diff --git a/.gitignore b/.gitignore
index 948133cd76e4c..90e30ff2d8351 100644
--- a/.gitignore
+++ b/.gitignore
@@ -46,8 +46,11 @@ no_llvm_build
 /inst/
 /llvm/
 /mingw-build/
+# Ignore all "build" directories, in the root and in subdirectories.
+# This makes it a bad idea to have Rust modules named "build", so there is a
+# tidy rule prohibiting that. Try to avoid adding exceptions to this, because
+# some tools don't properly handle negative .gitignore rules.
 build/
-!/compiler/rustc_mir_build/src/build/
 /build-rust-analyzer/
 /dist/
 /unicode-downloads
diff --git a/compiler/rustc_expand/src/build.rs b/compiler/rustc_expand/src/builder.rs
similarity index 100%
rename from compiler/rustc_expand/src/build.rs
rename to compiler/rustc_expand/src/builder.rs
diff --git a/compiler/rustc_expand/src/lib.rs b/compiler/rustc_expand/src/lib.rs
index 777044e3f33bf..695cdab60024e 100644
--- a/compiler/rustc_expand/src/lib.rs
+++ b/compiler/rustc_expand/src/lib.rs
@@ -18,7 +18,7 @@
 
 extern crate proc_macro as pm;
 
-mod build;
+mod builder;
 mod errors;
 // FIXME(Nilstrieb) Translate macro_rules diagnostics
 #[allow(rustc::untranslatable_diagnostic)]
diff --git a/compiler/rustc_mir_build/src/build/block.rs b/compiler/rustc_mir_build/src/builder/block.rs
similarity index 98%
rename from compiler/rustc_mir_build/src/build/block.rs
rename to compiler/rustc_mir_build/src/builder/block.rs
index 89e64015bc439..ba63a97de89fb 100644
--- a/compiler/rustc_mir_build/src/build/block.rs
+++ b/compiler/rustc_mir_build/src/builder/block.rs
@@ -5,9 +5,9 @@ use rustc_middle::{span_bug, ty};
 use rustc_span::Span;
 use tracing::debug;
 
-use crate::build::ForGuard::OutsideGuard;
-use crate::build::matches::{DeclareLetBindings, EmitStorageLive, ScheduleDrops};
-use crate::build::{BlockAnd, BlockAndExtension, BlockFrame, Builder};
+use crate::builder::ForGuard::OutsideGuard;
+use crate::builder::matches::{DeclareLetBindings, EmitStorageLive, ScheduleDrops};
+use crate::builder::{BlockAnd, BlockAndExtension, BlockFrame, Builder};
 
 impl<'a, 'tcx> Builder<'a, 'tcx> {
     pub(crate) fn ast_block(
diff --git a/compiler/rustc_mir_build/src/build/cfg.rs b/compiler/rustc_mir_build/src/builder/cfg.rs
similarity index 99%
rename from compiler/rustc_mir_build/src/build/cfg.rs
rename to compiler/rustc_mir_build/src/builder/cfg.rs
index 9c5ee5b0996e5..cca309115ba81 100644
--- a/compiler/rustc_mir_build/src/build/cfg.rs
+++ b/compiler/rustc_mir_build/src/builder/cfg.rs
@@ -4,7 +4,7 @@ use rustc_middle::mir::*;
 use rustc_middle::ty::TyCtxt;
 use tracing::debug;
 
-use crate::build::CFG;
+use crate::builder::CFG;
 
 impl<'tcx> CFG<'tcx> {
     pub(crate) fn block_data(&self, blk: BasicBlock) -> &BasicBlockData<'tcx> {
diff --git a/compiler/rustc_mir_build/src/build/coverageinfo.rs b/compiler/rustc_mir_build/src/builder/coverageinfo.rs
similarity index 99%
rename from compiler/rustc_mir_build/src/build/coverageinfo.rs
rename to compiler/rustc_mir_build/src/builder/coverageinfo.rs
index 52a4a4b4b510d..a80bd4f3c8009 100644
--- a/compiler/rustc_mir_build/src/build/coverageinfo.rs
+++ b/compiler/rustc_mir_build/src/builder/coverageinfo.rs
@@ -8,8 +8,8 @@ use rustc_middle::thir::{ExprId, ExprKind, Pat, Thir};
 use rustc_middle::ty::TyCtxt;
 use rustc_span::def_id::LocalDefId;
 
-use crate::build::coverageinfo::mcdc::MCDCInfoBuilder;
-use crate::build::{Builder, CFG};
+use crate::builder::coverageinfo::mcdc::MCDCInfoBuilder;
+use crate::builder::{Builder, CFG};
 
 mod mcdc;
 
diff --git a/compiler/rustc_mir_build/src/build/coverageinfo/mcdc.rs b/compiler/rustc_mir_build/src/builder/coverageinfo/mcdc.rs
similarity index 99%
rename from compiler/rustc_mir_build/src/build/coverageinfo/mcdc.rs
rename to compiler/rustc_mir_build/src/builder/coverageinfo/mcdc.rs
index 343d400004315..6b4871dc1fcc8 100644
--- a/compiler/rustc_mir_build/src/build/coverageinfo/mcdc.rs
+++ b/compiler/rustc_mir_build/src/builder/coverageinfo/mcdc.rs
@@ -9,7 +9,7 @@ use rustc_middle::thir::LogicalOp;
 use rustc_middle::ty::TyCtxt;
 use rustc_span::Span;
 
-use crate::build::Builder;
+use crate::builder::Builder;
 use crate::errors::MCDCExceedsConditionLimit;
 
 /// LLVM uses `i16` to represent condition id. Hence `i16::MAX` is the hard limit for number of
diff --git a/compiler/rustc_mir_build/src/build/custom/mod.rs b/compiler/rustc_mir_build/src/builder/custom/mod.rs
similarity index 100%
rename from compiler/rustc_mir_build/src/build/custom/mod.rs
rename to compiler/rustc_mir_build/src/builder/custom/mod.rs
diff --git a/compiler/rustc_mir_build/src/build/custom/parse.rs b/compiler/rustc_mir_build/src/builder/custom/parse.rs
similarity index 100%
rename from compiler/rustc_mir_build/src/build/custom/parse.rs
rename to compiler/rustc_mir_build/src/builder/custom/parse.rs
diff --git a/compiler/rustc_mir_build/src/build/custom/parse/instruction.rs b/compiler/rustc_mir_build/src/builder/custom/parse/instruction.rs
similarity index 99%
rename from compiler/rustc_mir_build/src/build/custom/parse/instruction.rs
rename to compiler/rustc_mir_build/src/builder/custom/parse/instruction.rs
index c3e9bd302deb9..b26df01460dbf 100644
--- a/compiler/rustc_mir_build/src/build/custom/parse/instruction.rs
+++ b/compiler/rustc_mir_build/src/builder/custom/parse/instruction.rs
@@ -9,8 +9,8 @@ use rustc_span::Span;
 use rustc_span::source_map::Spanned;
 
 use super::{PResult, ParseCtxt, parse_by_kind};
-use crate::build::custom::ParseError;
-use crate::build::expr::as_constant::as_constant_inner;
+use crate::builder::custom::ParseError;
+use crate::builder::expr::as_constant::as_constant_inner;
 
 impl<'a, 'tcx> ParseCtxt<'a, 'tcx> {
     pub(crate) fn parse_statement(&self, expr_id: ExprId) -> PResult<StatementKind<'tcx>> {
diff --git a/compiler/rustc_mir_build/src/build/expr/as_constant.rs b/compiler/rustc_mir_build/src/builder/expr/as_constant.rs
similarity index 99%
rename from compiler/rustc_mir_build/src/build/expr/as_constant.rs
rename to compiler/rustc_mir_build/src/builder/expr/as_constant.rs
index 640408cb9c808..177c1e33a83bb 100644
--- a/compiler/rustc_mir_build/src/build/expr/as_constant.rs
+++ b/compiler/rustc_mir_build/src/builder/expr/as_constant.rs
@@ -14,7 +14,7 @@ use rustc_middle::ty::{
 use rustc_middle::{bug, mir, span_bug};
 use tracing::{instrument, trace};
 
-use crate::build::{Builder, parse_float_into_constval};
+use crate::builder::{Builder, parse_float_into_constval};
 
 impl<'a, 'tcx> Builder<'a, 'tcx> {
     /// Compile `expr`, yielding a compile-time constant. Assumes that
diff --git a/compiler/rustc_mir_build/src/build/expr/as_operand.rs b/compiler/rustc_mir_build/src/builder/expr/as_operand.rs
similarity index 98%
rename from compiler/rustc_mir_build/src/build/expr/as_operand.rs
rename to compiler/rustc_mir_build/src/builder/expr/as_operand.rs
index 0cab853196b8e..0ac2b3940c80f 100644
--- a/compiler/rustc_mir_build/src/build/expr/as_operand.rs
+++ b/compiler/rustc_mir_build/src/builder/expr/as_operand.rs
@@ -4,8 +4,8 @@ use rustc_middle::mir::*;
 use rustc_middle::thir::*;
 use tracing::{debug, instrument};
 
-use crate::build::expr::category::Category;
-use crate::build::{BlockAnd, BlockAndExtension, Builder, NeedsTemporary};
+use crate::builder::expr::category::Category;
+use crate::builder::{BlockAnd, BlockAndExtension, Builder, NeedsTemporary};
 
 impl<'a, 'tcx> Builder<'a, 'tcx> {
     /// Construct a temporary lifetime restricted to just the local scope
diff --git a/compiler/rustc_mir_build/src/build/expr/as_place.rs b/compiler/rustc_mir_build/src/builder/expr/as_place.rs
similarity index 98%
rename from compiler/rustc_mir_build/src/build/expr/as_place.rs
rename to compiler/rustc_mir_build/src/builder/expr/as_place.rs
index 6ce88cdc39d0c..b1851e79d5c65 100644
--- a/compiler/rustc_mir_build/src/build/expr/as_place.rs
+++ b/compiler/rustc_mir_build/src/builder/expr/as_place.rs
@@ -14,9 +14,9 @@ use rustc_middle::{bug, span_bug};
 use rustc_span::Span;
 use tracing::{debug, instrument, trace};
 
-use crate::build::ForGuard::{OutsideGuard, RefWithinGuard};
-use crate::build::expr::category::Category;
-use crate::build::{BlockAnd, BlockAndExtension, Builder, Capture, CaptureMap};
+use crate::builder::ForGuard::{OutsideGuard, RefWithinGuard};
+use crate::builder::expr::category::Category;
+use crate::builder::{BlockAnd, BlockAndExtension, Builder, Capture, CaptureMap};
 
 /// The "outermost" place that holds this value.
 #[derive(Copy, Clone, Debug, PartialEq)]
@@ -68,7 +68,7 @@ pub(crate) enum PlaceBase {
 /// This is used internally when building a place for an expression like `a.b.c`. The fields `b`
 /// and `c` can be progressively pushed onto the place builder that is created when converting `a`.
 #[derive(Clone, Debug, PartialEq)]
-pub(in crate::build) struct PlaceBuilder<'tcx> {
+pub(in crate::builder) struct PlaceBuilder<'tcx> {
     base: PlaceBase,
     projection: Vec<PlaceElem<'tcx>>,
 }
@@ -249,7 +249,7 @@ fn strip_prefix<'a, 'tcx>(
 }
 
 impl<'tcx> PlaceBuilder<'tcx> {
-    pub(in crate::build) fn to_place(&self, cx: &Builder<'_, 'tcx>) -> Place<'tcx> {
+    pub(in crate::builder) fn to_place(&self, cx: &Builder<'_, 'tcx>) -> Place<'tcx> {
         self.try_to_place(cx).unwrap_or_else(|| match self.base {
             PlaceBase::Local(local) => span_bug!(
                 cx.local_decls[local].source_info.span,
@@ -265,7 +265,7 @@ impl<'tcx> PlaceBuilder<'tcx> {
     }
 
     /// Creates a `Place` or returns `None` if an upvar cannot be resolved
-    pub(in crate::build) fn try_to_place(&self, cx: &Builder<'_, 'tcx>) -> Option<Place<'tcx>> {
+    pub(in crate::builder) fn try_to_place(&self, cx: &Builder<'_, 'tcx>) -> Option<Place<'tcx>> {
         let resolved = self.resolve_upvar(cx);
         let builder = resolved.as_ref().unwrap_or(self);
         let PlaceBase::Local(local) = builder.base else { return None };
@@ -283,7 +283,7 @@ impl<'tcx> PlaceBuilder<'tcx> {
     /// not captured. This can happen because the final mir that will be
     /// generated doesn't require a read for this place. Failures will only
     /// happen inside closures.
-    pub(in crate::build) fn resolve_upvar(
+    pub(in crate::builder) fn resolve_upvar(
         &self,
         cx: &Builder<'_, 'tcx>,
     ) -> Option<PlaceBuilder<'tcx>> {
diff --git a/compiler/rustc_mir_build/src/build/expr/as_rvalue.rs b/compiler/rustc_mir_build/src/builder/expr/as_rvalue.rs
similarity index 99%
rename from compiler/rustc_mir_build/src/build/expr/as_rvalue.rs
rename to compiler/rustc_mir_build/src/builder/expr/as_rvalue.rs
index 3f89e337781e5..9961c2488ef42 100644
--- a/compiler/rustc_mir_build/src/build/expr/as_rvalue.rs
+++ b/compiler/rustc_mir_build/src/builder/expr/as_rvalue.rs
@@ -16,9 +16,9 @@ use rustc_span::source_map::Spanned;
 use rustc_span::{DUMMY_SP, Span};
 use tracing::debug;
 
-use crate::build::expr::as_place::PlaceBase;
-use crate::build::expr::category::{Category, RvalueFunc};
-use crate::build::{BlockAnd, BlockAndExtension, Builder, NeedsTemporary};
+use crate::builder::expr::as_place::PlaceBase;
+use crate::builder::expr::category::{Category, RvalueFunc};
+use crate::builder::{BlockAnd, BlockAndExtension, Builder, NeedsTemporary};
 
 impl<'a, 'tcx> Builder<'a, 'tcx> {
     /// Returns an rvalue suitable for use until the end of the current
@@ -231,7 +231,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                             if range.start <= range.end { BinOp::BitAnd } else { BinOp::BitOr };
 
                         let mut comparer = |range: u128, bin_op: BinOp| -> Place<'tcx> {
-                            // We can use `ty::TypingEnv::fully_monomorphized()`` here
+                            // We can use `ty::TypingEnv::fully_monomorphized()` here
                             // as we only need it to compute the layout of a primitive.
                             let range_val = Const::from_bits(
                                 this.tcx,
diff --git a/compiler/rustc_mir_build/src/build/expr/as_temp.rs b/compiler/rustc_mir_build/src/builder/expr/as_temp.rs
similarity index 98%
rename from compiler/rustc_mir_build/src/build/expr/as_temp.rs
rename to compiler/rustc_mir_build/src/builder/expr/as_temp.rs
index 466f67b1ba4d8..5e3a24e18fb13 100644
--- a/compiler/rustc_mir_build/src/build/expr/as_temp.rs
+++ b/compiler/rustc_mir_build/src/builder/expr/as_temp.rs
@@ -7,8 +7,8 @@ use rustc_middle::mir::*;
 use rustc_middle::thir::*;
 use tracing::{debug, instrument};
 
-use crate::build::scope::DropKind;
-use crate::build::{BlockAnd, BlockAndExtension, Builder};
+use crate::builder::scope::DropKind;
+use crate::builder::{BlockAnd, BlockAndExtension, Builder};
 
 impl<'a, 'tcx> Builder<'a, 'tcx> {
     /// Compile `expr` into a fresh temporary. This is used when building
diff --git a/compiler/rustc_mir_build/src/build/expr/category.rs b/compiler/rustc_mir_build/src/builder/expr/category.rs
similarity index 100%
rename from compiler/rustc_mir_build/src/build/expr/category.rs
rename to compiler/rustc_mir_build/src/builder/expr/category.rs
diff --git a/compiler/rustc_mir_build/src/build/expr/into.rs b/compiler/rustc_mir_build/src/builder/expr/into.rs
similarity index 99%
rename from compiler/rustc_mir_build/src/build/expr/into.rs
rename to compiler/rustc_mir_build/src/builder/expr/into.rs
index bebb44faba614..b5ec5d8f38d68 100644
--- a/compiler/rustc_mir_build/src/build/expr/into.rs
+++ b/compiler/rustc_mir_build/src/builder/expr/into.rs
@@ -13,9 +13,9 @@ use rustc_middle::ty::CanonicalUserTypeAnnotation;
 use rustc_span::source_map::Spanned;
 use tracing::{debug, instrument};
 
-use crate::build::expr::category::{Category, RvalueFunc};
-use crate::build::matches::DeclareLetBindings;
-use crate::build::{BlockAnd, BlockAndExtension, BlockFrame, Builder, NeedsTemporary};
+use crate::builder::expr::category::{Category, RvalueFunc};
+use crate::builder::matches::DeclareLetBindings;
+use crate::builder::{BlockAnd, BlockAndExtension, BlockFrame, Builder, NeedsTemporary};
 
 impl<'a, 'tcx> Builder<'a, 'tcx> {
     /// Compile `expr`, storing the result into `destination`, which
diff --git a/compiler/rustc_mir_build/src/build/expr/mod.rs b/compiler/rustc_mir_build/src/builder/expr/mod.rs
similarity index 100%
rename from compiler/rustc_mir_build/src/build/expr/mod.rs
rename to compiler/rustc_mir_build/src/builder/expr/mod.rs
diff --git a/compiler/rustc_mir_build/src/build/expr/stmt.rs b/compiler/rustc_mir_build/src/builder/expr/stmt.rs
similarity index 98%
rename from compiler/rustc_mir_build/src/build/expr/stmt.rs
rename to compiler/rustc_mir_build/src/builder/expr/stmt.rs
index 15ee6dd014ce9..4ae3536d9c24b 100644
--- a/compiler/rustc_mir_build/src/build/expr/stmt.rs
+++ b/compiler/rustc_mir_build/src/builder/expr/stmt.rs
@@ -5,8 +5,8 @@ use rustc_middle::thir::*;
 use rustc_span::source_map::Spanned;
 use tracing::debug;
 
-use crate::build::scope::BreakableTarget;
-use crate::build::{BlockAnd, BlockAndExtension, BlockFrame, Builder};
+use crate::builder::scope::BreakableTarget;
+use crate::builder::{BlockAnd, BlockAndExtension, BlockFrame, Builder};
 
 impl<'a, 'tcx> Builder<'a, 'tcx> {
     /// Builds a block of MIR statements to evaluate the THIR `expr`.
diff --git a/compiler/rustc_mir_build/src/build/matches/match_pair.rs b/compiler/rustc_mir_build/src/builder/matches/match_pair.rs
similarity index 98%
rename from compiler/rustc_mir_build/src/build/matches/match_pair.rs
rename to compiler/rustc_mir_build/src/builder/matches/match_pair.rs
index 2815b39037513..e7507c0475483 100644
--- a/compiler/rustc_mir_build/src/build/matches/match_pair.rs
+++ b/compiler/rustc_mir_build/src/builder/matches/match_pair.rs
@@ -2,9 +2,9 @@ use rustc_middle::mir::*;
 use rustc_middle::thir::{self, *};
 use rustc_middle::ty::{self, Ty, TypeVisitableExt};
 
-use crate::build::Builder;
-use crate::build::expr::as_place::{PlaceBase, PlaceBuilder};
-use crate::build::matches::{FlatPat, MatchPairTree, TestCase};
+use crate::builder::Builder;
+use crate::builder::expr::as_place::{PlaceBase, PlaceBuilder};
+use crate::builder::matches::{FlatPat, MatchPairTree, TestCase};
 
 impl<'a, 'tcx> Builder<'a, 'tcx> {
     /// Builds and returns [`MatchPairTree`] subtrees, one for each pattern in
@@ -86,7 +86,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
 impl<'pat, 'tcx> MatchPairTree<'pat, 'tcx> {
     /// Recursively builds a match pair tree for the given pattern and its
     /// subpatterns.
-    pub(in crate::build) fn for_pattern(
+    pub(in crate::builder) fn for_pattern(
         mut place_builder: PlaceBuilder<'tcx>,
         pattern: &'pat Pat<'tcx>,
         cx: &mut Builder<'_, 'tcx>,
diff --git a/compiler/rustc_mir_build/src/build/matches/mod.rs b/compiler/rustc_mir_build/src/builder/matches/mod.rs
similarity index 99%
rename from compiler/rustc_mir_build/src/build/matches/mod.rs
rename to compiler/rustc_mir_build/src/builder/matches/mod.rs
index 5791460a6b1c8..edbe9c6910918 100644
--- a/compiler/rustc_mir_build/src/build/matches/mod.rs
+++ b/compiler/rustc_mir_build/src/builder/matches/mod.rs
@@ -18,10 +18,10 @@ use rustc_span::symbol::Symbol;
 use rustc_span::{BytePos, Pos, Span};
 use tracing::{debug, instrument};
 
-use crate::build::ForGuard::{self, OutsideGuard, RefWithinGuard};
-use crate::build::expr::as_place::PlaceBuilder;
-use crate::build::scope::DropKind;
-use crate::build::{
+use crate::builder::ForGuard::{self, OutsideGuard, RefWithinGuard};
+use crate::builder::expr::as_place::PlaceBuilder;
+use crate::builder::scope::DropKind;
+use crate::builder::{
     BlockAnd, BlockAndExtension, Builder, GuardFrame, GuardFrameLocal, LocalsForNode,
 };
 
diff --git a/compiler/rustc_mir_build/src/build/matches/simplify.rs b/compiler/rustc_mir_build/src/builder/matches/simplify.rs
similarity index 96%
rename from compiler/rustc_mir_build/src/build/matches/simplify.rs
rename to compiler/rustc_mir_build/src/builder/matches/simplify.rs
index 5b402604395af..ebaed1e431bbc 100644
--- a/compiler/rustc_mir_build/src/build/matches/simplify.rs
+++ b/compiler/rustc_mir_build/src/builder/matches/simplify.rs
@@ -16,8 +16,8 @@ use std::mem;
 
 use tracing::{debug, instrument};
 
-use crate::build::Builder;
-use crate::build::matches::{MatchPairTree, PatternExtraData, TestCase};
+use crate::builder::Builder;
+use crate::builder::matches::{MatchPairTree, PatternExtraData, TestCase};
 
 impl<'a, 'tcx> Builder<'a, 'tcx> {
     /// Simplify a list of match pairs so they all require a test. Stores relevant bindings and
diff --git a/compiler/rustc_mir_build/src/build/matches/test.rs b/compiler/rustc_mir_build/src/builder/matches/test.rs
similarity index 99%
rename from compiler/rustc_mir_build/src/build/matches/test.rs
rename to compiler/rustc_mir_build/src/builder/matches/test.rs
index 4f7bbc4ce3e69..596d525f5d8ab 100644
--- a/compiler/rustc_mir_build/src/build/matches/test.rs
+++ b/compiler/rustc_mir_build/src/builder/matches/test.rs
@@ -20,8 +20,8 @@ use rustc_span::symbol::{Symbol, sym};
 use rustc_span::{DUMMY_SP, Span};
 use tracing::{debug, instrument};
 
-use crate::build::Builder;
-use crate::build::matches::{Candidate, MatchPairTree, Test, TestBranch, TestCase, TestKind};
+use crate::builder::Builder;
+use crate::builder::matches::{Candidate, MatchPairTree, Test, TestBranch, TestCase, TestKind};
 
 impl<'a, 'tcx> Builder<'a, 'tcx> {
     /// Identifies what test is needed to decide if `match_pair` is applicable.
diff --git a/compiler/rustc_mir_build/src/build/matches/util.rs b/compiler/rustc_mir_build/src/builder/matches/util.rs
similarity index 98%
rename from compiler/rustc_mir_build/src/build/matches/util.rs
rename to compiler/rustc_mir_build/src/builder/matches/util.rs
index 555684ded81a1..1bd399e511b39 100644
--- a/compiler/rustc_mir_build/src/build/matches/util.rs
+++ b/compiler/rustc_mir_build/src/builder/matches/util.rs
@@ -4,9 +4,9 @@ use rustc_middle::ty::Ty;
 use rustc_span::Span;
 use tracing::debug;
 
-use crate::build::Builder;
-use crate::build::expr::as_place::PlaceBase;
-use crate::build::matches::{Binding, Candidate, FlatPat, MatchPairTree, TestCase};
+use crate::builder::Builder;
+use crate::builder::expr::as_place::PlaceBase;
+use crate::builder::matches::{Binding, Candidate, FlatPat, MatchPairTree, TestCase};
 
 impl<'a, 'tcx> Builder<'a, 'tcx> {
     /// Creates a false edge to `imaginary_target` and a real edge to
diff --git a/compiler/rustc_mir_build/src/build/misc.rs b/compiler/rustc_mir_build/src/builder/misc.rs
similarity index 98%
rename from compiler/rustc_mir_build/src/build/misc.rs
rename to compiler/rustc_mir_build/src/builder/misc.rs
index a14dcad6573ce..a53ae05e84f3c 100644
--- a/compiler/rustc_mir_build/src/build/misc.rs
+++ b/compiler/rustc_mir_build/src/builder/misc.rs
@@ -7,7 +7,7 @@ use rustc_span::Span;
 use rustc_trait_selection::infer::InferCtxtExt;
 use tracing::debug;
 
-use crate::build::Builder;
+use crate::builder::Builder;
 
 impl<'a, 'tcx> Builder<'a, 'tcx> {
     /// Adds a new temporary value of type `ty` storing the result of
diff --git a/compiler/rustc_mir_build/src/build/mod.rs b/compiler/rustc_mir_build/src/builder/mod.rs
similarity index 99%
rename from compiler/rustc_mir_build/src/build/mod.rs
rename to compiler/rustc_mir_build/src/builder/mod.rs
index 3317f3b7f8acb..5d1029528dcfb 100644
--- a/compiler/rustc_mir_build/src/build/mod.rs
+++ b/compiler/rustc_mir_build/src/builder/mod.rs
@@ -23,8 +23,8 @@ use rustc_span::symbol::sym;
 use rustc_span::{Span, Symbol};
 
 use super::lints;
-use crate::build::expr::as_place::PlaceBuilder;
-use crate::build::scope::DropKind;
+use crate::builder::expr::as_place::PlaceBuilder;
+use crate::builder::scope::DropKind;
 
 pub(crate) fn closure_saved_names_of_captured_variables<'tcx>(
     tcx: TyCtxt<'tcx>,
diff --git a/compiler/rustc_mir_build/src/build/scope.rs b/compiler/rustc_mir_build/src/builder/scope.rs
similarity index 99%
rename from compiler/rustc_mir_build/src/build/scope.rs
rename to compiler/rustc_mir_build/src/builder/scope.rs
index 636e47b7ad2f2..882e29de46d34 100644
--- a/compiler/rustc_mir_build/src/build/scope.rs
+++ b/compiler/rustc_mir_build/src/builder/scope.rs
@@ -95,7 +95,7 @@ use rustc_span::source_map::Spanned;
 use rustc_span::{DUMMY_SP, Span};
 use tracing::{debug, instrument};
 
-use crate::build::{BlockAnd, BlockAndExtension, BlockFrame, Builder, CFG};
+use crate::builder::{BlockAnd, BlockAndExtension, BlockFrame, Builder, CFG};
 
 #[derive(Debug)]
 pub(crate) struct Scopes<'tcx> {
diff --git a/compiler/rustc_mir_build/src/check_unsafety.rs b/compiler/rustc_mir_build/src/check_unsafety.rs
index f37b3f977fa25..6e255f851e22f 100644
--- a/compiler/rustc_mir_build/src/check_unsafety.rs
+++ b/compiler/rustc_mir_build/src/check_unsafety.rs
@@ -18,7 +18,7 @@ use rustc_span::def_id::{DefId, LocalDefId};
 use rustc_span::symbol::Symbol;
 use rustc_span::{Span, sym};
 
-use crate::build::ExprCategory;
+use crate::builder::ExprCategory;
 use crate::errors::*;
 
 struct UnsafetyVisitor<'a, 'tcx> {
diff --git a/compiler/rustc_mir_build/src/lib.rs b/compiler/rustc_mir_build/src/lib.rs
index 3dbb552cdbbef..1c82dd33ae7d0 100644
--- a/compiler/rustc_mir_build/src/lib.rs
+++ b/compiler/rustc_mir_build/src/lib.rs
@@ -11,7 +11,7 @@
 #![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
-mod build;
+mod builder;
 mod check_unsafety;
 mod errors;
 pub mod lints;
@@ -24,9 +24,9 @@ rustc_fluent_macro::fluent_messages! { "../messages.ftl" }
 pub fn provide(providers: &mut Providers) {
     providers.check_match = thir::pattern::check_match;
     providers.lit_to_const = thir::constant::lit_to_const;
-    providers.hooks.build_mir = build::mir_build;
+    providers.hooks.build_mir = builder::mir_build;
     providers.closure_saved_names_of_captured_variables =
-        build::closure_saved_names_of_captured_variables;
+        builder::closure_saved_names_of_captured_variables;
     providers.check_unsafety = check_unsafety::check_unsafety;
     providers.thir_body = thir::cx::thir_body;
     providers.hooks.thir_tree = thir::print::thir_tree;
diff --git a/compiler/rustc_mir_build/src/thir/constant.rs b/compiler/rustc_mir_build/src/thir/constant.rs
index 3fa0e4def82e6..62d3948edf547 100644
--- a/compiler/rustc_mir_build/src/thir/constant.rs
+++ b/compiler/rustc_mir_build/src/thir/constant.rs
@@ -5,7 +5,7 @@ use rustc_middle::mir::interpret::{LitToConstError, LitToConstInput};
 use rustc_middle::ty::{self, ScalarInt, TyCtxt};
 use tracing::trace;
 
-use crate::build::parse_float_into_scalar;
+use crate::builder::parse_float_into_scalar;
 
 pub(crate) fn lit_to_const<'tcx>(
     tcx: TyCtxt<'tcx>,
diff --git a/compiler/rustc_next_trait_solver/src/solve/inspect/build.rs b/compiler/rustc_next_trait_solver/src/solve/inspect/builder.rs
similarity index 100%
rename from compiler/rustc_next_trait_solver/src/solve/inspect/build.rs
rename to compiler/rustc_next_trait_solver/src/solve/inspect/builder.rs
diff --git a/compiler/rustc_next_trait_solver/src/solve/inspect/mod.rs b/compiler/rustc_next_trait_solver/src/solve/inspect/mod.rs
index 0d8c00601269b..cd1038475454e 100644
--- a/compiler/rustc_next_trait_solver/src/solve/inspect/mod.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/inspect/mod.rs
@@ -1,6 +1,6 @@
 pub use rustc_type_ir::solve::inspect::*;
 
-mod build;
-pub(in crate::solve) use build::*;
+mod builder;
+pub(in crate::solve) use builder::*;
 
 pub use crate::solve::eval_ctxt::canonical::instantiate_canonical_state;
diff --git a/src/tools/tidy/src/style.rs b/src/tools/tidy/src/style.rs
index 6394a1463b0da..fee3971042a96 100644
--- a/src/tools/tidy/src/style.rs
+++ b/src/tools/tidy/src/style.rs
@@ -343,7 +343,8 @@ pub fn check(path: &Path, bad: &mut bool) {
         let filename = file.file_name().unwrap().to_string_lossy();
 
         let is_style_file = filename.ends_with(".css");
-        let under_rustfmt = filename.ends_with(".rs") &&
+        let is_rust_file = filename.ends_with(".rs");
+        let under_rustfmt = is_rust_file &&
             // This list should ideally be sourced from rustfmt.toml but we don't want to add a toml
             // parser to tidy.
             !file.ancestors().any(|a| {
@@ -485,6 +486,21 @@ pub fn check(path: &Path, bad: &mut bool) {
                     err("Don't use magic numbers that spell things (consider 0x12345678)");
                 }
             }
+
+            // Forbid `mod build;` and `mod build {`, because they imply the
+            // possible existence of a source directory named "build" that would
+            // be ignored by our blanket .gitignore rule for `build/`.
+            // (Bear in mind that some tools don't support negated .gitignore rules.)
+            // This also has the side-effect of forbidding `build.rs` files that
+            // aren't build scripts.
+            if is_rust_file
+                && !trimmed.starts_with("//")
+                && static_regex!(r"\bmod\s+build\s*[;{]").is_match(trimmed)
+            {
+                err("modules named `build` can cause problems for git-adjacent tools; \
+                    use a different name (e.g. `builder`)");
+            }
+
             // for now we just check libcore
             if trimmed.contains("unsafe {")
                 && !trimmed.starts_with("//")