Auto merge of #104975 - JakobDegen:custom_mir_let, r=oli-obk

`#![custom_mir]`: Various improvements

This PR makes a bunch of improvements to `#![custom_mir]`. Ideally this would be 4 PRs, one for each commit, but those would take forever to get merged and be a pain to juggle. Should still be reviewed one commit at a time though.

### Commit 1: Support arbitrary `let`

Before this change, all locals used in the body need to be declared at the top of the `mir!` invocation, which is rather annoying. We attempt to change that.

Unfortunately, we still have the requirement that the output of the `mir!` macro must resolve, typecheck, etc. Because of that, we can't just accept this in the THIR -> MIR parser because something like
```rust
{
    let x = 0;
    Goto(other)
}
other = {
    RET = x;
    Return()
}
```
will fail to resolve. Instead, the implementation does macro shenanigans to find the let declarations and extract them as part of the `mir!` macro. That *works*, but it is fairly complicated and degrades debuginfo by quite a bit. Specifically, the spans for any statements and declarations that are affected by this are completely wrong. My guess is that this is a net improvement though.

One way to recover some of the debuginfo would be to not support type annotations in the `let` statements, which would allow us to parse like `let $stmt:stmt`. That seems quite surprising though.

### Commit 2: Parse consts

Reuses most of the const parsing from regular Mir building for building custom mir

### Commit 3: Parse statics

Statics are slightly weird because the Mir primitive associated with them is a reference/pointer to them, so this is factored out separately.

### Commit 4: Fix some spans

A bunch of the spans were non-ideal, so we adjust them to be much more helpful.

r? `@oli-obk`

9 files changed, 152 insertions, 17 deletions

diff --git a/src/test/mir-opt/building/custom/arbitrary_let.arbitrary_let.built.after.mir b/src/test/mir-opt/building/custom/arbitrary_let.arbitrary_let.built.after.mir
new file mode 100644
index 00000000000..20dd251e7e3
--- /dev/null
+++ b/src/test/mir-opt/building/custom/arbitrary_let.arbitrary_let.built.after.mir
@@ -0,0 +1,22 @@
+// MIR for `arbitrary_let` after built
+
+fn arbitrary_let(_1: i32) -> i32 {
+    let mut _0: i32;                     // return place in scope 0 at $DIR/arbitrary_let.rs:+0:29: +0:32
+    let mut _2: i32;                     // in scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
+    let mut _3: i32;                     // in scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
+
+    bb0: {
+        _2 = _1;                         // scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
+        goto -> bb2;                     // scope 0 at $DIR/arbitrary_let.rs:+4:13: +4:25
+    }
+
+    bb1: {
+        _0 = _3;                         // scope 0 at $DIR/arbitrary_let.rs:+7:13: +7:20
+        return;                          // scope 0 at $DIR/arbitrary_let.rs:+8:13: +8:21
+    }
+
+    bb2: {
+        _3 = _2;                         // scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
+        goto -> bb1;                     // scope 0 at $DIR/arbitrary_let.rs:+12:13: +12:24
+    }
+}
diff --git a/src/test/mir-opt/building/custom/arbitrary_let.rs b/src/test/mir-opt/building/custom/arbitrary_let.rs
new file mode 100644
index 00000000000..776df3151ff
--- /dev/null
+++ b/src/test/mir-opt/building/custom/arbitrary_let.rs
@@ -0,0 +1,28 @@
+#![feature(custom_mir, core_intrinsics)]
+
+extern crate core;
+use core::intrinsics::mir::*;
+use core::ptr::{addr_of, addr_of_mut};
+
+// EMIT_MIR arbitrary_let.arbitrary_let.built.after.mir
+#[custom_mir(dialect = "built")]
+fn arbitrary_let(x: i32) -> i32 {
+    mir!(
+        {
+            let y = x;
+            Goto(second)
+        }
+        third = {
+            RET = z;
+            Return()
+        }
+        second = {
+            let z = y;
+            Goto(third)
+        }
+    )
+}
+
+fn main() {
+    assert_eq!(arbitrary_let(5), 5);
+}
diff --git a/src/test/mir-opt/building/custom/consts.consts.built.after.mir b/src/test/mir-opt/building/custom/consts.consts.built.after.mir
new file mode 100644
index 00000000000..ba753cfc20c
--- /dev/null
+++ b/src/test/mir-opt/building/custom/consts.consts.built.after.mir
@@ -0,0 +1,22 @@
+// MIR for `consts` after built
+
+fn consts() -> () {
+    let mut _0: ();                      // return place in scope 0 at $DIR/consts.rs:+0:27: +0:27
+    let mut _1: u8;                      // in scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
+    let mut _2: i8;                      // in scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
+    let mut _3: u32;                     // in scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
+    let mut _4: i32;                     // in scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
+    let mut _5: fn() {consts::<10>};     // in scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
+
+    bb0: {
+        _1 = const 5_u8;                 // scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
+        _2 = const _;                    // scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
+        _3 = const C;                    // scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
+        _4 = const _;                    // scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
+        _5 = consts::<10>;               // scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
+                                         // mir::Constant
+                                         // + span: $DIR/consts.rs:16:18: 16:30
+                                         // + literal: Const { ty: fn() {consts::<10>}, val: Value(<ZST>) }
+        return;                          // scope 0 at $DIR/consts.rs:+7:9: +7:17
+    }
+}
diff --git a/src/test/mir-opt/building/custom/consts.rs b/src/test/mir-opt/building/custom/consts.rs
new file mode 100644
index 00000000000..ff4fe1a9324
--- /dev/null
+++ b/src/test/mir-opt/building/custom/consts.rs
@@ -0,0 +1,36 @@
+#![feature(custom_mir, core_intrinsics, inline_const)]
+
+extern crate core;
+use core::intrinsics::mir::*;
+
+const D: i32 = 5;
+
+// EMIT_MIR consts.consts.built.after.mir
+#[custom_mir(dialect = "built")]
+fn consts<const C: u32>() {
+    mir!({
+        let _a = 5_u8;
+        let _b = const { 5_i8 };
+        let _c = C;
+        let _d = D;
+        let _e = consts::<10>;
+        Return()
+    })
+}
+
+static S: i32 = 5;
+static mut T: i32 = 10;
+// EMIT_MIR consts.statics.built.after.mir
+#[custom_mir(dialect = "built")]
+fn statics() {
+    mir!({
+        let _a: &i32 = Static(S);
+        let _b: *mut i32 = StaticMut(T);
+        Return()
+    })
+}
+
+fn main() {
+    consts::<5>();
+    statics();
+}
diff --git a/src/test/mir-opt/building/custom/consts.statics.built.after.mir b/src/test/mir-opt/building/custom/consts.statics.built.after.mir
new file mode 100644
index 00000000000..ee768e263ec
--- /dev/null
+++ b/src/test/mir-opt/building/custom/consts.statics.built.after.mir
@@ -0,0 +1,27 @@
+// MIR for `statics` after built
+
+fn statics() -> () {
+    let mut _0: ();                      // return place in scope 0 at $DIR/consts.rs:+0:14: +0:14
+    let mut _1: &i32;                    // in scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
+    let mut _2: *mut i32;                // in scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
+
+    bb0: {
+        _1 = const {alloc1: &i32};       // scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
+                                         // mir::Constant
+                                         // + span: $DIR/consts.rs:27:31: 27:32
+                                         // + literal: Const { ty: &i32, val: Value(Scalar(alloc1)) }
+        _2 = const {alloc2: *mut i32};   // scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
+                                         // mir::Constant
+                                         // + span: $DIR/consts.rs:28:38: 28:39
+                                         // + literal: Const { ty: *mut i32, val: Value(Scalar(alloc2)) }
+        return;                          // scope 0 at $DIR/consts.rs:+4:9: +4:17
+    }
+}
+
+alloc2 (static: T, size: 4, align: 4) {
+    0a 00 00 00                                     │ ....
+}
+
+alloc1 (static: S, size: 4, align: 4) {
+    05 00 00 00                                     │ ....
+}
diff --git a/src/test/mir-opt/building/custom/references.immut_ref.built.after.mir b/src/test/mir-opt/building/custom/references.immut_ref.built.after.mir
index 4a5ddde4081..4d38d45c0f4 100644
--- a/src/test/mir-opt/building/custom/references.immut_ref.built.after.mir
+++ b/src/test/mir-opt/building/custom/references.immut_ref.built.after.mir
@@ -5,10 +5,10 @@ fn immut_ref(_1: &i32) -> &i32 {
     let mut _2: *const i32;              // in scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
 
     bb0: {
-        _2 = &raw const (*_1);           // scope 0 at $DIR/references.rs:+0:1: +0:34
-        Retag([raw] _2);                 // scope 0 at $DIR/references.rs:+0:1: +0:34
-        _0 = &(*_2);                     // scope 0 at $DIR/references.rs:+0:1: +0:34
-        Retag(_0);                       // scope 0 at $DIR/references.rs:+0:1: +0:34
-        return;                          // scope 0 at $DIR/references.rs:+0:1: +0:34
+        _2 = &raw const (*_1);           // scope 0 at $DIR/references.rs:+5:13: +5:29
+        Retag([raw] _2);                 // scope 0 at $DIR/references.rs:+6:13: +6:24
+        _0 = &(*_2);                     // scope 0 at $DIR/references.rs:+7:13: +7:23
+        Retag(_0);                       // scope 0 at $DIR/references.rs:+8:13: +8:23
+        return;                          // scope 0 at $DIR/references.rs:+9:13: +9:21
     }
 }
diff --git a/src/test/mir-opt/building/custom/references.mut_ref.built.after.mir b/src/test/mir-opt/building/custom/references.mut_ref.built.after.mir
index ec8509f69d1..01bc8a9cd35 100644
--- a/src/test/mir-opt/building/custom/references.mut_ref.built.after.mir
+++ b/src/test/mir-opt/building/custom/references.mut_ref.built.after.mir
@@ -5,10 +5,10 @@ fn mut_ref(_1: &mut i32) -> &mut i32 {
     let mut _2: *mut i32;                // in scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
 
     bb0: {
-        _2 = &raw mut (*_1);             // scope 0 at $DIR/references.rs:+0:1: +0:40
-        Retag([raw] _2);                 // scope 0 at $DIR/references.rs:+0:1: +0:40
-        _0 = &mut (*_2);                 // scope 0 at $DIR/references.rs:+0:1: +0:40
-        Retag(_0);                       // scope 0 at $DIR/references.rs:+0:1: +0:40
-        return;                          // scope 0 at $DIR/references.rs:+0:1: +0:40
+        _2 = &raw mut (*_1);             // scope 0 at $DIR/references.rs:+5:13: +5:33
+        Retag([raw] _2);                 // scope 0 at $DIR/references.rs:+6:13: +6:24
+        _0 = &mut (*_2);                 // scope 0 at $DIR/references.rs:+7:13: +7:26
+        Retag(_0);                       // scope 0 at $DIR/references.rs:+8:13: +8:23
+        return;                          // scope 0 at $DIR/references.rs:+9:13: +9:21
     }
 }
diff --git a/src/test/mir-opt/building/custom/simple_assign.simple.built.after.mir b/src/test/mir-opt/building/custom/simple_assign.simple.built.after.mir
index a5a2834c2e1..d7560fde69c 100644
--- a/src/test/mir-opt/building/custom/simple_assign.simple.built.after.mir
+++ b/src/test/mir-opt/building/custom/simple_assign.simple.built.after.mir
@@ -6,13 +6,13 @@ fn simple(_1: i32) -> i32 {
     let mut _3: i32;                     // in scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
 
     bb0: {
-        _2 = _1;                         // scope 0 at $DIR/simple_assign.rs:+0:1: +0:29
-        goto -> bb1;                     // scope 0 at $DIR/simple_assign.rs:+0:1: +0:29
+        _2 = _1;                         // scope 0 at $DIR/simple_assign.rs:+6:13: +6:22
+        goto -> bb1;                     // scope 0 at $DIR/simple_assign.rs:+7:13: +7:23
     }
 
     bb1: {
-        _3 = move _2;                    // scope 0 at $DIR/simple_assign.rs:+0:1: +0:29
-        _0 = _3;                         // scope 0 at $DIR/simple_assign.rs:+0:1: +0:29
-        return;                          // scope 0 at $DIR/simple_assign.rs:+0:1: +0:29
+        _3 = move _2;                    // scope 0 at $DIR/simple_assign.rs:+11:13: +11:32
+        _0 = _3;                         // scope 0 at $DIR/simple_assign.rs:+12:13: +12:24
+        return;                          // scope 0 at $DIR/simple_assign.rs:+13:13: +13:21
     }
 }
diff --git a/src/test/mir-opt/building/custom/simple_assign.simple_ref.built.after.mir b/src/test/mir-opt/building/custom/simple_assign.simple_ref.built.after.mir
index 6c90f0130a2..2b0e8f1047b 100644
--- a/src/test/mir-opt/building/custom/simple_assign.simple_ref.built.after.mir
+++ b/src/test/mir-opt/building/custom/simple_assign.simple_ref.built.after.mir
@@ -4,7 +4,7 @@ fn simple_ref(_1: &mut i32) -> &mut i32 {
     let mut _0: &mut i32;                // return place in scope 0 at $DIR/simple_assign.rs:+0:35: +0:43
 
     bb0: {
-        _0 = move _1;                    // scope 0 at $DIR/simple_assign.rs:+0:1: +0:43
-        return;                          // scope 0 at $DIR/simple_assign.rs:+0:1: +0:43
+        _0 = move _1;                    // scope 0 at $DIR/simple_assign.rs:+2:9: +2:22
+        return;                          // scope 0 at $DIR/simple_assign.rs:+3:9: +3:17
     }
 }