[POC] {cwd} placeholder in --remap-path-prefix#157405
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Proof-of-concept for an alternative to the incremental-cache invalidation caused
by remapping the current working directory. Validates feasibility ahead of a
Major Change Proposal; not meant to merge as-is.
Today `-Zremap-cwd-prefix=V` resolves the absolute cwd at parse time and stores
it in `remap_path_prefix` (`[TRACKED_NO_CRATE_HASH]`), so the working directory
enters the incremental command-line hash and a build from a different directory
(e.g. a sandbox) purges the cache even though the remapped output is identical.
This lets a `--remap-path-prefix` FROM contain a `{cwd}` placeholder. The FROM is
stored verbatim (so `{cwd}` the placeholder and `{{cwd}}` the escaped literal stay
distinct in storage, with no new type needed), and the placeholder is expanded to
the working directory only when the `FilePathMapping` is built. The stored value
is therefore stable across build directories and keeps the incremental cache
valid. `-Zremap-cwd-prefix=V` becomes exactly sugar for
`--remap-path-prefix={cwd}=V`, providing a path to sunset it.
`{cwd}` uses `format!`-style escaping (`{{`/`}}`). Open questions for the MCP:
unknown-placeholder handling (kept literal here), the precise escaping rules, and
the relationship to remap-path-scope and RFC 3127. Adding placeholder semantics to
a stable flag is a stable-surface change and needs sign-off.
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Warning
Proof-of-concept, do not merge. Validates feasibility + CI ahead of a Major Change Proposal, since it adds placeholder semantics to the stable
--remap-path-prefixflag. Companion to #157348 (the minimal, unstable-only fix that can land now).Idea
Let a
--remap-path-prefixFROM contain a{cwd}placeholder:The FROM is stored verbatim in
remap_path_prefix, and the placeholder is expanded to the working directory only when theFilePathMappingis built (apply time). So:{cwd}/sub), not the absolute cwd → its dependency hash is stable across build directories → the incremental cache survives (fixes the cwd half of no-op remap_path_prefix change invalidates incremental cache #132132).{cwd}(placeholder) and{{cwd}}(escaped literal) distinct in storage, so a plainPathBufis sufficient.-Zremap-cwd-prefix=Vis reimplemented as exactly sugar for--remap-path-prefix={cwd}=V, giving a path to sunset that unstable flag.Escaping
format!-style: a path component{cwd}is the placeholder;{{/}}are literal{/}, so a literal{cwd}directory is written{{cwd}}.Why
{cwd}(vs<cwd>,%cwd%,\(cwd))format!syntax — native to Rust, self-escaping ({{/}}), doesn't look path-like.<cwd>/\(cwd): shell redirection / backslash issues (\is the Windows path separator).%cwd%: collides with cmd.exe%VAR%(%CD%).(Hand-typed it's quoted like any tool-level token; build systems pass argv with no shell, so it arrives verbatim.)
Open questions for the MCP
{cwd},{{/}}here).--remap-path-scope; relationship to RFC 3127.Tests
{cwd}/substored-verbatim + applied; escaped{{cwd}}stays literal;-Zremap-cwd-prefix→{cwd}equivalence.r? compiler