add decls.byLent for overload resolution

Author: timotheecour

changelog.md

@@ -119,6 +119,7 @@
 - add `macros.extractDocCommentsAndRunnables` helper
 
 - `strformat.fmt` and `strformat.&` support `= specifier`. `fmt"{expr=}"` now expands to `fmt"expr={expr}"`.
+- Add `decls.byLent` to turn an argument into a let param, useful for overload resolution.
 
 ## Language changes
 - In the newruntime it is now allowed to assign to the discriminator field

compiler/options.nim

@@ -13,6 +13,7 @@ import
 
 from terminal import isatty
 from times import utc, fromUnix, local, getTime, format, DateTime
+from std/decls import byLent
 
 const
   hasTinyCBackend* = defined(tinyc)
@@ -619,12 +620,6 @@ proc shortenDir*(conf: ConfigRef; dir: string): string {.
     return substr(dir, prefix.len)
   result = dir
 
-proc removeTrailingDirSep*(path: string): string =
-  if (path.len > 0) and (path[^1] == DirSep):
-    result = substr(path, 0, path.len - 2)
-  else:
-    result = path
-
 proc disableNimblePath*(conf: ConfigRef) =
   incl conf.globalOptions, optNoNimblePath
   conf.lazyPaths.setLen(0)
@@ -653,7 +648,7 @@ proc getNimcacheDir*(conf: ConfigRef): AbsoluteDir =
                (if isDefined(conf, "release") or isDefined(conf, "danger"): "_r" else: "_d"))
 
 proc pathSubs*(conf: ConfigRef; p, config: string): string =
-  let home = removeTrailingDirSep(os.getHomeDir())
+  let home = os.getHomeDir().normalizePathEnd
   result = unixToNativePath(p % [
     "nim", getPrefixDir(conf).string,
     "lib", conf.libpath.string,
@@ -668,7 +663,7 @@ iterator nimbleSubs*(conf: ConfigRef; p: string): string =
   let pl = p.toLowerAscii
   if "$nimblepath" in pl or "$nimbledir" in pl:
     for i in countdown(conf.nimblePaths.len-1, 0):
-      let nimblePath = removeTrailingDirSep(conf.nimblePaths[i].string)
+      let nimblePath = conf.nimblePaths[i].string.byLent.normalizePathEnd
       yield p % ["nimblepath", nimblePath, "nimbledir", nimblePath]
   else:
     yield p

lib/std/decls.nim

@@ -1,5 +1,11 @@
-# see `semLowerLetVarCustomPragma` for compiler support that enables these
-# lowerings
+#[
+keep this module dependency-light to be usable in low level modules.
+
+see `semLowerLetVarCustomPragma` for compiler support that enables these
+lowerings
+
+Experimental module, 
+]#
 
 template byaddr*(lhs, typ, ex) =
   ## Allows a syntax for lvalue reference, exact analog to
@@ -17,3 +23,15 @@ template byaddr*(lhs, typ, ex) =
   else:
     let tmp: ptr typ = addr(ex)
   template lhs: untyped = tmp[]
+
+proc byLent*[T](a: T): lent T {.inline.} =
+  ## Transforms `a` into a let param without copying; this is useful for overload
+  ## resolution
+  runnableExamples:
+    proc fn(a: int): int = result = a*2
+    proc fn(a: var int) = a = a*2
+    var x = 3
+    # x = fn(x)  # would give: Error: expression 'fn(x)' has no type (or is ambiguous)
+    x = fn(x.byLent) # works
+    doAssert x == 3*2
+  a

tests/js/taddr.nim

@@ -78,10 +78,11 @@ doAssert(someGlobalPtr[] == 5)
 someGlobalPtr[] = 10
 doAssert(someGlobal == 10)
 
+from std/decls import byLent
+
 block:
   # issue #14576
   # lots of these used to give: Error: internal error: genAddr: 2
-  proc byLent[T](a: T): lent T = a
   proc byPtr[T](a: T): ptr T = a.unsafeAddr
 
   block:
@@ -90,14 +91,10 @@ block:
     doAssert (x,y) == a
 
   block:
-    when defined(c) and defined(release):
-      # bug; pending https://github.com/nim-lang/Nim/issues/14578
-      discard
-    else:
-      let a = 10
-      doAssert byLent(a) == 10
-      let a2 = byLent(a)
-      doAssert a2 == 10
+    let a = 10
+    doAssert byLent(a) == 10
+    let a2 = byLent(a)
+    doAssert a2 == 10
 
   block:
     let a = [11,12]

tests/stdlib/tdecls.nim

@@ -1,6 +1,6 @@
 import std/decls
 
-block:
+block: # byaddr
   var s = @[10,11,12]
   var a {.byaddr.} = s[0]
   a+=100
@@ -34,23 +34,9 @@ block:
   doAssert compiles(block:
     var b2 {.byaddr.}: int = s[2])
 
-## We can define custom pragmas in user code
-template byUnsafeAddr(lhs, typ, expr) =
-  when typ is type(nil):
-    let tmp = unsafeAddr(expr)
-  else:
-    let tmp: ptr typ = unsafeAddr(expr)
-  template lhs: untyped = tmp[]
-
-block:
-  let s = @["foo", "bar"]
-  let a {.byUnsafeAddr.} = s[0]
-  doAssert a == "foo"
-  doAssert a[0].unsafeAddr == s[0][0].unsafeAddr
-
-block: # nkAccQuoted
-  # shows using a keyword, which requires nkAccQuoted
-  template `cast`(lhs, typ, expr) =
+block: # custom var pragmas
+  ## We can define custom pragmas in user code
+  template byUnsafeAddr(lhs, typ, expr) =
     when typ is type(nil):
       let tmp = unsafeAddr(expr)
     else:
@@ -59,12 +45,34 @@ block: # nkAccQuoted
 
   block:
     let s = @["foo", "bar"]
-    let a {.`byUnsafeAddr`.} = s[0]
+    let a {.byUnsafeAddr.} = s[0]
     doAssert a == "foo"
     doAssert a[0].unsafeAddr == s[0][0].unsafeAddr
 
-  block:
-    let s = @["foo", "bar"]
-    let a {.`cast`.} = s[0]
-    doAssert a == "foo"
-    doAssert a[0].unsafeAddr == s[0][0].unsafeAddr
+  block: # nkAccQuoted
+    # shows using a keyword, which requires nkAccQuoted
+    template `cast`(lhs, typ, expr) =
+      when typ is type(nil):
+        let tmp = unsafeAddr(expr)
+      else:
+        let tmp: ptr typ = unsafeAddr(expr)
+      template lhs: untyped = tmp[]
+
+    block:
+      let s = @["foo", "bar"]
+      let a {.`byUnsafeAddr`.} = s[0]
+      doAssert a == "foo"
+      doAssert a[0].unsafeAddr == s[0][0].unsafeAddr
+
+    block:
+      let s = @["foo", "bar"]
+      let a {.`cast`.} = s[0]
+      doAssert a == "foo"
+      doAssert a[0].unsafeAddr == s[0][0].unsafeAddr
+
+block: # byLent
+  proc fn(a: int): int = result = a*2
+  proc fn(a: var int) = a = a*5
+  var x = 3
+  x = fn(x.byLent)
+  doAssert x == 3*2