diff --git a/src/librustc/arena.rs b/src/librustc/arena.rs
index 5a5919d786638..70ac6b91b2ba2 100644
--- a/src/librustc/arena.rs
+++ b/src/librustc/arena.rs
@@ -23,9 +23,9 @@ macro_rules! arena_types {
             [] generics: rustc::ty::Generics,
             [] trait_def: rustc::ty::TraitDef,
             [] adt_def: rustc::ty::AdtDef,
-            [] steal_mir: rustc::ty::steal::Steal<rustc::mir::Body<$tcx>>,
+            [] steal_mir: rustc_data_structures::steal::Steal<rustc::mir::Body<$tcx>>,
             [] mir: rustc::mir::Body<$tcx>,
-            [] steal_promoted: rustc::ty::steal::Steal<
+            [] steal_promoted: rustc_data_structures::steal::Steal<
                 rustc_index::vec::IndexVec<
                     rustc::mir::Promoted,
                     rustc::mir::Body<$tcx>
diff --git a/src/librustc/hir/lowering.rs b/src/librustc/hir/lowering.rs
index 2238a56b29d04..7c362657b8612 100644
--- a/src/librustc/hir/lowering.rs
+++ b/src/librustc/hir/lowering.rs
@@ -2079,6 +2079,7 @@ impl<'a> LoweringContext<'a> {
             span: l.span,
             attrs: l.attrs.clone(),
             source: hir::LocalSource::Normal,
+            interp_tag: l.interp_tag.clone(),
         }, ids)
     }
 
@@ -3048,6 +3049,7 @@ impl<'a> LoweringContext<'a> {
             source,
             span,
             ty: None,
+            interp_tag: None,
         };
         self.stmt(span, hir::StmtKind::Local(P(local)))
     }
diff --git a/src/librustc/hir/mod.rs b/src/librustc/hir/mod.rs
index 9ae661f0952a4..bcb7ee11aab97 100644
--- a/src/librustc/hir/mod.rs
+++ b/src/librustc/hir/mod.rs
@@ -1273,6 +1273,8 @@ pub struct Local {
     /// Can be `ForLoopDesugar` if the `let` statement is part of a `for` loop
     /// desugaring. Otherwise will be `Normal`.
     pub source: LocalSource,
+    /// See comment on `syntax::ast::Local`.
+    pub interp_tag: Option<ast::LocalInterpTag>,
 }
 
 /// Represents a single arm of a `match` expression, e.g.
diff --git a/src/librustc/ich/impls_hir.rs b/src/librustc/ich/impls_hir.rs
index c0255e5b8a481..82b5532bd62b8 100644
--- a/src/librustc/ich/impls_hir.rs
+++ b/src/librustc/ich/impls_hir.rs
@@ -392,3 +392,14 @@ impl<'hir> HashStable<StableHashingContext<'hir>> for attr::OptimizeAttr {
         mem::discriminant(self).hash_stable(hcx, hasher);
     }
 }
+
+impl_stable_hash_for!(enum ast::LocalInterpState {
+    Uninitialized,
+    Set,
+    Moved,
+});
+
+impl_stable_hash_for!(struct ast::LocalInterpTag {
+    id,
+    state,
+});
diff --git a/src/librustc/ich/impls_ty.rs b/src/librustc/ich/impls_ty.rs
index c643baf11254c..b562d96bed786 100644
--- a/src/librustc/ich/impls_ty.rs
+++ b/src/librustc/ich/impls_ty.rs
@@ -1,14 +1,15 @@
 //! This module contains `HashStable` implementations for various data types
-//! from rustc::ty in no particular order.
+//! from `rustc::ty` in no particular order.
 
 use crate::ich::{Fingerprint, StableHashingContext, NodeIdHashingMode};
+use crate::middle::region;
+use crate::mir;
+use crate::ty;
+
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::stable_hasher::{HashStable, ToStableHashKey, StableHasher};
 use std::cell::RefCell;
 use std::mem;
-use crate::middle::region;
-use crate::ty;
-use crate::mir;
 
 impl<'a, 'tcx, T> HashStable<StableHashingContext<'a>> for &'tcx ty::List<T>
 where
@@ -197,15 +198,6 @@ impl<'a> HashStable<StableHashingContext<'a>> for ty::FloatVid {
     }
 }
 
-impl<'a, T> HashStable<StableHashingContext<'a>> for ty::steal::Steal<T>
-where
-    T: HashStable<StableHashingContext<'a>>,
-{
-    fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
-        self.borrow().hash_stable(hcx, hasher);
-    }
-}
-
 impl<'a> HashStable<StableHashingContext<'a>>
 for crate::middle::privacy::AccessLevels {
     fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
diff --git a/src/librustc/query/mod.rs b/src/librustc/query/mod.rs
index e463810b7af5f..03aa5a9809f2e 100644
--- a/src/librustc/query/mod.rs
+++ b/src/librustc/query/mod.rs
@@ -1120,8 +1120,7 @@ rustc_queries! {
         }
 
         // Get an estimate of the size of an InstanceDef based on its MIR for CGU partitioning.
-        query instance_def_size_estimate(def: ty::InstanceDef<'tcx>)
-            -> usize {
+        query instance_def_size_estimate(def: ty::InstanceDef<'tcx>) -> usize {
             no_force
             desc { |tcx| "estimating size for `{}`", tcx.def_path_str(def.def_id()) }
         }
@@ -1130,5 +1129,10 @@ rustc_queries! {
             eval_always
             desc { "looking up enabled feature gates" }
         }
+
+        query interp_user_fn(_: CrateNum) -> DefId {
+            eval_always
+            desc { "locating interpreter user fn in HIR" }
+        }
     }
 }
diff --git a/src/librustc/session/config.rs b/src/librustc/session/config.rs
index 50aa036f723a0..8f2e4d975b33b 100644
--- a/src/librustc/session/config.rs
+++ b/src/librustc/session/config.rs
@@ -391,6 +391,7 @@ top_level_options!(
         // can influence whether overflow checks are done or not.
         debug_assertions: bool [TRACKED],
         debuginfo: DebugInfo [TRACKED],
+        interp_mode: bool [TRACKED],
         lint_opts: Vec<(String, lint::Level)> [TRACKED],
         lint_cap: Option<lint::Level> [TRACKED],
         describe_lints: bool [UNTRACKED],
@@ -599,6 +600,7 @@ impl Default for Options {
             crate_types: Vec::new(),
             optimize: OptLevel::No,
             debuginfo: DebugInfo::None,
+            interp_mode: false,
             lint_opts: Vec::new(),
             lint_cap: None,
             describe_lints: false,
@@ -2467,6 +2469,7 @@ pub fn build_session_options_and_crate_config(
             crate_types,
             optimize: opt_level,
             debuginfo,
+            interp_mode: false,
             lint_opts,
             lint_cap,
             describe_lints,
diff --git a/src/librustc/ty/context.rs b/src/librustc/ty/context.rs
index ffe1a11e81a11..31704d6c5fec3 100644
--- a/src/librustc/ty/context.rs
+++ b/src/librustc/ty/context.rs
@@ -38,7 +38,6 @@ use crate::ty::{InferConst, ParamConst};
 use crate::ty::GenericParamDefKind;
 use crate::ty::layout::{LayoutDetails, TargetDataLayout, VariantIdx};
 use crate::ty::query;
-use crate::ty::steal::Steal;
 use crate::ty::subst::{UserSubsts, GenericArgKind};
 use crate::ty::{BoundVar, BindingMode};
 use crate::ty::CanonicalPolyFnSig;
@@ -49,12 +48,13 @@ use crate::util::nodemap::{FxHashMap, FxHashSet};
 use errors::DiagnosticBuilder;
 use arena::SyncDroplessArena;
 use smallvec::SmallVec;
+use rustc_index::vec::{Idx, IndexVec};
 use rustc_data_structures::stable_hasher::{
     HashStable, StableHasher, StableVec, hash_stable_hashmap,
 };
-use rustc_index::vec::{Idx, IndexVec};
 use rustc_data_structures::sharded::ShardedHashMap;
 use rustc_data_structures::sync::{Lrc, Lock, WorkerLocal};
+use rustc_data_structures::steal::Steal;
 use std::any::Any;
 use std::borrow::Borrow;
 use std::cmp::Ordering;
@@ -2894,6 +2894,44 @@ fn ptr_eq<T, U>(t: *const T, u: *const U) -> bool {
     t as *const () == u as *const ()
 }
 
+/// In interpreter mode, locates the `DefId` of the user fn (a closure marked by an attribute named
+/// `rustc_interp_user_fn`) by visiting the local crate's HIR.
+fn find_interp_user_fn(tcx: TyCtxt<'_>) -> DefId {
+    use hir::intravisit::{self, Visitor, NestedVisitorMap};
+
+    struct InterpUserFnVisitor<'tcx> {
+        tcx: TyCtxt<'tcx>,
+        def_id: Option<DefId>,
+    }
+
+    impl<'tcx> Visitor<'tcx> for InterpUserFnVisitor<'tcx> {
+        fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
+            NestedVisitorMap::OnlyBodies(&self.tcx.hir())
+        }
+
+        fn visit_expr(&mut self, ex: &'tcx hir::Expr) {
+            if syntax::attr::contains_name(&ex.attrs, sym::rustc_interp_user_fn) {
+                self.def_id = Some(self.tcx.hir().local_def_id(ex.hir_id));
+                return;
+            }
+
+            intravisit::walk_expr(self, ex);
+        }
+    }
+
+    let mut visitor = InterpUserFnVisitor {
+        tcx,
+        def_id: None,
+    };
+    tcx.hir().krate().visit_all_item_likes(&mut visitor.as_deep_visitor());
+    visitor.def_id
+        .unwrap_or_else(|| tcx.sess.fatal(&format!(
+            "could not find interpreter user fn in HIR; it should be a closure expression \
+             marked with the `#[{}]` attribute",
+            sym::rustc_interp_user_fn
+        )))
+}
+
 pub fn provide(providers: &mut ty::query::Providers<'_>) {
     providers.in_scope_traits_map = |tcx, id| tcx.gcx.trait_map.get(&id);
     providers.module_exports = |tcx, id| tcx.gcx.export_map.get(&id).map(|v| &v[..]);
@@ -2971,4 +3009,8 @@ pub fn provide(providers: &mut ty::query::Providers<'_>) {
         assert_eq!(cnum, LOCAL_CRATE);
         attr::contains_name(tcx.hir().krate_attrs(), sym::compiler_builtins)
     };
+    providers.interp_user_fn = |tcx, cnum| {
+        assert_eq!(cnum, LOCAL_CRATE);
+        find_interp_user_fn(tcx)
+    };
 }
diff --git a/src/librustc/ty/mod.rs b/src/librustc/ty/mod.rs
index 4b9117f71be5c..58e99a8434612 100644
--- a/src/librustc/ty/mod.rs
+++ b/src/librustc/ty/mod.rs
@@ -105,7 +105,6 @@ pub mod outlives;
 pub mod print;
 pub mod query;
 pub mod relate;
-pub mod steal;
 pub mod subst;
 pub mod trait_def;
 pub mod walk;
diff --git a/src/librustc/ty/query/mod.rs b/src/librustc/ty/query/mod.rs
index 863721a5b4b79..9622f57c57cea 100644
--- a/src/librustc/ty/query/mod.rs
+++ b/src/librustc/ty/query/mod.rs
@@ -34,7 +34,7 @@ use crate::traits::query::outlives_bounds::OutlivesBound;
 use crate::traits::specialization_graph;
 use crate::traits::Clauses;
 use crate::ty::{self, CrateInherentImpls, ParamEnvAnd, Ty, TyCtxt, AdtSizedConstraint};
-use crate::ty::steal::Steal;
+use rustc_data_structures::steal::Steal;
 use crate::ty::util::NeedsDrop;
 use crate::ty::subst::SubstsRef;
 use crate::util::nodemap::{DefIdSet, DefIdMap, ItemLocalSet};
diff --git a/src/librustc_data_structures/lib.rs b/src/librustc_data_structures/lib.rs
index 474a42644d915..7b19755422050 100644
--- a/src/librustc_data_structures/lib.rs
+++ b/src/librustc_data_structures/lib.rs
@@ -94,6 +94,7 @@ pub use ena::unify;
 pub mod vec_linked_list;
 pub mod work_queue;
 pub mod fingerprint;
+pub mod steal;
 
 pub struct OnDrop<F: Fn()>(pub F);
 
diff --git a/src/librustc/ty/steal.rs b/src/librustc_data_structures/steal.rs
similarity index 74%
rename from src/librustc/ty/steal.rs
rename to src/librustc_data_structures/steal.rs
index 711e59dbcc9d2..39530ea48e8b5 100644
--- a/src/librustc/ty/steal.rs
+++ b/src/librustc_data_structures/steal.rs
@@ -1,7 +1,8 @@
-use rustc_data_structures::sync::{RwLock, ReadGuard, MappedReadGuard};
+use crate::stable_hasher::{StableHasher, HashStable};
+use crate::sync::{RwLock, ReadGuard, MappedReadGuard};
 
 /// The `Steal` struct is intended to used as the value for a query.
-/// Specifically, we sometimes have queries (*cough* MIR *cough*)
+/// Specifically, we sometimes have queries (in particular, for MIR)
 /// where we create a large, complex value that we want to iteratively
 /// update (e.g., optimize). We could clone the value for each
 /// optimization, but that'd be expensive. And yet we don't just want
@@ -26,21 +27,28 @@ pub struct Steal<T> {
 
 impl<T> Steal<T> {
     pub fn new(value: T) -> Self {
-        Steal {
+        Self {
             value: RwLock::new(Some(value))
         }
     }
 
     pub fn borrow(&self) -> MappedReadGuard<'_, T> {
         ReadGuard::map(self.value.borrow(), |opt| match *opt {
-            None => bug!("attempted to read from stolen value"),
+            None => panic!("attempted to read from stolen value"),
             Some(ref v) => v
         })
     }
 
     pub fn steal(&self) -> T {
-        let value_ref = &mut *self.value.try_write().expect("stealing value which is locked");
+        let value_ref = &mut *self.value.try_write()
+            .expect("stealing value that is locked");
         let value = value_ref.take();
         value.expect("attempt to read from stolen value")
     }
 }
+
+impl<T: HashStable<CTX>, CTX> HashStable<CTX> for Steal<T> {
+    fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher) {
+        self.borrow().hash_stable(hcx, hasher);
+    }
+}
diff --git a/src/librustc_interface/interface.rs b/src/librustc_interface/interface.rs
index fef60a47dc4e7..2edb07c5852a1 100644
--- a/src/librustc_interface/interface.rs
+++ b/src/librustc_interface/interface.rs
@@ -9,13 +9,15 @@ use rustc::session::{DiagnosticOutput, Session};
 use rustc::util::common::ErrorReported;
 use rustc_codegen_utils::codegen_backend::CodegenBackend;
 use rustc_data_structures::OnDrop;
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::sync::Lrc;
-use rustc_data_structures::fx::{FxHashSet, FxHashMap};
+use rustc_data_structures::steal::Steal;
 use rustc_metadata::cstore::CStore;
 use std::path::PathBuf;
 use std::result;
 use std::sync::{Arc, Mutex};
 use syntax;
+use syntax::parse::parser::InterpUserFn;
 use syntax::source_map::{FileLoader, SourceMap};
 use syntax_pos::edition;
 
@@ -35,9 +37,19 @@ pub struct Compiler {
     pub(crate) queries: Queries,
     pub(crate) cstore: Lrc<CStore>,
     pub(crate) crate_name: Option<String>,
+    /// In interpreter mode, the user fn to use when parsing.
+    pub(crate) interp_user_fn: Option<Steal<InterpUserFn>>,
 }
 
 impl Compiler {
+    pub fn set_interp_user_fn(
+        &mut self,
+        interp_user_fn: Option<InterpUserFn>,
+    ) -> &mut Self {
+        self.interp_user_fn = interp_user_fn.map(|user_fn| Steal::new(user_fn));
+        self
+    }
+
     pub fn session(&self) -> &Lrc<Session> {
         &self.sess
     }
@@ -61,12 +73,12 @@ impl Compiler {
     }
 }
 
-/// The compiler configuration
+/// The compiler configuration.
 pub struct Config {
-    /// Command line options
+    /// The command-line options.
     pub opts: config::Options,
 
-    /// cfg! configuration in addition to the default ones
+    /// `cfg!` configuration in addition to the default ones.
     pub crate_cfg: FxHashSet<(String, Option<String>)>,
 
     pub input: Input,
@@ -76,17 +88,14 @@ pub struct Config {
     pub file_loader: Option<Box<dyn FileLoader + Send + Sync>>,
     pub diagnostic_output: DiagnosticOutput,
 
-    /// Set to capture stderr output during compiler execution
+    /// `Some` to capture stderr output during compiler execution.
     pub stderr: Option<Arc<Mutex<Vec<u8>>>>,
 
     pub crate_name: Option<String>,
     pub lint_caps: FxHashMap<lint::LintId, lint::Level>,
 }
 
-pub fn run_compiler_in_existing_thread_pool<F, R>(config: Config, f: F) -> R
-where
-    F: FnOnce(&Compiler) -> R,
-{
+pub fn create_compiler(config: Config) -> Compiler {
     let (sess, codegen_backend, source_map) = util::create_session(
         config.opts,
         config.crate_cfg,
@@ -98,7 +107,7 @@ where
 
     let cstore = Lrc::new(CStore::new(codegen_backend.metadata_loader()));
 
-    let compiler = Compiler {
+    Compiler {
         sess,
         codegen_backend,
         source_map,
@@ -109,7 +118,15 @@ where
         output_file: config.output_file,
         queries: Default::default(),
         crate_name: config.crate_name,
-    };
+        interp_user_fn: None,
+    }
+}
+
+pub fn run_compiler_in_existing_thread_pool<R>(
+    config: Config,
+    f: impl FnOnce(&Compiler) -> R,
+) -> R {
+    let compiler = create_compiler(config);
 
     let _sess_abort_error = OnDrop(|| {
         compiler.sess.diagnostic().print_error_count(&util::diagnostics_registry());
diff --git a/src/librustc_interface/lib.rs b/src/librustc_interface/lib.rs
index 2e593d441553a..a4bcf02a11e19 100644
--- a/src/librustc_interface/lib.rs
+++ b/src/librustc_interface/lib.rs
@@ -19,3 +19,4 @@ mod proc_macro_decls;
 mod profile;
 
 pub use interface::{run_compiler, Config};
+pub use passes::BoxedGlobalCtxt;
diff --git a/src/librustc_interface/passes.rs b/src/librustc_interface/passes.rs
index 9874c0673cfc9..e4b860d380ba8 100644
--- a/src/librustc_interface/passes.rs
+++ b/src/librustc_interface/passes.rs
@@ -11,7 +11,7 @@ use rustc::lint;
 use rustc::middle::{self, reachable, resolve_lifetime, stability};
 use rustc::middle::cstore::CrateStore;
 use rustc::ty::{self, AllArenas, Resolutions, TyCtxt, GlobalCtxt};
-use rustc::ty::steal::Steal;
+use rustc_data_structures::steal::Steal;
 use rustc::traits;
 use rustc::util::common::{time, ErrorReported};
 use rustc::session::Session;
@@ -38,6 +38,7 @@ use syntax::early_buffered_lints::BufferedEarlyLint;
 use syntax::ext::base::{NamedSyntaxExtension, ExtCtxt};
 use syntax::mut_visit::MutVisitor;
 use syntax::parse::{self, PResult};
+use syntax::parse::parser::InterpUserFn;
 use syntax::util::node_count::NodeCounter;
 use syntax::symbol::Symbol;
 use syntax_pos::FileName;
@@ -56,16 +57,28 @@ use std::path::PathBuf;
 use std::cell::RefCell;
 use std::rc::Rc;
 
-pub fn parse<'a>(sess: &'a Session, input: &Input) -> PResult<'a, ast::Crate> {
+pub fn parse<'a>(
+    sess: &'a Session,
+    input: &Input,
+    interp_user_fn: Option<InterpUserFn>,
+) -> PResult<'a, ast::Crate> {
     sess.diagnostic()
         .set_continue_after_error(sess.opts.debugging_opts.continue_parse_after_error);
     sess.profiler(|p| p.start_activity("parsing"));
     let krate = time(sess, "parsing", || match *input {
-        Input::File(ref file) => parse::parse_crate_from_file(file, &sess.parse_sess),
+        Input::File(ref file) => parse::parse_crate_from_file(
+            file,
+            &sess.parse_sess,
+        ),
         Input::Str {
             ref input,
             ref name,
-        } => parse::parse_crate_from_source_str(name.clone(), input.clone(), &sess.parse_sess),
+        } => parse::parse_crate_from_source_str(
+            name.clone(),
+            input.clone(),
+            &sess.parse_sess,
+            interp_user_fn,
+        ),
     })?;
     sess.profiler(|p| p.end_activity("parsing"));
 
@@ -112,7 +125,7 @@ declare_box_region_type!(
 /// harness if one is to be provided, injection of a dependency on the
 /// standard library and prelude, and name resolution.
 ///
-/// Returns `None` if we're aborting after handling -W help.
+/// Returns `None` if we're aborting after handling `-W help`.
 pub fn configure_and_expand(
     sess: Lrc<Session>,
     cstore: Lrc<CStore>,
@@ -236,7 +249,7 @@ pub fn register_plugins<'a>(
         sess.edition(),
         &sess.opts.debugging_opts.allow_features,
     );
-    // these need to be set "early" so that expansion sees `quote` if enabled.
+    // These need to be set "early" so that expansion sees `quote` if enabled.
     sess.init_features(features);
 
     let crate_types = util::collect_crate_types(sess, &krate.attrs);
@@ -354,13 +367,13 @@ fn configure_and_expand_inner<'a>(
         &mut krate, &mut resolver, plugin_info.syntax_exts, sess.edition()
     );
 
-    // Expand all macros
+    // Expand all macros.
     sess.profiler(|p| p.start_activity("macro expansion"));
     krate = time(sess, "expansion", || {
-        // Windows dlls do not have rpaths, so they don't know how to find their
+        // Windows DLLs do not have rpaths, so they don't know how to find their
         // dependencies. It's up to us to tell the system where to find all the
-        // dependent dlls. Note that this uses cfg!(windows) as opposed to
-        // targ_cfg because syntax extensions are always loaded for the host
+        // dependent DLLs. Note that this uses `cfg!(windows)` as opposed to
+        // `targ_cfg` because syntax extensions are always loaded for the host
         // compiler, not for the target.
         //
         // This is somewhat of an inherently racy operation, however, as
@@ -389,7 +402,7 @@ fn configure_and_expand_inner<'a>(
             );
         }
 
-        // Create the config for macro expansion
+        // Create the config for macro expansion.
         let features = sess.features_untracked();
         let cfg = syntax::ext::expand::ExpansionConfig {
             features: Some(&features),
@@ -406,7 +419,7 @@ fn configure_and_expand_inner<'a>(
             ecx.monotonic_expander().expand_crate(krate)
         });
 
-        // The rest is error reporting
+        // The rest is error reporting.
 
         time(sess, "check unused macros", || {
             ecx.check_unused_macros();
@@ -447,7 +460,7 @@ fn configure_and_expand_inner<'a>(
     });
 
     // If we're actually rustdoc then there's no need to actually compile
-    // anything, so switch everything to just looping
+    // anything, so switch everything to just looping.
     if sess.opts.actually_rustdoc {
         util::ReplaceBodyWithLoop::new(sess).visit_crate(&mut krate);
     }
@@ -582,7 +595,7 @@ fn generated_output_paths(
                 out_filenames.push(p);
             },
             OutputType::DepInfo if sess.opts.debugging_opts.dep_info_omit_d_target => {
-                // Don't add the dep-info output when omitting it from dep-info targets
+                // Don't add the dep-info output when omitting it from dep-info targets.
             }
             _ => {
                 out_filenames.push(file);
@@ -640,7 +653,7 @@ fn escape_dep_filename(filename: &FileName) -> String {
 
 fn write_out_deps(compiler: &Compiler, outputs: &OutputFilenames, out_filenames: &[PathBuf]) {
     let sess = &compiler.sess;
-    // Write out dependency rules to the dep-info file if requested
+    // Write out dependency rules to the dep-info file if requested.
     if !sess.opts.output_types.contains_key(&OutputType::DepInfo) {
         return;
     }
@@ -648,7 +661,7 @@ fn write_out_deps(compiler: &Compiler, outputs: &OutputFilenames, out_filenames:
 
     let result = (|| -> io::Result<()> {
         // Build a list of files used to compile the output and
-        // write Makefile-compatible dependency rules
+        // write Makefile-compatible dependency rules.
         let mut files: Vec<String> = sess.source_map()
             .files()
             .iter()
@@ -680,7 +693,7 @@ fn write_out_deps(compiler: &Compiler, outputs: &OutputFilenames, out_filenames:
 
         // Emit a fake target for each input file to the compilation. This
         // prevents `make` from spitting out an error if a file is later
-        // deleted. For more info see #28735
+        // deleted. For more info see #28735.
         for path in files {
             writeln!(file, "{}:", path)?;
         }
@@ -710,7 +723,7 @@ pub fn prepare_outputs(
     krate: &ast::Crate,
     crate_name: &str
 ) -> Result<OutputFilenames> {
-    // FIXME: rustdoc passes &[] instead of &krate.attrs here
+    // FIXME: rustdoc passes `&[]` instead of `&krate.attrs` here.
     let outputs = util::build_output_filenames(
         &compiler.input,
         &compiler.output_dir,
@@ -731,8 +744,7 @@ pub fn prepare_outputs(
         if sess.opts.will_create_output_file() {
             if output_contains_path(&output_paths, input_path) {
                 sess.err(&format!(
-                    "the input file \"{}\" would be overwritten by the generated \
-                        executable",
+                    "the input file \"{}\" would be overwritten by the generated executable",
                     input_path.display()
                 ));
                 return Err(ErrorReported);
@@ -740,7 +752,7 @@ pub fn prepare_outputs(
             if let Some(dir_path) = output_conflicts_with_dir(&output_paths) {
                 sess.err(&format!(
                     "the generated executable for the input file \"{}\" conflicts with the \
-                        existing directory \"{}\"",
+                     existing directory \"{}\"",
                     input_path.display(),
                     dir_path.display()
                 ));
@@ -857,14 +869,14 @@ pub fn create_global_ctxt(
             hir_map,
             query_result_on_disk_cache,
             &crate_name,
-            &outputs
+            &outputs,
         );
 
         global_ctxt = Some(gcx);
         let gcx = global_ctxt.as_ref().unwrap();
 
         ty::tls::enter_global(gcx, |tcx| {
-            // Do some initialization of the DepGraph that can only be done with the
+            // Do some initialization of the `DepGraph` that can only be done with the
             // tcx available.
             time(tcx.sess, "dep graph tcx init", || rustc_incremental::dep_graph_tcx_init(tcx));
         });
@@ -911,7 +923,7 @@ fn analysis(tcx: TyCtxt<'_>, cnum: CrateNum) -> Result<()> {
         });
     });
 
-    // passes are timed inside typeck
+    // Passes are timed inside typeck.
     typeck::check_crate(tcx)?;
 
     time(sess, "misc checking 2", || {
@@ -925,12 +937,11 @@ fn analysis(tcx: TyCtxt<'_>, cnum: CrateNum) -> Result<()> {
         }, {
             time(sess, "liveness checking + intrinsic checking", || {
                 par_iter(&tcx.hir().krate().modules).for_each(|(&module, _)| {
-                    // this must run before MIR dump, because
+                    // This must run before MIR dump, because
                     // "not all control paths return a value" is reported here.
                     //
-                    // maybe move the check to a MIR pass?
+                    // FIXME: maybe move the check to a MIR pass?
                     let local_def_id = tcx.hir().local_def_id(module);
-
                     tcx.ensure().check_mod_liveness(local_def_id);
                     tcx.ensure().check_mod_intrinsics(local_def_id);
                 });
@@ -1055,8 +1066,7 @@ fn encode_and_write_metadata(
     (metadata, need_metadata_module)
 }
 
-/// Runs the codegen backend, after which the AST and analysis can
-/// be discarded.
+/// Runs the codegen backend, after which the AST and analysis can be discarded.
 pub fn start_codegen<'tcx>(
     codegen_backend: &dyn CodegenBackend,
     tcx: TyCtxt<'tcx>,
diff --git a/src/librustc_interface/queries.rs b/src/librustc_interface/queries.rs
index cd72dc9453c7e..5dcc9b4117ed4 100644
--- a/src/librustc_interface/queries.rs
+++ b/src/librustc_interface/queries.rs
@@ -6,15 +6,16 @@ use rustc::session::config::{OutputFilenames, OutputType};
 use rustc::util::common::{time, ErrorReported};
 use rustc::hir;
 use rustc::hir::def_id::LOCAL_CRATE;
-use rustc::ty::steal::Steal;
+use rustc_data_structures::steal::Steal;
 use rustc::dep_graph::DepGraph;
+use syntax::{self, ast};
+
 use std::cell::{Ref, RefMut, RefCell};
 use std::rc::Rc;
 use std::any::Any;
 use std::mem;
-use syntax::{self, ast};
 
-/// Represent the result of a query.
+/// Represents the result of a query.
 /// This result can be stolen with the `take` method and returned with the `give` method.
 pub struct Query<T> {
     result: RefCell<Option<Result<T>>>,
@@ -46,14 +47,14 @@ impl<T> Query<T> {
         *result = Some(Ok(value));
     }
 
-    /// Borrows the query result using the RefCell. Panics if the result is stolen.
+    /// Borrows the query result using the `RefCell`. Panics if the result is stolen.
     pub fn peek(&self) -> Ref<'_, T> {
         Ref::map(self.result.borrow(), |r| {
             r.as_ref().unwrap().as_ref().expect("missing query result")
         })
     }
 
-    /// Mutably borrows the query result using the RefCell. Panics if the result is stolen.
+    /// Mutably borrows the query result using the `RefCell`. Panics if the result is stolen.
     pub fn peek_mut(&self) -> RefMut<'_, T> {
         RefMut::map(self.result.borrow_mut(), |r| {
             r.as_mut().unwrap().as_mut().expect("missing query result")
@@ -97,11 +98,18 @@ impl Compiler {
 
     pub fn parse(&self) -> Result<&Query<ast::Crate>> {
         self.queries.parse.compute(|| {
-            passes::parse(self.session(), &self.input).map_err(
+            passes::parse(
+                self.session(),
+                &self.input,
+                // This is only run once, so the steal here is fine (unfortunately, the current
+                // architechture of rustc_interface is not great at exposing the nature of these
+                // queries as "linear").
+                self.interp_user_fn.as_ref().map(|s| s.steal()),
+            ).map_err(
                 |mut parse_error| {
                     parse_error.emit();
                     ErrorReported
-                },
+                }
             )
         })
     }
@@ -221,7 +229,8 @@ impl Compiler {
                 expansion.defs.steal(),
                 expansion.resolutions.steal(),
                 outputs,
-                &crate_name))
+                &crate_name,
+            ))
         })
     }
 
@@ -231,7 +240,7 @@ impl Compiler {
             self.global_ctxt()?.peek_mut().enter(|tcx| {
                 tcx.analysis(LOCAL_CRATE).ok();
 
-                // Don't do code generation if there were any errors
+                // Don't do code generation if there were any errors.
                 self.session().compile_status()?;
 
                 Ok(passes::start_codegen(
@@ -276,12 +285,12 @@ impl Compiler {
 
         self.global_ctxt()?;
 
-        // Drop AST after creating GlobalCtxt to free memory.
+        // Drop AST after creating `GlobalCtxt` to free memory.
         mem::drop(self.expansion()?.take());
 
         self.ongoing_codegen()?;
 
-        // Drop GlobalCtxt after starting codegen to free memory.
+        // Drop `GlobalCtxt` after starting codegen to free memory.
         mem::drop(self.global_ctxt()?.take());
 
         self.link().map(|_| ())
diff --git a/src/librustc_mir/transform/mod.rs b/src/librustc_mir/transform/mod.rs
index 7e06729c2c742..6c5a44e13f224 100644
--- a/src/librustc_mir/transform/mod.rs
+++ b/src/librustc_mir/transform/mod.rs
@@ -1,10 +1,11 @@
 use crate::{build, shim};
+
 use rustc_index::vec::IndexVec;
+use rustc_data_structures::steal::Steal;
 use rustc::hir::def_id::{CrateNum, DefId, LOCAL_CRATE};
 use rustc::mir::{Body, MirPhase, Promoted};
 use rustc::ty::{TyCtxt, InstanceDef};
 use rustc::ty::query::Providers;
-use rustc::ty::steal::Steal;
 use rustc::hir;
 use rustc::hir::intravisit::{self, Visitor, NestedVisitorMap};
 use rustc::util::nodemap::DefIdSet;
diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index 023952042e6d4..d3c572c3a4f41 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -905,7 +905,27 @@ pub enum MacStmtStyle {
     NoBraces,
 }
 
-/// Local represents a `let` statement, e.g., `let <pat>:<ty> = <expr>;`.
+/// The interpreter state of a `Local`.
+#[derive(Clone, Copy, PartialEq, Eq, RustcEncodable, RustcDecodable, Debug)]
+pub enum LocalInterpState {
+    /// The local was not uninitialized in any previous evaluation session.
+    Uninitialized,
+    /// The local was set to a value in a previous evaluation session.
+    Set,
+    /// The local was moved during a previous evaluation session.
+    Moved,
+}
+
+/// Extra information about a `Local` for the interpreter.
+#[derive(Clone, Copy, RustcEncodable, RustcDecodable, Debug)]
+pub struct LocalInterpTag {
+    /// The ID of this local, for use by the interpreter.
+    pub id: usize,
+    /// The state of the local.
+    pub state: LocalInterpState,
+}
+
+/// Represents a `let` statement, e.g., `let <pat>:<ty> = <expr>;`.
 #[derive(Clone, RustcEncodable, RustcDecodable, Debug)]
 pub struct Local {
     pub id: NodeId,
@@ -915,6 +935,8 @@ pub struct Local {
     pub init: Option<P<Expr>>,
     pub span: Span,
     pub attrs: ThinVec<Attribute>,
+    /// In interpreter mode, extra information about the local.
+    pub interp_tag: Option<LocalInterpTag>,
 }
 
 /// An arm of a 'match'.
diff --git a/src/libsyntax/ext/build.rs b/src/libsyntax/ext/build.rs
index 8c5289671c98e..9d826d1070251 100644
--- a/src/libsyntax/ext/build.rs
+++ b/src/libsyntax/ext/build.rs
@@ -190,6 +190,7 @@ impl<'a> ExtCtxt<'a> {
             id: ast::DUMMY_NODE_ID,
             span: sp,
             attrs: ThinVec::new(),
+            interp_tag: None,
         });
         ast::Stmt {
             id: ast::DUMMY_NODE_ID,
@@ -207,6 +208,7 @@ impl<'a> ExtCtxt<'a> {
             id: ast::DUMMY_NODE_ID,
             span,
             attrs: ThinVec::new(),
+            interp_tag: None,
         });
         ast::Stmt {
             id: ast::DUMMY_NODE_ID,
diff --git a/src/libsyntax/feature_gate/builtin_attrs.rs b/src/libsyntax/feature_gate/builtin_attrs.rs
index d14afc6deaa69..eb8175143ee00 100644
--- a/src/libsyntax/feature_gate/builtin_attrs.rs
+++ b/src/libsyntax/feature_gate/builtin_attrs.rs
@@ -337,7 +337,7 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     gated!(allow_internal_unsafe, Normal, template!(Word), EXPLAIN_ALLOW_INTERNAL_UNSAFE),
 
     // ==========================================================================
-    // Internal attributes: Type system related:
+    // Internal attributes: Type system:
     // ==========================================================================
 
     gated!(fundamental, Whitelisted, template!(Word), experimental!(fundamental)),
@@ -352,7 +352,7 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ),
 
     // ==========================================================================
-    // Internal attributes: Runtime related:
+    // Internal attributes: Runtime:
     // ==========================================================================
 
     rustc_attr!(rustc_allocator, Whitelisted, template!(Word), IMPL_DETAIL),
@@ -389,7 +389,7 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ),
 
     // ==========================================================================
-    // Internal attributes, Linkage:
+    // Internal attributes: Linkage:
     // ==========================================================================
 
     gated!(
@@ -399,7 +399,7 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     rustc_attr!(rustc_std_internal_symbol, Whitelisted, template!(Word), INTERAL_UNSTABLE),
 
     // ==========================================================================
-    // Internal attributes, Macro related:
+    // Internal attributes: Macros:
     // ==========================================================================
 
     rustc_attr!(rustc_builtin_macro, Whitelisted, template!(Word), IMPL_DETAIL),
@@ -411,7 +411,7 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ),
 
     // ==========================================================================
-    // Internal attributes, Diagnostics related:
+    // Internal attributes: Diagnostics:
     // ==========================================================================
 
     gated!(
@@ -427,7 +427,7 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     rustc_attr!(rustc_conversion_suggestion, Whitelisted, template!(Word), INTERAL_UNSTABLE),
 
     // ==========================================================================
-    // Internal attributes, Const related:
+    // Internal attributes: Const:
     // ==========================================================================
 
     rustc_attr!(rustc_promotable, Whitelisted, template!(Word), IMPL_DETAIL),
@@ -435,7 +435,7 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     rustc_attr!(rustc_args_required_const, Whitelisted, template!(List: "N"), INTERAL_UNSTABLE),
 
     // ==========================================================================
-    // Internal attributes, Layout related:
+    // Internal attributes: Layout related:
     // ==========================================================================
 
     rustc_attr!(
@@ -455,7 +455,7 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ),
 
     // ==========================================================================
-    // Internal attributes, Misc:
+    // Internal attributes: Miscellaneous:
     // ==========================================================================
     gated!(
         lang, Normal, template!(NameValueStr: "name"), lang_items,
@@ -489,7 +489,7 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ),
     gated!(
         rustc_paren_sugar, Normal, template!(Word), unboxed_closures,
-        "unboxed_closures are still evolving",
+        "the `unboxed_closures` feature is still evolving",
     ),
     rustc_attr!(
         rustc_inherit_overflow_checks, Whitelisted, template!(Word),
@@ -505,9 +505,14 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         rustc_test_marker, Normal, template!(Word),
         "the `#[rustc_test_marker]` attribute is used internally to track tests",
     ),
+    rustc_attr!(
+        // Used by interpreters:
+        rustc_interp_user_fn, Normal, template!(Word),
+        "`#[rustc_interp_user_fn]` is for use by interpreters only"
+    ),
 
     // ==========================================================================
-    // Internal attributes, Testing:
+    // Internal attributes: Testing:
     // ==========================================================================
 
     rustc_attr!(TEST, rustc_outlives, Normal, template!(Word)),
@@ -545,7 +550,7 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     rustc_attr!(TEST, rustc_dump_program_clauses, Whitelisted, template!(Word)),
     rustc_attr!(TEST, rustc_dump_env_program_clauses, Whitelisted, template!(Word)),
     rustc_attr!(TEST, rustc_object_lifetime_default, Whitelisted, template!(Word)),
-    rustc_attr!(TEST, rustc_dummy, Normal, template!(Word /* doesn't matter*/)),
+    rustc_attr!(TEST, rustc_dummy, Normal, template!(Word /* doesn't matter */)),
     gated!(
         omit_gdb_pretty_printer_section, Whitelisted, template!(Word),
         "the `#[omit_gdb_pretty_printer_section]` attribute is just used for the Rust test suite",
diff --git a/src/libsyntax/mut_visit.rs b/src/libsyntax/mut_visit.rs
index 3923b9f297b9f..841449d6e4648 100644
--- a/src/libsyntax/mut_visit.rs
+++ b/src/libsyntax/mut_visit.rs
@@ -540,7 +540,7 @@ pub fn noop_visit_parenthesized_parameter_data<T: MutVisitor>(args: &mut Parenth
 }
 
 pub fn noop_visit_local<T: MutVisitor>(local: &mut P<Local>, vis: &mut T) {
-    let Local { id, pat, ty, init, span, attrs } = local.deref_mut();
+    let Local { id, pat, ty, init, span, attrs, interp_tag: _ } = local.deref_mut();
     vis.visit_id(id);
     vis.visit_pat(pat);
     visit_opt(ty, |ty| vis.visit_ty(ty));
diff --git a/src/libsyntax/parse/diagnostics.rs b/src/libsyntax/parse/diagnostics.rs
index ec5d00e0952d7..247ee9b5f5673 100644
--- a/src/libsyntax/parse/diagnostics.rs
+++ b/src/libsyntax/parse/diagnostics.rs
@@ -1087,7 +1087,9 @@ impl<'a> Parser<'a> {
     /// statement. This is something of a best-effort heuristic.
     ///
     /// We terminate when we find an unmatched `}` (without consuming it).
-    crate fn recover_stmt(&mut self) {
+    //
+    // NOTE: needs to be `pub` for interpreter.
+    pub fn recover_stmt(&mut self) {
         self.recover_stmt_(SemiColonMode::Ignore, BlockMode::Ignore)
     }
 
diff --git a/src/libsyntax/parse/mod.rs b/src/libsyntax/parse/mod.rs
index fa4c10431228a..e39ce36a885cc 100644
--- a/src/libsyntax/parse/mod.rs
+++ b/src/libsyntax/parse/mod.rs
@@ -170,25 +170,40 @@ pub enum DirectoryOwnership {
 // uses a HOF to parse anything, and <source> includes file and
 // `source_str`.
 
-pub fn parse_crate_from_file<'a>(input: &Path, sess: &'a ParseSess) -> PResult<'a, ast::Crate> {
-    let mut parser = new_parser_from_file(sess, input);
-    parser.parse_crate_mod()
+pub fn parse_crate_from_file<'a>(
+    input: &Path,
+    sess: &'a ParseSess,
+) -> PResult<'a, ast::Crate> {
+    new_parser_from_file(sess, input)
+        .parse_crate_mod()
 }
 
-pub fn parse_crate_attrs_from_file<'a>(input: &Path, sess: &'a ParseSess)
-                                       -> PResult<'a, Vec<ast::Attribute>> {
-    let mut parser = new_parser_from_file(sess, input);
-    parser.parse_inner_attributes()
+pub fn parse_crate_attrs_from_file<'a>(
+    input: &Path,
+    sess: &'a ParseSess,
+) -> PResult<'a, Vec<ast::Attribute>> {
+    new_parser_from_file(sess, input)
+        .parse_inner_attributes()
 }
 
-pub fn parse_crate_from_source_str(name: FileName, source: String, sess: &ParseSess)
-                                       -> PResult<'_, ast::Crate> {
-    new_parser_from_source_str(sess, name, source).parse_crate_mod()
+pub fn parse_crate_from_source_str<'a>(
+    name: FileName,
+    source: String,
+    sess: &'a ParseSess,
+    interp_user_fn: Option<parser::InterpUserFn>,
+) -> PResult<'a, ast::Crate> {
+    new_parser_from_source_str(sess, name, source)
+        .set_interp_user_fn(interp_user_fn)
+        .parse_crate_mod()
 }
 
-pub fn parse_crate_attrs_from_source_str(name: FileName, source: String, sess: &ParseSess)
-                                             -> PResult<'_, Vec<ast::Attribute>> {
-    new_parser_from_source_str(sess, name, source).parse_inner_attributes()
+pub fn parse_crate_attrs_from_source_str<'a>(
+    name: FileName,
+    source: String,
+    sess: &'a ParseSess,
+) -> PResult<'a, Vec<ast::Attribute>> {
+    new_parser_from_source_str(sess, name, source)
+        .parse_inner_attributes()
 }
 
 pub fn parse_stream_from_source_str(
diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 2fb6f197dad7c..7c2d62c4ea082 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -107,6 +107,15 @@ enum PrevTokenKind {
 
 // NOTE: `Ident`s are handled by `common.rs`.
 
+/// In interpreter mode, information about the user fn (where the user-input code goes).
+#[derive(Clone)]
+pub struct InterpUserFn {
+    /// The placeholder name. The parser looks for this as a single ident within a block.
+    pub placeholder: Symbol,
+    /// The body to insert in place of the placeholder ident.
+    pub body: Option<P<ast::Block>>,
+}
+
 #[derive(Clone)]
 pub struct Parser<'a> {
     pub sess: &'a ParseSess,
@@ -151,6 +160,8 @@ pub struct Parser<'a> {
     crate last_type_ascription: Option<(Span, bool /* likely path typo */)>,
     /// If present, this `Parser` is not parsing Rust code but rather a macro call.
     crate subparser_name: Option<&'static str>,
+    /// In interpreter mode, the user fn to use.
+    pub interp_user_fn: Option<InterpUserFn>,
 }
 
 impl<'a> Drop for Parser<'a> {
@@ -368,6 +379,7 @@ impl<'a> Parser<'a> {
             last_unexpected_token_span: None,
             last_type_ascription: None,
             subparser_name,
+            interp_user_fn: None,
         };
 
         parser.token = parser.next_tok();
@@ -387,6 +399,11 @@ impl<'a> Parser<'a> {
         parser
     }
 
+    pub fn set_interp_user_fn(&mut self, interp_user_fn: Option<InterpUserFn>) -> &mut Self {
+        self.interp_user_fn = interp_user_fn;
+        self
+    }
+
     fn next_tok(&mut self) -> Token {
         let mut next = if self.desugar_doc_comments {
             self.token_cursor.next_desugared()
diff --git a/src/libsyntax/parse/parser/expr.rs b/src/libsyntax/parse/parser/expr.rs
index 23674ad589dc5..6d6474a3043e7 100644
--- a/src/libsyntax/parse/parser/expr.rs
+++ b/src/libsyntax/parse/parser/expr.rs
@@ -5,7 +5,7 @@ use super::{
 use super::pat::{GateOr, PARAM_EXPECTED};
 
 use crate::ast::{
-    self, DUMMY_NODE_ID, Attribute, AttrStyle, Ident, CaptureBy, BlockCheckMode,
+    self, DUMMY_NODE_ID, Attribute, AttrStyle, Ident, CaptureBy, Block, BlockCheckMode,
     Expr, ExprKind, RangeLimits, Label, Movability, IsAsync, Arm, Ty, TyKind,
     FunctionRetTy, Param, FnDecl, BinOpKind, BinOp, UnOp, Mac, AnonConst, Field,
 };
@@ -631,13 +631,13 @@ impl<'a> Parser<'a> {
         let mut e = e0;
         let mut hi;
         loop {
-            // expr?
+            // `expr?`
             while self.eat(&token::Question) {
                 let hi = self.prev_span;
                 e = self.mk_expr(lo.to(hi), ExprKind::Try(e), ThinVec::new());
             }
 
-            // expr.f
+            // `expr.f`
             if self.eat(&token::Dot) {
                 match self.token.kind {
                     token::Ident(..) => {
@@ -691,7 +691,7 @@ impl<'a> Parser<'a> {
             }
             if self.expr_is_complete(&e) { break; }
             match self.token.kind {
-                // expr(...)
+                // `expr(...)`
                 token::OpenDelim(token::Paren) => {
                     let seq = self.parse_paren_expr_seq().map(|es| {
                         let nd = self.mk_call(e, es);
@@ -701,7 +701,7 @@ impl<'a> Parser<'a> {
                     e = self.recover_seq_parse_error(token::Paren, lo, seq);
                 }
 
-                // expr[...]
+                // `expr[...]`
                 // Could be either an index expression or a slicing expression.
                 token::OpenDelim(token::Bracket) => {
                     self.bump();
@@ -1100,10 +1100,42 @@ impl<'a> Parser<'a> {
         let mut attrs = outer_attrs;
         attrs.extend(self.parse_inner_attributes()?);
 
-        let blk = self.parse_block_tail(lo, blk_mode)?;
+        let blk = if let Some(interp_blk) = self.parse_interp_user_fn_block()? {
+            interp_blk
+        } else {
+            self.parse_block_tail(lo, blk_mode)?
+        };
         Ok(self.mk_expr(blk.span, ExprKind::Block(blk, opt_label), attrs))
     }
 
+    /// In interpreter mode, parses the placeholder for the user fn body as the user-provided
+    /// block (not from the source).
+    /// Outside of interpreter mode, simply returns `Ok(None)`.
+    fn parse_interp_user_fn_block(&mut self) -> PResult<'a, Option<P<Block>>> {
+        if let Some(placeholder) = self.interp_user_fn.as_ref().map(|f| f.placeholder) {
+            if let token::Ident(name, _) = self.token.kind {
+                if name == placeholder {
+                    // We are in interpreter mode, and have found the block to use as the
+                    // "user fn" body. Insert the given body for the user fn instead of
+                    // parsing one.
+
+                    self.bump();
+                    self.expect(&token::CloseDelim(token::Brace))?;
+
+                    let blk = self.interp_user_fn.as_mut().and_then(|f| f.body.take())
+                        .ok_or_else(|| {
+                            self.diagnostic().struct_span_err(self.token.span, &format!(
+                                "encountered interpreter user fn placeholder `{}` more than once",
+                                placeholder
+                            ))
+                        })?;
+                    return Ok(Some(blk));
+                }
+            }
+        }
+        Ok(None)
+    }
+
     /// Parses a closure expression (e.g., `move |args| expr`).
     fn parse_closure_expr(&mut self, attrs: ThinVec<Attribute>) -> PResult<'a, P<Expr>> {
         let lo = self.token.span;
diff --git a/src/libsyntax/parse/parser/stmt.rs b/src/libsyntax/parse/parser/stmt.rs
index 855b03ddd6f6b..cd936d3168fd8 100644
--- a/src/libsyntax/parse/parser/stmt.rs
+++ b/src/libsyntax/parse/parser/stmt.rs
@@ -275,6 +275,7 @@ impl<'a> Parser<'a> {
             id: DUMMY_NODE_ID,
             span: lo.to(hi),
             attrs,
+            interp_tag: None,
         }))
     }
 
@@ -422,7 +423,9 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses a statement, including the trailing semicolon.
-    crate fn parse_full_stmt(&mut self, macro_legacy_warnings: bool) -> PResult<'a, Option<Stmt>> {
+    //
+    // NOTE: needs to be `pub` for interpreter.
+    pub fn parse_full_stmt(&mut self, macro_legacy_warnings: bool) -> PResult<'a, Option<Stmt>> {
         // Skip looking for a trailing semicolon when we have an interpolated statement.
         maybe_whole!(self, NtStmt, |x| Some(x));
 
diff --git a/src/libsyntax_ext/deriving/debug.rs b/src/libsyntax_ext/deriving/debug.rs
index 003c2423576eb..0b7eda7cce713 100644
--- a/src/libsyntax_ext/deriving/debug.rs
+++ b/src/libsyntax_ext/deriving/debug.rs
@@ -128,6 +128,7 @@ fn stmt_let_undescore(cx: &mut ExtCtxt<'_>, sp: Span, expr: P<ast::Expr>) -> ast
         id: ast::DUMMY_NODE_ID,
         span: sp,
         attrs: ThinVec::new(),
+        interp_tag: None,
     });
     ast::Stmt {
         id: ast::DUMMY_NODE_ID,
diff --git a/src/libsyntax_pos/span_encoding.rs b/src/libsyntax_pos/span_encoding.rs
index 525ec13623289..f9f22302e36e8 100644
--- a/src/libsyntax_pos/span_encoding.rs
+++ b/src/libsyntax_pos/span_encoding.rs
@@ -68,7 +68,7 @@ const LEN_TAG: u16 = 0b1000_0000_0000_0000;
 const MAX_LEN: u32 = 0b0111_1111_1111_1111;
 const MAX_CTXT: u32 = 0b1111_1111_1111_1111;
 
-/// Dummy span, both position and length are zero, syntax context is zero as well.
+/// Dummy span: both position and length are zero; syntax context is zero (empty) as well.
 pub const DUMMY_SP: Span = Span { base_or_index: 0, len_or_tag: 0, ctxt_or_zero: 0 };
 
 impl Span {
diff --git a/src/libsyntax_pos/symbol.rs b/src/libsyntax_pos/symbol.rs
index 1769135e7f21a..4e14596ff9b63 100644
--- a/src/libsyntax_pos/symbol.rs
+++ b/src/libsyntax_pos/symbol.rs
@@ -101,7 +101,7 @@ symbols! {
     }
 
     // Symbols that can be referred to with syntax_pos::sym::*. The symbol is
-    // the stringified identifier unless otherwise specified (e.g.
+    // the stringified identifier unless otherwise specified (e.g.,
     // `proc_dash_macro` represents "proc-macro").
     //
     // As well as the symbols listed, there are symbols for the the strings
@@ -581,6 +581,7 @@ symbols! {
         rustc_expected_cgu_reuse,
         rustc_if_this_changed,
         rustc_inherit_overflow_checks,
+        rustc_interp_user_fn,
         rustc_layout,
         rustc_layout_scalar_valid_range_end,
         rustc_layout_scalar_valid_range_start,
diff --git a/src/test/ui/interp-user-fn.rs b/src/test/ui/interp-user-fn.rs
new file mode 100644
index 0000000000000..a9a1b28c0ea45
--- /dev/null
+++ b/src/test/ui/interp-user-fn.rs
@@ -0,0 +1,7 @@
+#![feature(stmt_expr_attributes)]
+
+fn main() {
+    let user_body = #[rustc_interp_user_fn] || {};
+    //~^ ERROR `#[rustc_interp_user_fn]` is for use by interpreters only [E0658]
+    user_body();
+}
diff --git a/src/test/ui/interp-user-fn.stderr b/src/test/ui/interp-user-fn.stderr
new file mode 100644
index 0000000000000..bb2275fd1fba3
--- /dev/null
+++ b/src/test/ui/interp-user-fn.stderr
@@ -0,0 +1,12 @@
+error[E0658]: `#[rustc_interp_user_fn]` is for use by interpreters only
+  --> $DIR/interp-user-fn.rs:4:21
+   |
+LL |     let user_body = #[rustc_interp_user_fn] || {};
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/29642
+   = help: add `#![feature(rustc_attrs)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.