-Zprint-type-sizes: print the types of awaitees and unnamed coroutine locals.

This should assist comprehending the size of coroutines.
In particular, whenever a future is suspended while awaiting another
future, the latter is given the special name `__awaitee`, and now the
type of the awaited future will be printed, allowing identifying
caller/callee — er, I mean, poller/pollee — relationships.

It would be possible to include the type name in more cases, but I
thought that that might be overly verbose (`print-type-sizes` is already
a lot of text) and ordinary named fields or variables are easier for
readers to discover the types of.

1 files changed, 14 insertions, 4 deletions

diff --git a/compiler/rustc_session/src/code_stats.rs b/compiler/rustc_session/src/code_stats.rs
index 2553df33cc7..93839fa1186 100644
--- a/compiler/rustc_session/src/code_stats.rs
+++ b/compiler/rustc_session/src/code_stats.rs
@@ -44,6 +44,10 @@ pub struct FieldInfo {
     pub offset: u64,
     pub size: u64,
     pub align: u64,
+    /// Name of the type of this field.
+    /// Present only if the creator thought that this would be important for identifying the field,
+    /// typically because the field name is uninformative.
+    pub type_name: Option<Symbol>,
 }
 
 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
@@ -192,7 +196,7 @@ impl CodeStats {
                 fields.sort_by_key(|f| (f.offset, f.size));
 
                 for field in fields {
-                    let FieldInfo { kind, ref name, offset, size, align } = field;
+                    let FieldInfo { kind, ref name, offset, size, align, type_name } = field;
 
                     if offset > min_offset {
                         let pad = offset - min_offset;
@@ -201,21 +205,27 @@ impl CodeStats {
 
                     if offset < min_offset {
                         // If this happens it's probably a union.
-                        println!(
+                        print!(
                             "print-type-size {indent}{kind} `.{name}`: {size} bytes, \
                                   offset: {offset} bytes, \
                                   alignment: {align} bytes"
                         );
                     } else if info.packed || offset == min_offset {
-                        println!("print-type-size {indent}{kind} `.{name}`: {size} bytes");
+                        print!("print-type-size {indent}{kind} `.{name}`: {size} bytes");
                     } else {
                         // Include field alignment in output only if it caused padding injection
-                        println!(
+                        print!(
                             "print-type-size {indent}{kind} `.{name}`: {size} bytes, \
                                   alignment: {align} bytes"
                         );
                     }
 
+                    if let Some(type_name) = type_name {
+                        println!(", type: {type_name}");
+                    } else {
+                        println!();
+                    }
+
                     min_offset = offset + size;
                 }
             }