Auto merge of #13024 - jonas-schievink:hir-pretty, r=jonas-schievink

internal: Add an HIR pretty-printer

This improves the "View HIR" command by pretty-printing the HIR to make it much more readable.

Example function:

```rust
    fn newline(&mut self) {
        match self.buf.chars().rev().skip_while(|ch| *ch == ' ').next() {
            Some('\n') | None => {}
            _ => writeln!(self).unwrap(),
        }
    }
```

Previous output:

```
HIR expressions in the body of `newline`:
Idx::<Expr>(0): Path(Path { type_anchor: None, mod_path: ModPath { kind: Super(0), segments: [] }, generic_args: [] })
Idx::<Expr>(1): Field { expr: Idx::<Expr>(0), name: Name(Text("buf")) }
Idx::<Expr>(2): MethodCall { receiver: Idx::<Expr>(1), method_name: Name(Text("chars")), args: [], generic_args: None }
Idx::<Expr>(3): MethodCall { receiver: Idx::<Expr>(2), method_name: Name(Text("rev")), args: [], generic_args: None }
Idx::<Expr>(4): Path(Path { type_anchor: None, mod_path: ModPath { kind: Plain, segments: [Name(Text("ch"))] }, generic_args: [None] })
Idx::<Expr>(5): UnaryOp { expr: Idx::<Expr>(4), op: Deref }
Idx::<Expr>(6): Literal(Char(' '))
Idx::<Expr>(7): BinaryOp { lhs: Idx::<Expr>(5), rhs: Idx::<Expr>(6), op: Some(CmpOp(Eq { negated: false })) }
Idx::<Expr>(8): Closure { args: [Idx::<Pat>(1)], arg_types: [None], ret_type: None, body: Idx::<Expr>(7) }
Idx::<Expr>(9): MethodCall { receiver: Idx::<Expr>(3), method_name: Name(Text("skip_while")), args: [Idx::<Expr>(8)], generic_args: None }
Idx::<Expr>(10): MethodCall { receiver: Idx::<Expr>(9), method_name: Name(Text("next")), args: [], generic_args: None }
Idx::<Expr>(11): Literal(Char('\n'))
Idx::<Expr>(12): Block { id: BlockId(37), statements: [], tail: None, label: None }
Idx::<Expr>(13): Path(Path { type_anchor: None, mod_path: ModPath { kind: Super(0), segments: [] }, generic_args: [] })
Idx::<Expr>(14): Path(Path { type_anchor: None, mod_path: ModPath { kind: Plain, segments: [Name(Text("std")), Name(Text("fmt")), Name(Text("Arguments")), Name(Text("new_v1"))] }, generic_args: [None, None, None, None] })
Idx::<Expr>(15): Array(ElementList { elements: [], is_assignee_expr: false })
Idx::<Expr>(16): Ref { expr: Idx::<Expr>(15), rawness: Ref, mutability: Shared }
Idx::<Expr>(17): Array(ElementList { elements: [], is_assignee_expr: false })
Idx::<Expr>(18): Ref { expr: Idx::<Expr>(17), rawness: Ref, mutability: Shared }
Idx::<Expr>(19): Call { callee: Idx::<Expr>(14), args: [Idx::<Expr>(16), Idx::<Expr>(18)], is_assignee_expr: false }
Idx::<Expr>(20): MethodCall { receiver: Idx::<Expr>(13), method_name: Name(Text("write_fmt")), args: [Idx::<Expr>(19)], generic_args: None }
Idx::<Expr>(21): MacroStmts { statements: [], tail: Some(Idx::<Expr>(20)) }
Idx::<Expr>(22): MethodCall { receiver: Idx::<Expr>(21), method_name: Name(Text("unwrap")), args: [], generic_args: None }
Idx::<Expr>(23): Match { expr: Idx::<Expr>(10), arms: [MatchArm { pat: Idx::<Pat>(5), guard: None, expr: Idx::<Expr>(12) }, MatchArm { pat: Idx::<Pat>(6), guard: None, expr: Idx::<Expr>(22) }] }
Idx::<Expr>(24): Block { id: BlockId(36), statements: [], tail: Some(Idx::<Expr>(23)), label: None }
```

Output after this PR:

```rust
fn newline(…) {
    match self.buf.chars().rev().skip_while(
        |ch| (*ch) == (' '),
    ).next() {
        Some('\n') | None => {},
        _ => { // macro statements
            self.write_fmt(
                std::fmt::Arguments::new_v1(
                    &[],
                    &[],
                ),
            )
        }.unwrap(),
    }
}
```

It also works for consts and statics now.

This should make debugging HIR-lowering related issues like #12940 much easier.

crates/hir-def/src/body.rs

@@ -4,6 +4,7 @@ mod lower;
 #[cfg(test)]
 mod tests;
 pub mod scope;
+mod pretty;
 
 use std::{ops::Index, sync::Arc};
 
@@ -352,6 +353,10 @@ impl Body {
         }
     }
 
+    pub fn pretty_print(&self, db: &dyn DefDatabase, owner: DefWithBodyId) -> String {
+        pretty::print_body_hir(db, self, owner)
+    }
+
     fn new(
         db: &dyn DefDatabase,
         expander: Expander,