diff --git a/compiler/rustc_driver/src/pretty.rs b/compiler/rustc_driver/src/pretty.rs
index ae3ac8625b1..a7c040397a1 100644
--- a/compiler/rustc_driver/src/pretty.rs
+++ b/compiler/rustc_driver/src/pretty.rs
@@ -498,6 +498,21 @@ fn print_with_analysis(tcx: TyCtxt<'_>, ppm: PpMode) -> Result<(), ErrorGuarante
             out
         }
 
+        ThirFlat => {
+            let mut out = String::new();
+            abort_on_err(rustc_hir_analysis::check_crate(tcx), tcx.sess);
+            debug!("pretty printing THIR flat");
+            for did in tcx.hir().body_owners() {
+                let _ = writeln!(
+                    out,
+                    "{:?}:\n{}\n",
+                    did,
+                    tcx.thir_flat(ty::WithOptConstParam::unknown(did))
+                );
+            }
+            out
+        }
+
         _ => unreachable!(),
     };
 
diff --git a/compiler/rustc_middle/src/query/mod.rs b/compiler/rustc_middle/src/query/mod.rs
index b1e0d380d04..2543614318f 100644
--- a/compiler/rustc_middle/src/query/mod.rs
+++ b/compiler/rustc_middle/src/query/mod.rs
@@ -361,6 +361,13 @@ rustc_queries! {
         desc { |tcx| "constructing THIR tree for `{}`", tcx.def_path_str(key.did.to_def_id()) }
     }
 
+    /// Create a list-like THIR representation for debugging.
+    query thir_flat(key: ty::WithOptConstParam<LocalDefId>) -> String {
+        no_hash
+        arena_cache
+        desc { |tcx| "constructing flat THIR representation for `{}`", tcx.def_path_str(key.did.to_def_id()) }
+    }
+
     /// Set of all the `DefId`s in this crate that have MIR associated with
     /// them. This includes all the body owners, but also things like struct
     /// constructors.
diff --git a/compiler/rustc_mir_build/src/lib.rs b/compiler/rustc_mir_build/src/lib.rs
index a428180a4fa..94dae36154c 100644
--- a/compiler/rustc_mir_build/src/lib.rs
+++ b/compiler/rustc_mir_build/src/lib.rs
@@ -34,4 +34,5 @@ pub fn provide(providers: &mut Providers) {
     providers.thir_check_unsafety_for_const_arg = check_unsafety::thir_check_unsafety_for_const_arg;
     providers.thir_body = thir::cx::thir_body;
     providers.thir_tree = thir::cx::thir_tree;
+    providers.thir_flat = thir::cx::thir_flat;
 }
diff --git a/compiler/rustc_mir_build/src/thir/cx/mod.rs b/compiler/rustc_mir_build/src/thir/cx/mod.rs
index 52ecc67524b..10df4b22952 100644
--- a/compiler/rustc_mir_build/src/thir/cx/mod.rs
+++ b/compiler/rustc_mir_build/src/thir/cx/mod.rs
@@ -62,6 +62,13 @@ pub(crate) fn thir_tree(tcx: TyCtxt<'_>, owner_def: ty::WithOptConstParam<LocalD
     }
 }
 
+pub(crate) fn thir_flat(tcx: TyCtxt<'_>, owner_def: ty::WithOptConstParam<LocalDefId>) -> String {
+    match thir_body(tcx, owner_def) {
+        Ok((thir, _)) => format!("{:#?}", thir.steal()),
+        Err(_) => "error".into(),
+    }
+}
+
 struct Cx<'tcx> {
     tcx: TyCtxt<'tcx>,
     thir: Thir<'tcx>,
diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index 586454f7657..1abe5d24249 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -2577,6 +2577,7 @@ fn parse_pretty(unstable_opts: &UnstableOptions, efmt: ErrorOutputType) -> Optio
         "hir,typed" => Hir(PpHirMode::Typed),
         "hir-tree" => HirTree,
         "thir-tree" => ThirTree,
+        "thir-flat" => ThirFlat,
         "mir" => Mir,
         "mir-cfg" => MirCFG,
         name => early_error(
@@ -2585,7 +2586,8 @@ fn parse_pretty(unstable_opts: &UnstableOptions, efmt: ErrorOutputType) -> Optio
                 "argument to `unpretty` must be one of `normal`, `identified`, \
                             `expanded`, `expanded,identified`, `expanded,hygiene`, \
                             `ast-tree`, `ast-tree,expanded`, `hir`, `hir,identified`, \
-                            `hir,typed`, `hir-tree`, `thir-tree`, `mir` or `mir-cfg`; got {name}"
+                            `hir,typed`, `hir-tree`, `thir-tree`, `thir-flat`, `mir` or \
+                            `mir-cfg`; got {name}"
             ),
         ),
     };
@@ -2740,6 +2742,8 @@ pub enum PpMode {
     HirTree,
     /// `-Zunpretty=thir-tree`
     ThirTree,
+    /// `-Zunpretty=`thir-flat`
+    ThirFlat,
     /// `-Zunpretty=mir`
     Mir,
     /// `-Zunpretty=mir-cfg`
@@ -2758,6 +2762,7 @@ impl PpMode {
             | Hir(_)
             | HirTree
             | ThirTree
+            | ThirFlat
             | Mir
             | MirCFG => true,
         }
@@ -2767,13 +2772,13 @@ impl PpMode {
         match *self {
             Source(_) | AstTree(_) => false,
 
-            Hir(_) | HirTree | ThirTree | Mir | MirCFG => true,
+            Hir(_) | HirTree | ThirTree | ThirFlat | Mir | MirCFG => true,
         }
     }
 
     pub fn needs_analysis(&self) -> bool {
         use PpMode::*;
-        matches!(*self, Mir | MirCFG | ThirTree)
+        matches!(*self, Mir | MirCFG | ThirTree | ThirFlat)
     }
 }