diff --git a/compiler/rustc_borrowck/src/nll.rs b/compiler/rustc_borrowck/src/nll.rs
index 4a12e1b1b92..e379e647062 100644
--- a/compiler/rustc_borrowck/src/nll.rs
+++ b/compiler/rustc_borrowck/src/nll.rs
@@ -73,7 +73,7 @@ pub(crate) fn replace_regions_in_mir<'tcx>(
     // Replace all remaining regions with fresh inference variables.
     renumber::renumber_mir(infcx, body, promoted);
 
-    dump_mir(infcx.tcx, None, "renumber", &0, body, |_, _| Ok(()));
+    dump_mir(infcx.tcx, false, "renumber", &0, body, |_, _| Ok(()));
 
     universal_regions
 }
@@ -331,7 +331,7 @@ pub(super) fn dump_mir_results<'tcx>(
         return;
     }
 
-    dump_mir(infcx.tcx, None, "nll", &0, body, |pass_where, out| {
+    dump_mir(infcx.tcx, false, "nll", &0, body, |pass_where, out| {
         match pass_where {
             // Before the CFG, dump out the values for each region variable.
             PassWhere::BeforeCFG => {
@@ -358,15 +358,13 @@ pub(super) fn dump_mir_results<'tcx>(
 
     // Also dump the inference graph constraints as a graphviz file.
     let _: io::Result<()> = try {
-        let mut file =
-            create_dump_file(infcx.tcx, "regioncx.all.dot", None, "nll", &0, body.source)?;
+        let mut file = create_dump_file(infcx.tcx, "regioncx.all.dot", false, "nll", &0, body)?;
         regioncx.dump_graphviz_raw_constraints(&mut file)?;
     };
 
     // Also dump the inference graph constraints as a graphviz file.
     let _: io::Result<()> = try {
-        let mut file =
-            create_dump_file(infcx.tcx, "regioncx.scc.dot", None, "nll", &0, body.source)?;
+        let mut file = create_dump_file(infcx.tcx, "regioncx.scc.dot", false, "nll", &0, body)?;
         regioncx.dump_graphviz_scc_constraints(&mut file)?;
     };
 }
diff --git a/compiler/rustc_middle/src/mir/mod.rs b/compiler/rustc_middle/src/mir/mod.rs
index 20dde64e51b..a513444e1e0 100644
--- a/compiler/rustc_middle/src/mir/mod.rs
+++ b/compiler/rustc_middle/src/mir/mod.rs
@@ -100,13 +100,9 @@ impl<'tcx> HasLocalDecls<'tcx> for Body<'tcx> {
 /// pass will be named after the type, and it will consist of a main
 /// loop that goes over each available MIR and applies `run_pass`.
 pub trait MirPass<'tcx> {
-    fn name(&self) -> Cow<'_, str> {
+    fn name(&self) -> &str {
         let name = std::any::type_name::<Self>();
-        if let Some(tail) = name.rfind(':') {
-            Cow::from(&name[tail + 1..])
-        } else {
-            Cow::from(name)
-        }
+        if let Some((_, tail)) = name.rsplit_once(':') { tail } else { name }
     }
 
     /// Returns `true` if this pass is enabled with the current combination of compiler flags.
@@ -182,35 +178,6 @@ impl RuntimePhase {
     }
 }
 
-impl Display for MirPhase {
-    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
-        match self {
-            MirPhase::Built => write!(f, "built"),
-            MirPhase::Analysis(p) => write!(f, "analysis-{}", p),
-            MirPhase::Runtime(p) => write!(f, "runtime-{}", p),
-        }
-    }
-}
-
-impl Display for AnalysisPhase {
-    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
-        match self {
-            AnalysisPhase::Initial => write!(f, "initial"),
-            AnalysisPhase::PostCleanup => write!(f, "post_cleanup"),
-        }
-    }
-}
-
-impl Display for RuntimePhase {
-    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
-        match self {
-            RuntimePhase::Initial => write!(f, "initial"),
-            RuntimePhase::PostCleanup => write!(f, "post_cleanup"),
-            RuntimePhase::Optimized => write!(f, "optimized"),
-        }
-    }
-}
-
 /// Where a specific `mir::Body` comes from.
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 #[derive(HashStable, TyEncodable, TyDecodable, TypeFoldable, TypeVisitable)]
@@ -368,7 +335,7 @@ impl<'tcx> Body<'tcx> {
 
         let mut body = Body {
             phase: MirPhase::Built,
-            pass_count: 1,
+            pass_count: 0,
             source,
             basic_blocks: BasicBlocks::new(basic_blocks),
             source_scopes,
@@ -403,7 +370,7 @@ impl<'tcx> Body<'tcx> {
     pub fn new_cfg_only(basic_blocks: IndexVec<BasicBlock, BasicBlockData<'tcx>>) -> Self {
         let mut body = Body {
             phase: MirPhase::Built,
-            pass_count: 1,
+            pass_count: 0,
             source: MirSource::item(CRATE_DEF_ID.to_def_id()),
             basic_blocks: BasicBlocks::new(basic_blocks),
             source_scopes: IndexVec::new(),
diff --git a/compiler/rustc_middle/src/mir/pretty.rs b/compiler/rustc_middle/src/mir/pretty.rs
index 1cac656674d..2a4ff4b8810 100644
--- a/compiler/rustc_middle/src/mir/pretty.rs
+++ b/compiler/rustc_middle/src/mir/pretty.rs
@@ -16,7 +16,6 @@ use rustc_middle::mir::interpret::{
     Pointer, Provenance,
 };
 use rustc_middle::mir::visit::Visitor;
-use rustc_middle::mir::MirSource;
 use rustc_middle::mir::*;
 use rustc_middle::ty::{self, TyCtxt};
 use rustc_target::abi::Size;
@@ -74,7 +73,7 @@ pub enum PassWhere {
 #[inline]
 pub fn dump_mir<'tcx, F>(
     tcx: TyCtxt<'tcx>,
-    pass_num: Option<&dyn Display>,
+    pass_num: bool,
     pass_name: &str,
     disambiguator: &dyn Display,
     body: &Body<'tcx>,
@@ -111,7 +110,7 @@ pub fn dump_enabled<'tcx>(tcx: TyCtxt<'tcx>, pass_name: &str, def_id: DefId) ->
 
 fn dump_matched_mir_node<'tcx, F>(
     tcx: TyCtxt<'tcx>,
-    pass_num: Option<&dyn Display>,
+    pass_num: bool,
     pass_name: &str,
     disambiguator: &dyn Display,
     body: &Body<'tcx>,
@@ -120,8 +119,7 @@ fn dump_matched_mir_node<'tcx, F>(
     F: FnMut(PassWhere, &mut dyn Write) -> io::Result<()>,
 {
     let _: io::Result<()> = try {
-        let mut file =
-            create_dump_file(tcx, "mir", pass_num, pass_name, disambiguator, body.source)?;
+        let mut file = create_dump_file(tcx, "mir", pass_num, pass_name, disambiguator, body)?;
         // see notes on #41697 above
         let def_path =
             ty::print::with_forced_impl_filename_line!(tcx.def_path_str(body.source.def_id()));
@@ -143,16 +141,14 @@ fn dump_matched_mir_node<'tcx, F>(
 
     if tcx.sess.opts.unstable_opts.dump_mir_graphviz {
         let _: io::Result<()> = try {
-            let mut file =
-                create_dump_file(tcx, "dot", pass_num, pass_name, disambiguator, body.source)?;
+            let mut file = create_dump_file(tcx, "dot", pass_num, pass_name, disambiguator, body)?;
             write_mir_fn_graphviz(tcx, body, false, &mut file)?;
         };
     }
 
     if let Some(spanview) = tcx.sess.opts.unstable_opts.dump_mir_spanview {
         let _: io::Result<()> = try {
-            let file_basename =
-                dump_file_basename(tcx, pass_num, pass_name, disambiguator, body.source);
+            let file_basename = dump_file_basename(tcx, pass_num, pass_name, disambiguator, body);
             let mut file = create_dump_file_with_basename(tcx, &file_basename, "html")?;
             if body.source.def_id().is_local() {
                 write_mir_fn_spanview(tcx, body, spanview, &file_basename, &mut file)?;
@@ -165,11 +161,12 @@ fn dump_matched_mir_node<'tcx, F>(
 /// where we should dump a MIR representation output files.
 fn dump_file_basename<'tcx>(
     tcx: TyCtxt<'tcx>,
-    pass_num: Option<&dyn Display>,
+    pass_num: bool,
     pass_name: &str,
     disambiguator: &dyn Display,
-    source: MirSource<'tcx>,
+    body: &Body<'tcx>,
 ) -> String {
+    let source = body.source;
     let promotion_id = match source.promoted {
         Some(id) => format!("-{:?}", id),
         None => String::new(),
@@ -178,9 +175,10 @@ fn dump_file_basename<'tcx>(
     let pass_num = if tcx.sess.opts.unstable_opts.dump_mir_exclude_pass_number {
         String::new()
     } else {
-        match pass_num {
-            None => ".-------".to_string(),
-            Some(pass_num) => format!(".{}", pass_num),
+        if pass_num {
+            format!(".{:03}-{:03}", body.phase.phase_index(), body.pass_count)
+        } else {
+            ".-------".to_string()
         }
     };
 
@@ -250,14 +248,14 @@ fn create_dump_file_with_basename(
 pub fn create_dump_file<'tcx>(
     tcx: TyCtxt<'tcx>,
     extension: &str,
-    pass_num: Option<&dyn Display>,
+    pass_num: bool,
     pass_name: &str,
     disambiguator: &dyn Display,
-    source: MirSource<'tcx>,
+    body: &Body<'tcx>,
 ) -> io::Result<io::BufWriter<fs::File>> {
     create_dump_file_with_basename(
         tcx,
-        &dump_file_basename(tcx, pass_num, pass_name, disambiguator, source),
+        &dump_file_basename(tcx, pass_num, pass_name, disambiguator, body),
         extension,
     )
 }
diff --git a/compiler/rustc_middle/src/mir/syntax.rs b/compiler/rustc_middle/src/mir/syntax.rs
index f2030b91b9b..7d2a6bda569 100644
--- a/compiler/rustc_middle/src/mir/syntax.rs
+++ b/compiler/rustc_middle/src/mir/syntax.rs
@@ -89,6 +89,19 @@ pub enum MirPhase {
     Runtime(RuntimePhase),
 }
 
+impl MirPhase {
+    pub fn name(&self) -> &'static str {
+        match *self {
+            MirPhase::Built => "built",
+            MirPhase::Analysis(AnalysisPhase::Initial) => "analysis",
+            MirPhase::Analysis(AnalysisPhase::PostCleanup) => "analysis-post-cleanup",
+            MirPhase::Runtime(RuntimePhase::Initial) => "runtime",
+            MirPhase::Runtime(RuntimePhase::PostCleanup) => "runtime-post-cleanup",
+            MirPhase::Runtime(RuntimePhase::Optimized) => "runtime-optimized",
+        }
+    }
+}
+
 /// See [`MirPhase::Analysis`].
 #[derive(Copy, Clone, TyEncodable, TyDecodable, Debug, PartialEq, Eq, PartialOrd, Ord)]
 #[derive(HashStable)]
diff --git a/compiler/rustc_mir_build/src/build/custom/mod.rs b/compiler/rustc_mir_build/src/build/custom/mod.rs
index 2412824efeb..eb021f47757 100644
--- a/compiler/rustc_mir_build/src/build/custom/mod.rs
+++ b/compiler/rustc_mir_build/src/build/custom/mod.rs
@@ -57,7 +57,7 @@ pub(super) fn build_custom_mir<'tcx>(
         is_polymorphic: false,
         tainted_by_errors: None,
         injection_phase: None,
-        pass_count: 1,
+        pass_count: 0,
     };
 
     body.local_decls.push(LocalDecl::new(return_ty, return_ty_span));
diff --git a/compiler/rustc_mir_dataflow/src/framework/engine.rs b/compiler/rustc_mir_dataflow/src/framework/engine.rs
index bc75645e7c9..6ddbe69e17e 100644
--- a/compiler/rustc_mir_dataflow/src/framework/engine.rs
+++ b/compiler/rustc_mir_dataflow/src/framework/engine.rs
@@ -294,14 +294,7 @@ where
         None if tcx.sess.opts.unstable_opts.dump_mir_dataflow
             && dump_enabled(tcx, A::NAME, def_id) =>
         {
-            create_dump_file(
-                tcx,
-                ".dot",
-                None,
-                A::NAME,
-                &pass_name.unwrap_or("-----"),
-                body.source,
-            )?
+            create_dump_file(tcx, ".dot", false, A::NAME, &pass_name.unwrap_or("-----"), body)?
         }
 
         _ => return Ok(()),
diff --git a/compiler/rustc_mir_transform/src/coverage/debug.rs b/compiler/rustc_mir_transform/src/coverage/debug.rs
index 0f8679b0bd6..d6a298fade4 100644
--- a/compiler/rustc_mir_transform/src/coverage/debug.rs
+++ b/compiler/rustc_mir_transform/src/coverage/debug.rs
@@ -638,7 +638,7 @@ pub(super) fn dump_coverage_spanview<'tcx>(
     let def_id = mir_source.def_id();
 
     let span_viewables = span_viewables(tcx, mir_body, basic_coverage_blocks, &coverage_spans);
-    let mut file = create_dump_file(tcx, "html", None, pass_name, &0, mir_source)
+    let mut file = create_dump_file(tcx, "html", false, pass_name, &0, mir_body)
         .expect("Unexpected error creating MIR spanview HTML file");
     let crate_name = tcx.crate_name(def_id.krate);
     let item_name = tcx.def_path(def_id).to_filename_friendly_no_crate();
@@ -739,7 +739,7 @@ pub(super) fn dump_coverage_graphviz<'tcx>(
                 .join("\n  ")
         ));
     }
-    let mut file = create_dump_file(tcx, "dot", None, pass_name, &0, mir_source)
+    let mut file = create_dump_file(tcx, "dot", false, pass_name, &0, mir_body)
         .expect("Unexpected error creating BasicCoverageBlock graphviz DOT file");
     graphviz_writer
         .write_graphviz(tcx, &mut file)
diff --git a/compiler/rustc_mir_transform/src/dest_prop.rs b/compiler/rustc_mir_transform/src/dest_prop.rs
index 8cd44ab82cc..97485c4f57b 100644
--- a/compiler/rustc_mir_transform/src/dest_prop.rs
+++ b/compiler/rustc_mir_transform/src/dest_prop.rs
@@ -787,7 +787,7 @@ fn dest_prop_mir_dump<'body, 'tcx>(
     round: usize,
 ) {
     let mut reachable = None;
-    dump_mir(tcx, None, "DestinationPropagation-dataflow", &round, body, |pass_where, w| {
+    dump_mir(tcx, false, "DestinationPropagation-dataflow", &round, body, |pass_where, w| {
         let reachable = reachable.get_or_insert_with(|| traversal::reachable_as_bitset(body));
 
         match pass_where {
diff --git a/compiler/rustc_mir_transform/src/dump_mir.rs b/compiler/rustc_mir_transform/src/dump_mir.rs
index 6b995141a2b..778ae63c5a4 100644
--- a/compiler/rustc_mir_transform/src/dump_mir.rs
+++ b/compiler/rustc_mir_transform/src/dump_mir.rs
@@ -1,6 +1,5 @@
 //! This pass just dumps MIR at a specified point.
 
-use std::borrow::Cow;
 use std::fs::File;
 use std::io;
 
@@ -13,8 +12,8 @@ use rustc_session::config::{OutputFilenames, OutputType};
 pub struct Marker(pub &'static str);
 
 impl<'tcx> MirPass<'tcx> for Marker {
-    fn name(&self) -> Cow<'_, str> {
-        Cow::Borrowed(self.0)
+    fn name(&self) -> &str {
+        self.0
     }
 
     fn run_pass(&self, _tcx: TyCtxt<'tcx>, _body: &mut Body<'tcx>) {}
diff --git a/compiler/rustc_mir_transform/src/generator.rs b/compiler/rustc_mir_transform/src/generator.rs
index ffe4d43bc88..8922298ecaf 100644
--- a/compiler/rustc_mir_transform/src/generator.rs
+++ b/compiler/rustc_mir_transform/src/generator.rs
@@ -1000,7 +1000,7 @@ fn create_generator_drop_shim<'tcx>(
     // unrelated code from the resume part of the function
     simplify::remove_dead_blocks(tcx, &mut body);
 
-    dump_mir(tcx, None, "generator_drop", &0, &body, |_, _| Ok(()));
+    dump_mir(tcx, false, "generator_drop", &0, &body, |_, _| Ok(()));
 
     body
 }
@@ -1171,7 +1171,7 @@ fn create_generator_resume_function<'tcx>(
     // unrelated code from the drop part of the function
     simplify::remove_dead_blocks(tcx, body);
 
-    dump_mir(tcx, None, "generator_resume", &0, body, |_, _| Ok(()));
+    dump_mir(tcx, false, "generator_resume", &0, body, |_, _| Ok(()));
 }
 
 fn insert_clean_drop(body: &mut Body<'_>) -> BasicBlock {
@@ -1394,14 +1394,14 @@ impl<'tcx> MirPass<'tcx> for StateTransform {
         // This is expanded to a drop ladder in `elaborate_generator_drops`.
         let drop_clean = insert_clean_drop(body);
 
-        dump_mir(tcx, None, "generator_pre-elab", &0, body, |_, _| Ok(()));
+        dump_mir(tcx, false, "generator_pre-elab", &0, body, |_, _| Ok(()));
 
         // Expand `drop(generator_struct)` to a drop ladder which destroys upvars.
         // If any upvars are moved out of, drop elaboration will handle upvar destruction.
         // However we need to also elaborate the code generated by `insert_clean_drop`.
         elaborate_generator_drops(tcx, body);
 
-        dump_mir(tcx, None, "generator_post-transform", &0, body, |_, _| Ok(()));
+        dump_mir(tcx, false, "generator_post-transform", &0, body, |_, _| Ok(()));
 
         // Create a copy of our MIR and use it to create the drop shim for the generator
         let drop_shim = create_generator_drop_shim(tcx, &transform, gen_ty, body, drop_clean);
diff --git a/compiler/rustc_mir_transform/src/pass_manager.rs b/compiler/rustc_mir_transform/src/pass_manager.rs
index 27dbc3e22c9..e1b65823a5a 100644
--- a/compiler/rustc_mir_transform/src/pass_manager.rs
+++ b/compiler/rustc_mir_transform/src/pass_manager.rs
@@ -1,5 +1,3 @@
-use std::borrow::Cow;
-
 use rustc_middle::mir::{self, Body, MirPhase, RuntimePhase};
 use rustc_middle::ty::TyCtxt;
 use rustc_session::Session;
@@ -8,13 +6,9 @@ use crate::{validate, MirPass};
 
 /// Just like `MirPass`, except it cannot mutate `Body`.
 pub trait MirLint<'tcx> {
-    fn name(&self) -> Cow<'_, str> {
+    fn name(&self) -> &str {
         let name = std::any::type_name::<Self>();
-        if let Some(tail) = name.rfind(':') {
-            Cow::from(&name[tail + 1..])
-        } else {
-            Cow::from(name)
-        }
+        if let Some((_, tail)) = name.rsplit_once(':') { tail } else { name }
     }
 
     fn is_enabled(&self, _sess: &Session) -> bool {
@@ -32,7 +26,7 @@ impl<'tcx, T> MirPass<'tcx> for Lint<T>
 where
     T: MirLint<'tcx>,
 {
-    fn name(&self) -> Cow<'_, str> {
+    fn name(&self) -> &str {
         self.0.name()
     }
 
@@ -55,7 +49,7 @@ impl<'tcx, T> MirPass<'tcx> for WithMinOptLevel<T>
 where
     T: MirPass<'tcx>,
 {
-    fn name(&self) -> Cow<'_, str> {
+    fn name(&self) -> &str {
         self.1.name()
     }
 
@@ -146,10 +140,11 @@ fn run_passes_inner<'tcx>(
         }
 
         body.phase = new_phase;
+        body.pass_count = 0;
 
         dump_mir_for_phase_change(tcx, body);
         if validate || new_phase == MirPhase::Runtime(RuntimePhase::Optimized) {
-            validate_body(tcx, body, format!("after phase change to {}", new_phase));
+            validate_body(tcx, body, format!("after phase change to {}", new_phase.name()));
         }
 
         body.pass_count = 1;
@@ -166,11 +161,9 @@ pub fn dump_mir_for_pass<'tcx>(
     pass_name: &str,
     is_after: bool,
 ) {
-    let phase_index = body.phase.phase_index();
-
     mir::dump_mir(
         tcx,
-        Some(&format_args!("{:03}-{:03}", phase_index, body.pass_count)),
+        true,
         pass_name,
         if is_after { &"after" } else { &"before" },
         body,
@@ -179,14 +172,6 @@ pub fn dump_mir_for_pass<'tcx>(
 }
 
 pub fn dump_mir_for_phase_change<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>) {
-    let phase_index = body.phase.phase_index();
-
-    mir::dump_mir(
-        tcx,
-        Some(&format_args!("{:03}-000", phase_index)),
-        &format!("{}", body.phase),
-        &"after",
-        body,
-        |_, _| Ok(()),
-    )
+    assert_eq!(body.pass_count, 0);
+    mir::dump_mir(tcx, true, body.phase.name(), &"after", body, |_, _| Ok(()))
 }
diff --git a/compiler/rustc_mir_transform/src/simplify.rs b/compiler/rustc_mir_transform/src/simplify.rs
index 57d372fda56..475e2ec9a1d 100644
--- a/compiler/rustc_mir_transform/src/simplify.rs
+++ b/compiler/rustc_mir_transform/src/simplify.rs
@@ -35,7 +35,6 @@ use rustc_middle::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Vis
 use rustc_middle::mir::*;
 use rustc_middle::ty::TyCtxt;
 use smallvec::SmallVec;
-use std::borrow::Cow;
 use std::convert::TryInto;
 
 pub struct SimplifyCfg {
@@ -57,8 +56,8 @@ pub fn simplify_cfg<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
 }
 
 impl<'tcx> MirPass<'tcx> for SimplifyCfg {
-    fn name(&self) -> Cow<'_, str> {
-        Cow::Borrowed(&self.label)
+    fn name(&self) -> &str {
+        &self.label
     }
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
diff --git a/compiler/rustc_mir_transform/src/simplify_branches.rs b/compiler/rustc_mir_transform/src/simplify_branches.rs
index 3bbae5b8976..405ebce4d22 100644
--- a/compiler/rustc_mir_transform/src/simplify_branches.rs
+++ b/compiler/rustc_mir_transform/src/simplify_branches.rs
@@ -2,8 +2,6 @@ use crate::MirPass;
 use rustc_middle::mir::*;
 use rustc_middle::ty::TyCtxt;
 
-use std::borrow::Cow;
-
 /// A pass that replaces a branch with a goto when its condition is known.
 pub struct SimplifyConstCondition {
     label: String,
@@ -16,8 +14,8 @@ impl SimplifyConstCondition {
 }
 
 impl<'tcx> MirPass<'tcx> for SimplifyConstCondition {
-    fn name(&self) -> Cow<'_, str> {
-        Cow::Borrowed(&self.label)
+    fn name(&self) -> &str {
+        &self.label
     }
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {