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`

compiler/rustc_mir_build/src/build/custom/mod.rs

@@ -74,7 +74,7 @@ pub(super) fn build_custom_mir<'tcx>(
     let mut pctxt = ParseCtxt {
         tcx,
         thir,
-        source_info: SourceInfo { span, scope: OUTERMOST_SOURCE_SCOPE },
+        source_scope: OUTERMOST_SOURCE_SCOPE,
         body: &mut body,
         local_map: FxHashMap::default(),
         block_map: FxHashMap::default(),
@@ -128,7 +128,7 @@ fn parse_attribute(attr: &Attribute) -> MirPhase {
 struct ParseCtxt<'tcx, 'body> {
     tcx: TyCtxt<'tcx>,
     thir: &'body Thir<'tcx>,
-    source_info: SourceInfo,
+    source_scope: SourceScope,
 
     body: &'body mut Body<'tcx>,
     local_map: FxHashMap<LocalVarId, Local>,

compiler/rustc_mir_build/src/build/custom/parse.rs

@@ -23,6 +23,7 @@ macro_rules! parse_by_kind {
     (
         $self:ident,
         $expr_id:expr,
+        $expr_name:pat,
         $expected:literal,
         $(
             @call($name:literal, $args:ident) => $call_expr:expr,
@@ -33,6 +34,8 @@ macro_rules! parse_by_kind {
     ) => {{
         let expr_id = $self.preparse($expr_id);
         let expr = &$self.thir[expr_id];
+        debug!("Trying to parse {:?} as {}", expr.kind, $expected);
+        let $expr_name = expr;
         match &expr.kind {
             $(
                 ExprKind::Call { ty, fun: _, args: $args, .. } if {
@@ -137,26 +140,26 @@ impl<'tcx, 'body> ParseCtxt<'tcx, 'body> {
     /// This allows us to easily parse the basic blocks declarations, local declarations, and
     /// basic block definitions in order.
     pub fn parse_body(&mut self, expr_id: ExprId) -> PResult<()> {
-        let body = parse_by_kind!(self, expr_id, "whole body",
+        let body = parse_by_kind!(self, expr_id, _, "whole body",
             ExprKind::Block { block } => self.thir[*block].expr.unwrap(),
         );
-        let (block_decls, rest) = parse_by_kind!(self, body, "body with block decls",
+        let (block_decls, rest) = parse_by_kind!(self, body, _, "body with block decls",
             ExprKind::Block { block } => {
                 let block = &self.thir[*block];
                 (&block.stmts, block.expr.unwrap())
             },
         );
         self.parse_block_decls(block_decls.iter().copied())?;
 
-        let (local_decls, rest) = parse_by_kind!(self, rest, "body with local decls",
+        let (local_decls, rest) = parse_by_kind!(self, rest, _, "body with local decls",
             ExprKind::Block { block } => {
                 let block = &self.thir[*block];
                 (&block.stmts, block.expr.unwrap())
             },
         );
         self.parse_local_decls(local_decls.iter().copied())?;
 
-        let block_defs = parse_by_kind!(self, rest, "body with block defs",
+        let block_defs = parse_by_kind!(self, rest, _, "body with block defs",
             ExprKind::Block { block } => &self.thir[*block].stmts,
         );
         for (i, block_def) in block_defs.iter().enumerate() {
@@ -223,22 +226,30 @@ impl<'tcx, 'body> ParseCtxt<'tcx, 'body> {
     }
 
     fn parse_block_def(&self, expr_id: ExprId) -> PResult<BasicBlockData<'tcx>> {
-        let block = parse_by_kind!(self, expr_id, "basic block",
+        let block = parse_by_kind!(self, expr_id, _, "basic block",
             ExprKind::Block { block } => &self.thir[*block],
         );
 
         let mut data = BasicBlockData::new(None);
         for stmt_id in &*block.stmts {
             let stmt = self.statement_as_expr(*stmt_id)?;
+            let span = self.thir[stmt].span;
             let statement = self.parse_statement(stmt)?;
-            data.statements.push(Statement { source_info: self.source_info, kind: statement });
+            data.statements.push(Statement {
+                source_info: SourceInfo { span, scope: self.source_scope },
+                kind: statement,
+            });
         }
 
         let Some(trailing) = block.expr else {
             return Err(self.expr_error(expr_id, "terminator"))
         };
+        let span = self.thir[trailing].span;
         let terminator = self.parse_terminator(trailing)?;
-        data.terminator = Some(Terminator { source_info: self.source_info, kind: terminator });
+        data.terminator = Some(Terminator {
+            source_info: SourceInfo { span, scope: self.source_scope },
+            kind: terminator,
+        });
 
         Ok(data)
     }

compiler/rustc_mir_build/src/build/custom/parse/instruction.rs

@@ -1,10 +1,11 @@
+use rustc_middle::mir::interpret::{ConstValue, Scalar};
 use rustc_middle::{mir::*, thir::*, ty};
 
 use super::{parse_by_kind, PResult, ParseCtxt};
 
 impl<'tcx, 'body> ParseCtxt<'tcx, 'body> {
     pub fn parse_statement(&self, expr_id: ExprId) -> PResult<StatementKind<'tcx>> {
-        parse_by_kind!(self, expr_id, "statement",
+        parse_by_kind!(self, expr_id, _, "statement",
             @call("mir_retag", args) => {
                 Ok(StatementKind::Retag(RetagKind::Default, Box::new(self.parse_place(args[0])?)))
             },
@@ -20,7 +21,7 @@ impl<'tcx, 'body> ParseCtxt<'tcx, 'body> {
     }
 
     pub fn parse_terminator(&self, expr_id: ExprId) -> PResult<TerminatorKind<'tcx>> {
-        parse_by_kind!(self, expr_id, "terminator",
+        parse_by_kind!(self, expr_id, _, "terminator",
             @call("mir_return", _args) => {
                 Ok(TerminatorKind::Return)
             },
@@ -31,7 +32,7 @@ impl<'tcx, 'body> ParseCtxt<'tcx, 'body> {
     }
 
     fn parse_rvalue(&self, expr_id: ExprId) -> PResult<Rvalue<'tcx>> {
-        parse_by_kind!(self, expr_id, "rvalue",
+        parse_by_kind!(self, expr_id, _, "rvalue",
             ExprKind::Borrow { borrow_kind, arg } => Ok(
                 Rvalue::Ref(self.tcx.lifetimes.re_erased, *borrow_kind, self.parse_place(*arg)?)
             ),
@@ -43,14 +44,26 @@ impl<'tcx, 'body> ParseCtxt<'tcx, 'body> {
     }
 
     fn parse_operand(&self, expr_id: ExprId) -> PResult<Operand<'tcx>> {
-        parse_by_kind!(self, expr_id, "operand",
+        parse_by_kind!(self, expr_id, expr, "operand",
             @call("mir_move", args) => self.parse_place(args[0]).map(Operand::Move),
+            @call("mir_static", args) => self.parse_static(args[0]),
+            @call("mir_static_mut", args) => self.parse_static(args[0]),
+            ExprKind::Literal { .. }
+            | ExprKind::NamedConst { .. }
+            | ExprKind::NonHirLiteral { .. }
+            | ExprKind::ZstLiteral { .. }
+            | ExprKind::ConstParam { .. }
+            | ExprKind::ConstBlock { .. } => {
+                Ok(Operand::Constant(Box::new(
+                    crate::build::expr::as_constant::as_constant_inner(expr, |_| None, self.tcx)
+                )))
+            },
             _ => self.parse_place(expr_id).map(Operand::Copy),
         )
     }
 
     fn parse_place(&self, expr_id: ExprId) -> PResult<Place<'tcx>> {
-        parse_by_kind!(self, expr_id, "place",
+        parse_by_kind!(self, expr_id, _, "place",
             ExprKind::Deref { arg } => Ok(
                 self.parse_place(*arg)?.project_deeper(&[PlaceElem::Deref], self.tcx)
             ),
@@ -59,14 +72,34 @@ impl<'tcx, 'body> ParseCtxt<'tcx, 'body> {
     }
 
     fn parse_local(&self, expr_id: ExprId) -> PResult<Local> {
-        parse_by_kind!(self, expr_id, "local",
+        parse_by_kind!(self, expr_id, _, "local",
             ExprKind::VarRef { id } => Ok(self.local_map[id]),
         )
     }
 
     fn parse_block(&self, expr_id: ExprId) -> PResult<BasicBlock> {
-        parse_by_kind!(self, expr_id, "basic block",
+        parse_by_kind!(self, expr_id, _, "basic block",
             ExprKind::VarRef { id } => Ok(self.block_map[id]),
         )
     }
+
+    fn parse_static(&self, expr_id: ExprId) -> PResult<Operand<'tcx>> {
+        let expr_id = parse_by_kind!(self, expr_id, _, "static",
+            ExprKind::Deref { arg } => *arg,
+        );
+
+        parse_by_kind!(self, expr_id, expr, "static",
+            ExprKind::StaticRef { alloc_id, ty, .. } => {
+                let const_val =
+                    ConstValue::Scalar(Scalar::from_pointer((*alloc_id).into(), &self.tcx));
+                let literal = ConstantKind::Val(const_val, *ty);
+
+                Ok(Operand::Constant(Box::new(Constant {
+                    span: expr.span,
+                    user_ty: None,
+                    literal
+                })))
+            },
+        )
+    }
 }

compiler/rustc_mir_build/src/build/expr/as_constant.rs

@@ -8,7 +8,9 @@ use rustc_middle::mir::interpret::{
 };
 use rustc_middle::mir::*;
 use rustc_middle::thir::*;
-use rustc_middle::ty::{self, CanonicalUserTypeAnnotation, TyCtxt};
+use rustc_middle::ty::{
+    self, CanonicalUserType, CanonicalUserTypeAnnotation, TyCtxt, UserTypeAnnotationIndex,
+};
 use rustc_span::DUMMY_SP;
 use rustc_target::abi::Size;
 
@@ -19,84 +21,87 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         let this = self;
         let tcx = this.tcx;
         let Expr { ty, temp_lifetime: _, span, ref kind } = *expr;
-        match *kind {
+        match kind {
             ExprKind::Scope { region_scope: _, lint_level: _, value } => {
-                this.as_constant(&this.thir[value])
-            }
-            ExprKind::Literal { lit, neg } => {
-                let literal =
-                    match lit_to_mir_constant(tcx, LitToConstInput { lit: &lit.node, ty, neg }) {
-                        Ok(c) => c,
-                        Err(LitToConstError::Reported(guar)) => {
-                            ConstantKind::Ty(tcx.const_error_with_guaranteed(ty, guar))
-                        }
-                        Err(LitToConstError::TypeError) => {
-                            bug!("encountered type error in `lit_to_mir_constant")
-                        }
-                    };
-
-                Constant { span, user_ty: None, literal }
+                this.as_constant(&this.thir[*value])
             }
-            ExprKind::NonHirLiteral { lit, ref user_ty } => {
-                let user_ty = user_ty.as_ref().map(|user_ty| {
-                    this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
+            _ => as_constant_inner(
+                expr,
+                |user_ty| {
+                    Some(this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
                         span,
                         user_ty: user_ty.clone(),
                         inferred_ty: ty,
-                    })
-                });
-                let literal = ConstantKind::Val(ConstValue::Scalar(Scalar::Int(lit)), ty);
+                    }))
+                },
+                tcx,
+            ),
+        }
+    }
+}
 
-                Constant { span, user_ty: user_ty, literal }
-            }
-            ExprKind::ZstLiteral { ref user_ty } => {
-                let user_ty = user_ty.as_ref().map(|user_ty| {
-                    this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
-                        span,
-                        user_ty: user_ty.clone(),
-                        inferred_ty: ty,
-                    })
-                });
-                let literal = ConstantKind::Val(ConstValue::ZeroSized, ty);
+pub fn as_constant_inner<'tcx>(
+    expr: &Expr<'tcx>,
+    push_cuta: impl FnMut(&Box<CanonicalUserType<'tcx>>) -> Option<UserTypeAnnotationIndex>,
+    tcx: TyCtxt<'tcx>,
+) -> Constant<'tcx> {
+    let Expr { ty, temp_lifetime: _, span, ref kind } = *expr;
+    match *kind {
+        ExprKind::Literal { lit, neg } => {
+            let literal =
+                match lit_to_mir_constant(tcx, LitToConstInput { lit: &lit.node, ty, neg }) {
+                    Ok(c) => c,
+                    Err(LitToConstError::Reported(guar)) => {
+                        ConstantKind::Ty(tcx.const_error_with_guaranteed(ty, guar))
+                    }
+                    Err(LitToConstError::TypeError) => {
+                        bug!("encountered type error in `lit_to_mir_constant")
+                    }
+                };
 
-                Constant { span, user_ty: user_ty, literal }
-            }
-            ExprKind::NamedConst { def_id, substs, ref user_ty } => {
-                let user_ty = user_ty.as_ref().map(|user_ty| {
-                    this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
-                        span,
-                        user_ty: user_ty.clone(),
-                        inferred_ty: ty,
-                    })
-                });
+            Constant { span, user_ty: None, literal }
+        }
+        ExprKind::NonHirLiteral { lit, ref user_ty } => {
+            let user_ty = user_ty.as_ref().map(push_cuta).flatten();
 
-                let uneval =
-                    mir::UnevaluatedConst::new(ty::WithOptConstParam::unknown(def_id), substs);
-                let literal = ConstantKind::Unevaluated(uneval, ty);
+            let literal = ConstantKind::Val(ConstValue::Scalar(Scalar::Int(lit)), ty);
 
-                Constant { user_ty, span, literal }
-            }
-            ExprKind::ConstParam { param, def_id: _ } => {
-                let const_param = tcx.mk_const(param, expr.ty);
-                let literal = ConstantKind::Ty(const_param);
+            Constant { span, user_ty: user_ty, literal }
+        }
+        ExprKind::ZstLiteral { ref user_ty } => {
+            let user_ty = user_ty.as_ref().map(push_cuta).flatten();
 
-                Constant { user_ty: None, span, literal }
-            }
-            ExprKind::ConstBlock { did: def_id, substs } => {
-                let uneval =
-                    mir::UnevaluatedConst::new(ty::WithOptConstParam::unknown(def_id), substs);
-                let literal = ConstantKind::Unevaluated(uneval, ty);
+            let literal = ConstantKind::Val(ConstValue::ZeroSized, ty);
 
-                Constant { user_ty: None, span, literal }
-            }
-            ExprKind::StaticRef { alloc_id, ty, .. } => {
-                let const_val = ConstValue::Scalar(Scalar::from_pointer(alloc_id.into(), &tcx));
-                let literal = ConstantKind::Val(const_val, ty);
+            Constant { span, user_ty: user_ty, literal }
+        }
+        ExprKind::NamedConst { def_id, substs, ref user_ty } => {
+            let user_ty = user_ty.as_ref().map(push_cuta).flatten();
 
-                Constant { span, user_ty: None, literal }
-            }
-            _ => span_bug!(span, "expression is not a valid constant {:?}", kind),
+            let uneval = mir::UnevaluatedConst::new(ty::WithOptConstParam::unknown(def_id), substs);
+            let literal = ConstantKind::Unevaluated(uneval, ty);
+
+            Constant { user_ty, span, literal }
+        }
+        ExprKind::ConstParam { param, def_id: _ } => {
+            let const_param = tcx.mk_const(ty::ConstKind::Param(param), expr.ty);
+            let literal = ConstantKind::Ty(const_param);
+
+            Constant { user_ty: None, span, literal }
+        }
+        ExprKind::ConstBlock { did: def_id, substs } => {
+            let uneval = mir::UnevaluatedConst::new(ty::WithOptConstParam::unknown(def_id), substs);
+            let literal = ConstantKind::Unevaluated(uneval, ty);
+
+            Constant { user_ty: None, span, literal }
+        }
+        ExprKind::StaticRef { alloc_id, ty, .. } => {
+            let const_val = ConstValue::Scalar(Scalar::from_pointer(alloc_id.into(), &tcx));
+            let literal = ConstantKind::Val(const_val, ty);
+
+            Constant { span, user_ty: None, literal }
         }
+        _ => span_bug!(span, "expression is not a valid constant {:?}", kind),
     }
 }
 

compiler/rustc_mir_build/src/build/mod.rs

@@ -492,7 +492,7 @@ fn construct_fn<'tcx>(
             arguments,
             return_ty,
             return_ty_span,
-            span,
+            span_with_body,
             custom_mir_attr,
         );
     }