`loop_match`: suggest extracting to a `const` item

if the expression cannot be evaluated in a straightforward way

2 files changed, 232 insertions, 0 deletions

diff --git a/tests/ui/loop-match/suggest-const-item.rs b/tests/ui/loop-match/suggest-const-item.rs
new file mode 100644
index 00000000000..f921b430b8c
--- /dev/null
+++ b/tests/ui/loop-match/suggest-const-item.rs
@@ -0,0 +1,174 @@
+#![allow(incomplete_features)]
+#![feature(loop_match)]
+#![feature(generic_const_items)]
+#![crate_type = "lib"]
+
+const fn const_fn() -> i32 {
+    1
+}
+
+#[unsafe(no_mangle)]
+fn suggest_const_block<const N: i32>() -> i32 {
+    let mut state = 0;
+    #[loop_match]
+    loop {
+        state = 'blk: {
+            match state {
+                0 => {
+                    #[const_continue]
+                    break 'blk const_fn();
+                    //~^ ERROR could not determine the target branch for this `#[const_continue]`
+                }
+                1 => {
+                    #[const_continue]
+                    break 'blk const { const_fn() };
+                    //~^ ERROR could not determine the target branch for this `#[const_continue]`
+                }
+                2 => {
+                    #[const_continue]
+                    break 'blk N;
+                    //~^ ERROR could not determine the target branch for this `#[const_continue]`
+                }
+                _ => {
+                    #[const_continue]
+                    break 'blk 1 + 1;
+                    //~^ ERROR could not determine the target branch for this `#[const_continue]`
+                }
+            }
+        }
+    }
+    state
+}
+
+struct S;
+
+impl S {
+    const M: usize = 42;
+
+    fn g() {
+        let mut state = 0;
+        #[loop_match]
+        loop {
+            state = 'blk: {
+                match state {
+                    0 => {
+                        #[const_continue]
+                        break 'blk Self::M;
+                    }
+                    _ => panic!(),
+                }
+            }
+        }
+    }
+}
+
+trait T {
+    const N: usize;
+
+    fn f() {
+        let mut state = 0;
+        #[loop_match]
+        loop {
+            state = 'blk: {
+                match state {
+                    0 => {
+                        #[const_continue]
+                        break 'blk Self::N;
+                        //~^ ERROR could not determine the target branch for this `#[const_continue]`
+                    }
+                    _ => panic!(),
+                }
+            }
+        }
+    }
+}
+
+impl T for S {
+    const N: usize = 1;
+}
+
+impl S {
+    fn h() {
+        let mut state = 0;
+        #[loop_match]
+        loop {
+            state = 'blk: {
+                match state {
+                    0 => {
+                        #[const_continue]
+                        break 'blk Self::N;
+                    }
+                    _ => panic!(),
+                }
+            }
+        }
+    }
+}
+
+trait T2<U> {
+    const L: u32;
+
+    fn p() {
+        let mut state = 0;
+        #[loop_match]
+        loop {
+            state = 'blk: {
+                match state {
+                    0 => {
+                        #[const_continue]
+                        break 'blk Self::L;
+                        //~^ ERROR could not determine the target branch for this `#[const_continue]`
+                    }
+                    _ => panic!(),
+                }
+            }
+        }
+    }
+}
+
+const SIZE_OF<T>: usize = size_of::<T>();
+
+fn q<T>() {
+    let mut state = 0;
+    #[loop_match]
+    loop {
+        state = 'blk: {
+            match state {
+                0 => {
+                    #[const_continue]
+                    break 'blk SIZE_OF::<T>;
+                    //~^ ERROR could not determine the target branch for this `#[const_continue]`
+                }
+                _ => panic!(),
+            }
+        }
+    }
+}
+
+trait Trait<T> {
+    const X: usize = 9000;
+    const Y: usize = size_of::<T>();
+}
+
+impl<T> Trait<T> for () {}
+
+fn r<T>() {
+    let mut state = 0;
+    #[loop_match]
+    loop {
+        state = 'blk: {
+            match state {
+                0 => {
+                    #[const_continue]
+                    break 'blk <() as Trait<T>>::X;
+                }
+                1 => {
+                    #[const_continue]
+                    break 'blk <() as Trait<T>>::Y;
+                    //~^ ERROR could not determine the target branch for this `#[const_continue]`
+                }
+                _ => panic!(),
+            }
+        }
+    }
+}
diff --git a/tests/ui/loop-match/suggest-const-item.stderr b/tests/ui/loop-match/suggest-const-item.stderr
new file mode 100644
index 00000000000..787474479ad
--- /dev/null
+++ b/tests/ui/loop-match/suggest-const-item.stderr
@@ -0,0 +1,58 @@
+error: could not determine the target branch for this `#[const_continue]`
+  --> $DIR/suggest-const-item.rs:19:32
+   |
+LL |                     break 'blk const_fn();
+   |                                ^^^^^^^^^^ this value must be a literal or a monomorphic const
+   |
+   = help: try extracting the expression into a `const` item
+
+error: could not determine the target branch for this `#[const_continue]`
+  --> $DIR/suggest-const-item.rs:24:32
+   |
+LL |                     break 'blk const { const_fn() };
+   |                                ^^^^^^^^^^^^^^^^^^^^ `const` blocks may use generics, and are not evaluated early enough
+   |
+   = help: try extracting the expression into a `const` item
+
+error: could not determine the target branch for this `#[const_continue]`
+  --> $DIR/suggest-const-item.rs:29:32
+   |
+LL |                     break 'blk N;
+   |                                ^ constant parameters may use generics, and are not evaluated early enough
+   |
+   = help: try extracting the expression into a `const` item
+
+error: could not determine the target branch for this `#[const_continue]`
+  --> $DIR/suggest-const-item.rs:34:32
+   |
+LL |                     break 'blk 1 + 1;
+   |                                ^^^^^ this value must be a literal or a monomorphic const
+   |
+   = help: try extracting the expression into a `const` item
+
+error: could not determine the target branch for this `#[const_continue]`
+  --> $DIR/suggest-const-item.rs:76:36
+   |
+LL |                         break 'blk Self::N;
+   |                                    ^^^^^^^ this value is too generic
+
+error: could not determine the target branch for this `#[const_continue]`
+  --> $DIR/suggest-const-item.rs:119:36
+   |
+LL |                         break 'blk Self::L;
+   |                                    ^^^^^^^ this value is too generic
+
+error: could not determine the target branch for this `#[const_continue]`
+  --> $DIR/suggest-const-item.rs:139:32
+   |
+LL |                     break 'blk SIZE_OF::<T>;
+   |                                ^^^^^^^^^^^^ this value is too generic
+
+error: could not determine the target branch for this `#[const_continue]`
+  --> $DIR/suggest-const-item.rs:167:32
+   |
+LL |                     break 'blk <() as Trait<T>>::Y;
+   |                                ^^^^^^^^^^^^^^^^^^^ this value is too generic
+
+error: aborting due to 8 previous errors
+