ops {
             type Target: ?Sized;
             fn deref(&self) -> &Self::Target;
         }
+        // region:deref_mut
+        #[lang = "deref_mut"]
+        pub trait DerefMut: Deref {
+            fn deref_mut(&mut self) -> &mut Self::Target;
+        }
+        // endregion:deref_mut
     }
     pub use self::deref::Deref;
-    // endregion:deref
+    pub use self::deref::DerefMut; //:deref_mut
+                                   // endregion:deref
 
     // region:range
     mod range {
@@ -104,6 +113,26 @@ pub mod ops {
     pub use self::range::{Range, RangeFrom, RangeFull, RangeTo};
     pub use self::range::{RangeInclusive, RangeToInclusive};
     // endregion:range
+
+    // region:fn
+    mod function {
+        #[lang = "fn"]
+        #[fundamental]
+        pub trait Fn<Args>: FnMut<Args> {}
+
+        #[lang = "fn_mut"]
+        #[fundamental]
+        pub trait FnMut<Args>: FnOnce<Args> {}
+
+        #[lang = "fn_once"]
+        #[fundamental]
+        pub trait FnOnce<Args> {
+            #[lang = "fn_once_output"]
+            type Output;
+        }
+    }
+    pub use self::function::{Fn, FnMut, FnOnce};
+    // endregion:fn
 }
 
 // region:slice
@@ -181,6 +210,7 @@ pub mod prelude {
     pub mod v1 {
         pub use crate::{
             marker::Sized,                      // :sized
+            ops::{Fn, FnMut, FnOnce},           // :fn
             option::Option::{self, None, Some}, // :option
             result::Result::{self, Err, Ok},    // :result
         };