Reorganize, bless and add tests for const control flow

This creates a new test directory, `ui/consts/control-flow` to hold
tests related to control flow in a const context. It also blesses all
existing tests with the new error messages, and adds new tests for the
`const_if_match` feature.

src/test/ui/borrowck/issue-64453.stderr

@@ -1,11 +1,13 @@
-error[E0744]: `match` is not allowed in a `static`
+error[E0658]: `match` is not allowed in a `static`
   --> $DIR/issue-64453.rs:4:31
    |
 LL | static settings_dir: String = format!("");
    |                               ^^^^^^^^^^^
    |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/49146
+   = help: add `#![feature(const_if_match)]` to the crate attributes to enable
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0744`.
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/consts/const-eval/infinite_loop.stderr

@@ -9,11 +9,14 @@ LL | |
 LL | |         }
    | |_________^
 
-error[E0744]: `if` is not allowed in a `const`
+error[E0658]: `if` is not allowed in a `const`
   --> $DIR/infinite_loop.rs:9:17
    |
 LL |             n = if n % 2 == 0 { n/2 } else { 3*n + 1 };
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/49146
+   = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 warning: Constant evaluating a complex constant, this might take some time
   --> $DIR/infinite_loop.rs:4:18
@@ -36,5 +39,5 @@ LL |             n = if n % 2 == 0 { n/2 } else { 3*n + 1 };
 
 error: aborting due to 3 previous errors
 
-Some errors have detailed explanations: E0080, E0744.
+Some errors have detailed explanations: E0080, E0658, E0744.
 For more information about an error, try `rustc --explain E0080`.

src/test/ui/consts/const-eval/match-test-ptr-null.stderr

@@ -1,4 +1,4 @@
-error[E0744]: `match` is not allowed in a `const`
+error[E0658]: `match` is not allowed in a `const`
   --> $DIR/match-test-ptr-null.rs:6:9
    |
 LL | /         match &1 as *const i32 as usize {
@@ -9,6 +9,9 @@ LL | |             0 => 42,
 LL | |             n => n,
 LL | |         }
    | |_________^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/49146
+   = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 error[E0658]: casting pointers to integers in constants is unstable
   --> $DIR/match-test-ptr-null.rs:6:15
@@ -27,5 +30,5 @@ LL |         match &1 as *const i32 as usize {
 
 error: aborting due to 3 previous errors
 
-Some errors have detailed explanations: E0080, E0658, E0744.
+Some errors have detailed explanations: E0080, E0658.
 For more information about an error, try `rustc --explain E0080`.

src/test/ui/consts/const-match-pattern-arm.stderr

@@ -1,4 +1,4 @@
-error[E0744]: `match` is not allowed in a `const`
+error[E0658]: `match` is not allowed in a `const`
   --> $DIR/const-match-pattern-arm.rs:3:17
    |
 LL |   const x: bool = match Some(true) {
@@ -7,16 +7,22 @@ LL | |     Some(value) => true,
 LL | |     _ => false
 LL | | };
    | |_^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/49146
+   = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
-error[E0744]: `match` is not allowed in a `const`
+error[E0658]: `match` is not allowed in a `const`
   --> $DIR/const-match-pattern-arm.rs:9:5
    |
 LL | /     match Some(true) {
 LL | |         Some(value) => true,
 LL | |         _ => false
 LL | |     }
    | |_____^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/49146
+   = help: add `#![feature(const_if_match)]` to the crate attributes to enable
 
 error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0744`.
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/consts/control-flow/assert.both.stderr

@@ -0,0 +1,13 @@
+error: any use of this value will cause an error
+  --> $DIR/assert.rs:12:15
+   |
+LL | const _: () = assert!(false);
+   | --------------^^^^^^^^^^^^^^-
+   |               |
+   |               the evaluated program panicked at 'assertion failed: false', $DIR/assert.rs:12:15
+   |
+   = note: `#[deny(const_err)]` on by default
+   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
+
+error: aborting due to previous error
+

src/test/ui/consts/control-flow/assert.if_match.stderr

@@ -0,0 +1,23 @@
+error[E0658]: panicking in constants is unstable
+  --> $DIR/assert.rs:8:15
+   |
+LL | const _: () = assert!(true);
+   |               ^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/51999
+   = help: add `#![feature(const_panic)]` to the crate attributes to enable
+   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
+
+error[E0658]: panicking in constants is unstable
+  --> $DIR/assert.rs:12:15
+   |
+LL | const _: () = assert!(false);
+   |               ^^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/51999
+   = help: add `#![feature(const_panic)]` to the crate attributes to enable
+   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/consts/control-flow/assert.panic.stderr

@@ -0,0 +1,21 @@
+error[E0658]: `if` is not allowed in a `const`
+  --> $DIR/assert.rs:8:15
+   |
+LL | const _: () = assert!(true);
+   |               ^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/49146
+   = help: add `#![feature(const_if_match)]` to the crate attributes to enable
+
+error[E0658]: `if` is not allowed in a `const`
+  --> $DIR/assert.rs:12:15
+   |
+LL | const _: () = assert!(false);
+   |               ^^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/49146
+   = help: add `#![feature(const_if_match)]` to the crate attributes to enable
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/consts/control-flow/assert.rs

@@ -0,0 +1,17 @@
+// Test that `assert` works only when both `const_if_match` and `const_panic` are enabled.
+
+// revisions: stock if_match panic both
+
+#![cfg_attr(any(both, if_match), feature(const_if_match))]
+#![cfg_attr(any(both, panic), feature(const_panic))]
+
+const _: () = assert!(true);
+//[stock,panic]~^ ERROR `if` is not allowed in a `const`
+//[if_match]~^^ ERROR panicking in constants is unstable
+
+const _: () = assert!(false);
+//[stock,panic]~^ ERROR `if` is not allowed in a `const`
+//[if_match]~^^ ERROR panicking in constants is unstable
+//[both]~^^^ ERROR any use of this value will cause an error
+
+fn main() {}

src/test/ui/consts/control-flow/assert.stock.stderr

@@ -0,0 +1,21 @@
+error[E0658]: `if` is not allowed in a `const`
+  --> $DIR/assert.rs:8:15
+   |
+LL | const _: () = assert!(true);
+   |               ^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/49146
+   = help: add `#![feature(const_if_match)]` to the crate attributes to enable
+
+error[E0658]: `if` is not allowed in a `const`
+  --> $DIR/assert.rs:12:15
+   |
+LL | const _: () = assert!(false);
+   |               ^^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/49146
+   = help: add `#![feature(const_if_match)]` to the crate attributes to enable
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/consts/control-flow/basics.rs

@@ -0,0 +1,43 @@
+// Test basic functionality of `if` and `match` in a const context.
+
+// run-pass
+
+#![feature(const_panic)]
+#![feature(const_if_match)]
+
+const X: u32 = 4;
+const Y: u32 = 5;
+
+const ABS_DIFF: u32 = if X < Y {
+    Y - X
+} else {
+    X - Y
+};
+
+const fn abs_diff(a: u32, b: u32) -> u32 {
+    match (a, b) {
+        (big, little) if big > little => big - little,
+        (little, big) => big - little,
+    }
+}
+
+const fn gcd(a: u32, b: u32) -> u32 {
+    if b == 0 {
+        return a;
+    }
+
+    gcd(b, a % b)
+}
+
+fn main() {
+    const _: () = assert!(abs_diff(4, 5) == abs_diff(5, 4));
+    assert_eq!(abs_diff(4, 5), abs_diff(5, 4));
+
+    const _: () = assert!(ABS_DIFF == abs_diff(5, 4));
+    assert_eq!(ABS_DIFF, abs_diff(5, 4));
+
+    const _: () = assert!(gcd(48, 18) == 6);
+    const _: () = assert!(gcd(18, 48) == 6);
+    assert_eq!(gcd(48, 18), 6);
+    assert_eq!(gcd(18, 48), 6);
+}